Architectural Glass

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All about usage of glass in Architecture




Architectural glass
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Jump to: navigation, search Architectural glass has been used in buildings since the 11th century. Glass is typically used in buildings as a transparent glazing material for windows in the building envelope. Glass is also used in glazed internal partitions and as an architectural feature. Glass in buildings is often of a safety type, including toughened and laminated glasses

Bullseye: The earliest style of glass window
The uneven surface of old glass is visible in the reflection on this window pane. It is not, however, a bullseye window. The earliest method of glass window manufature was the bullseye method. Hot blown glass was cut open opposite the pipe, then rapidly spun on a table before it could cool. Centrifugal force forced the hot globe of glass into a round, flat sheet. The sheet would then be broken off the pipe and trimmed to form a rectangular window to fit into a frame. At the center of a bullseye window, a thick remnent of the original blown bottle neck would remain, hence the name "bullseye." Optical distortions produced by the bullseye could be reduced by grinding the glass. The bullseye method for manufacturing flat glass panels was very expensive and could not be used to make large panes. It was replaced in the 19th century by the cylinder, sheet and rolled plate processes, but it is still used in traditional constrution and restoration.

Cylinder glass
In this manufacturing process glass is blown into a cylindrical iron mould. The ends are cut off and a cut is made down the side of the cylinder. The cut cylinder is then placed in an oven where the cylinder unrolls into a flat glass sheet. William J. Blenko used this method in the early 1900s to make stained glass. These imperfect panes have led to the misconception that glass is actually a high-viscosity liquid at room temperature, which is not the case.

Sheet glass

Sheet glass (sometimes called window glass or drawn glass) was made by dipping a leader into a vat of molten glass then pulling that leader straight up while a film of glass hardened just out of the vat. This film or ribbon was pulled up continuously held by tractors on both edges while it cooled. After 12 meters or so it was cut off the vertical ribbon and tipped down to be further cut. This glass is clear but has thickness variations due to small temperature changes just out of the vat as it was hardening. These variations cause lines of slight distortions. You may still see this glass in older houses. Float glass replaced this process.

Rolled plate glass
The glass is taken from the furnace in large iron ladles, which are carried upon slings running on overhead rails; from the ladle the glass is thrown upon the cast-iron bed of a rolling-table; and is rolled into sheet by an iron roller, the process being similar to that employed in making plate-glass, but on a smaller scale. The sheet thus rolled is roughly trimmed while hot and soft, so as to remove those portions of glass which have been spoilt by immediate contact with the ladle, and the sheet, still soft, is pushed into the open mouth of an annealing tunnel or lehr, down which it is carried by a system of rollers.

Polished plate glass
The plate glass process starts with sheet or rolled plate glass. This glass is dimensionally inaccurate and often created visual distortions. These rough panes were ground flat then polished clear. This was a fairly expensive process. Before the float process, mirrors were plate glass as sheet glass had visual distortions that were akin to those seen in amusement park or fun-fair mirrors.

Float glass
90% of the world's flat glass is produced by the float glass process invented in the 1950s by Sir Alastair Pilkington of Pilkington Glass, in which molten glass is poured onto one end of a molten tin bath. The glass floats on the tin, and levels out as it spreads along the bath, giving a smooth face to both sides. The glass cools and slowly solidifies as it travels over the molten tin and leaves the tin bath in a continuous ribbon. The glass is then annealed by cooling in a temperature controlled oven called a "lehr". The finished product has near-perfect parallel surfaces. A very small amount of the tin is embedded into the glass on the side it touched. The tin side is easier to make into a mirror. This "feature" quickened the switch from plate to float glass. The tin side of glass is also softer and easier to scratch. Glass is produced in standard metric thicknesses of 2, 3, 4, 5, 6, 8, 10, 12, 15, 19 and 22 mm. Molten glass floating on tin in a nitrogen/hydrogen atmosphere will spread out to a thickness of about 6 mm and stop due to surface tension. Thinner glass is made by

stretching the glass while it floats on the tin and cools. Similarly, thicker glass is pushed back and not permitted to expand as it cools on the tin.

Annealed glass
Annealed glass is glass without internal stresses caused by heat treatment (ie toughening or heat strengthening). Glass becomes annealed if it becomes heated above a transition point, then allowed to cool slowly, through not being quenched. Thus glass made using the float glass process is annealed by the process of manufacture. Because of this, where glass is simply described or specified as "float glass", this usually means annealed float glass, although in fact most toughened glass is strictly float glass, as it has been made using the float glass process. Annealed glass is the common glass that breaks into large, jagged shards that can cause serious injury, hence annealed glass is considered a hazard in architectural applications. Building codes in many parts of the world restrict the use of annealed glass in areas where there is a high risk of breakage and injury, for example in bathrooms, in door panels, fire exits and at low heights in schools.

Figure rolled glass
The elaborate patterns found on figure rolled glass are produced by in a similar fashion to the rolled plate glass process except that the plate is cast between two moving rollers. The pattern is impressed upon the sheet by a printing roller which is brought down upon the glass as it leaves the main rolls while still soft. This glass shows a pattern in high relief. The glass is then annealed in a lehr. The glass used for this purpose is typically whiter in colour than the clear glasses used for other applications. This glass can be laminated or toughened depending on the depth of the pattern to produce a safety glass.

Laminated glass
Laminated glass is a type of safety glass that holds together when shattered. In the event of breakage, it is held in place by an interlayer, typically of PVB, between its two or more layers of glass. The interlayer keeps the layers of glass bonded even when broken, and its high strength prevents the glass from breaking up into large sharp pieces. This produces a characteristic "spider web" cracking pattern when the impact is not enough to completely pierce the glass. Laminated glass is normally used when there is a possibility of human impact or where the glass could fall if shattered. Shopfront glazing and windshields are typically laminated glasses. The PVB interlayer also gives the glass a much higher sound insulation rating, due to the damping effect, and also blocks 99% of transmitted UV light. Using toughened glass on windshields would be a problem when a small stone hits the

windshield at speed, if it were toughened and the stone hit with enough force the whole windshield would shatter into the small squares making visibility difficult and it would also be likely that the wind would blow the small squares into the driver and passengers. Laminated glass was invented in 1903 by the French chemist Edouard Benedictus, inspired by a laboratory accident. A glass flask had become coated with the plastic cellulose nitrate and when dropped shattered but did not break into pieces. Benedictus fabricated a glass-plastic composite to reduce injuries in car accidents. However, it was not immediately adopted by automobile manufacturers, and the first widespread use of laminated glass was in the eyepieces of gas masks during World War I. Today, laminated glass is produced by bonding two or more layers of ordinary annealed glass together with a plastic interlayer, usually polyvinyl butyral (PVB). The PVB is sandwiched by the glass which is passed through rollers to expel any air pockets and form the initial bond then heated to around 70 °C in a pressurized oil bath. The tint at the top of some car windshields is in the PVB. A typical laminated makeup would be 3 mm glass / 0.38 mm interlayer / 3 mm glass. This gives a final product that would be referred to as 6.38 laminated glass. Multiple laminates and thicker glass increases the strength. Bulletproof glass is often made of several float glass, toughened glass and Perspex panels, and can be as thick as 100 mm. A similar glass is often used in airliners on the front windows, often three sheets of 6 mm toughened glass with thick PVB between them.

Toughened glass (Tempered glass)
Toughened glass (also known as tempered glass) is a type of safety glass that has increased strength and will usually shatter in small, square pieces when broken. It is used when strength, thermal resistance and safety are important considerations. At home you are likely to find toughened glass in shower and sliding glass patio doors. In commercial structures it is used in unframed assemblies such as frameless doors, structurally loaded applications and any glass where these is a danger of human impact. Using toughened glass can pose a security risk in some situations due to the tendency the glass has to shatter utterly upon hard impact. Toughened glass is typically four to six times the strength of annealed glass. However, this strength comes with a penalty. Due to the balanced stresses in the glass, damage to the glass will eventually result in the glass shattering into thumbnail sized pieces. Although toughened glass is most susceptible to breakage via edge damage, breakage can also occur from impacts in the centre of the glass pane. Shattering may not happen when the damage originally occurs and can be triggered by a minor stress like heat or small impact that would not normally affect the toughened glass. If any toughened glass shows any damage it must be replaced.

Toughened glass must be cut to size or pressed to shape before toughening and cannot be re-worked once toughened. Polishing the edges or drilling holes in the glass is carried out before the toughening process starts. Also, ironically, the toughened glass surface is not as hard as annealed glass and is slightly more susceptible to scratching. Toughened glass is made from annealed glass via a thermal tempering process. The glass is placed onto a roller table, taking it through a furnace which heats it to above its annealing point of 600 °C. The glass is then rapidly cooled with forced draughts of air. This rapidly cools the glass surface below its annealing point, causing it to harden and contract, while the inner portion of the glass remains free to flow for a short time. The final contraction of the inner layer induces compressive stresses in the surface of the glass balanced by tensile stresses in the body of the glass. This compressive stress on the surface of the glass is typically as high as 50 MPa. It is this compressive stress that gives the toughened glass an increased strength. This is because any surface flaws tend to be pressed closed by the retained compressive forces, while the core layer remains relatively free of the defects which could cause a crack to begin. The pattern of cooling during the process can be revealed by observing the glass with polarized light, which shows the strain pattern in the glass. Though the underlying mechanism was not known at the time, the effects of "tempering" glass have been known for centuries. In the 1640s, Prince Rupert of Bavaria (1619– 1682), who was grandson of James I of England, and nephew of Charles I, brought the discovery of what are now known as "Prince Rupert's Drops" to the attention of the King. These are remarkable teardrop shaped bits of glass which are produced by allowing a molten drop of glass to fall into a bucket of water, thereby rapidly cooling it. These were often used by the King as a practical joke.

Heat-strengthened glass
Heat-strengthened glass is glass that has been heat treated to induce surface compression, but not to the extent of causing it to "dice" on breaking in the manner of tempered glass. On breaking, heat-strengthened glass breaks into sharp pieces that are typically somewhat smaller than those found on breaking annealed glass, and is intermediate in strength between annealed and toughened glasses.

Chemically strengthened glass
Chemically strengthened glass is a type of glass that has increased strength. When broken it still shatters in long pointed splinters similar to float (annealed) glass. For this reason, it is not considered a safety glass and must be laminated if a safety glass is required. Chemically strengthened glass is typically six to eight times the strength of annealed glass.

The glass is chemically strengthened by submersing the glass in a bath containing a potassium salt (typically potassium nitrate) at 450 °C. This causes sodium ions in the glass surface to be replaced by potassium ions from the bath solution. These potassium ions are larger than the sodium ions and therefore wedge into the gaps left by the smaller sodium ions when they migrate to the potassium nitrate solution. This replacement of ions causes the surface of the glass to be in a state of compression and the core in compensating tension. The surface compression of chemically strengthened glass may reach up to 690 MPa. There also exists a more advanced two-stage process for making chemically strengthened glass, in which the glass article is first immersed in a sodium nitrate bath at 450 °C, which enriches the surface with sodium ions. This leaves more sodium ions on the glass for the immersion in potassium nitrate to replace with potassium ions. In this way, the use of a sodium nitrate bath increases the potential for surface compression in the finished article. Chemical strengthening results in a strengthening similar to toughened glass, however the process does not use extreme variations of temperature and therefore chemically strengthened glass has little or no bow or warp, optical distortion or strain pattern. This differs from toughened glass, in which slender pieces can often be significantly bowed. Also unlike toughened glass, chemically strengthened glass may be cut after strengthening, but loses its added strength within the region of approximately 20 mm of the cut. Similarly, when the surface of chemically strengthened glass is deeply scratched, this area loses its additional strength. Chemically strengthened glass was used on some fighter aircraft canopies.

Low-emissivity glass Self-cleaning glass
A recent innovation is so-called self-cleaning glass, aimed at building, automotive and other technical applications. A 50 nanometre coating of titanium dioxide on the outer surface of glass introduces two mechanisms which lead to the self-cleaning property. The first is a photo-catalytic effect, in which ultra-violet rays catalyse the breakdown of organic compounds on the window surface; the second is a hydrophilic effect in which water is attracted to the surface of the glass, forming a thin sheet which washes away the broken-down organic compounds.

Insulated glazing
Insulated glazing, or double glazing is a piece of glazing consisting of two or more layers of glazing separated by a spacer along the edge and sealed to create a dead air space between the layers. This type of glazing has functions of thermal insulation and noise mitigation.

Evacuated glazing
Another recent innovation for insulated glazing is evacuated glass, which as yet is produced commercially only in Japan. The extreme thinness of evacuated glazing offers many new architectural possibilities, particularly in building conservation and historicist architecture, where evacuated glazing can replace traditional (much less energy-efficient) single glazing. An evacuated glazing unit is made by sealing the edges of two glass sheets, typically by using a solder glass, and evacuating the space inside with a vacuum pump. The evacuated space between the two sheets can be very shallow and yet be a good insulator, yielding insulative window glass with nominal thicknesses as low as 6 mm overall. The reasons for this low thickness are deceptively complex, but the potential insulation is good essentially because there can be no convection or gaseous conduction in a vacuum. Unfortunately, evacuated glazing does have some disadvantages; its manufacture is complicated and difficult. For example, a necessary stage in the manufacture of evacuated glazing is outgassing; that is, heating it to liberate any gases adsorbed on the inner surfaces, which could otherwise later escape and destroy the vacuum. This heating process currently means that evacuated glazing cannot be toughened or heatstrengthened. If an evacuated safety glass is required, the glass must be laminated. The high temperatures necessary for outgassing also tend to destroy the highly effective "soft" low-emissivity coatings that are often applied to one or both of the internal surfaces (i.e. the ones facing the air gap) of other forms of modern insulative glazing, in order to prevent loss of heat through infrared radiation. Slightly less effective "hard" coatings are still suitable for evacuated glazing, however. Furthermore, because of the atmospheric pressure present on the outside of an evacuated glazing unit, its two glass sheets must somehow be held apart in order to prevent them flexing together and touching each other, which would defeat the object of evacuating the unit. The task of holding the panes apart is performed by a grid of spacers, which typically consist of small stainless steel discs that are placed around 20 mm apart. The spacers are small enough that they are visible only at very close distances, typically up to 1 m. However, the fact that the spacers will conduct some heat often leads in cold weather to the formation of temporary, grid-shaped patterns on the surface of an evacuated window, consisting either of small circles of interior condensation centred around the spacers, where the glass is slightly colder than average, or, when there is dew outside, small circles on the exterior face of the glass, in which the dew is absent because the spacers make the glass near them slightly warmer. The conduction of heat between the panes, caused by the spacers, tends to limit evacuated glazing’s overall insulative effectiveness. Nevertheless, evacuated glazing is still as insulative as much thicker conventional double glazing and tends to be stronger, since the two constituent glass sheets are pressed together by the atmosphere, and hence react practically as one thick sheet to bending forces. Evacuated glazing also offers very good sound insulation in comparison with other popular types of window glazing.

1)Pilkington plc is the largest glass manufacturer in the United Kingdom. It is based in St Helens, Merseyside. It was formerly an independent company listed on the London Stock Exchange and a constituent of the FTSE 100 Index, but in 2006 is was taken over by Nippon Sheet Glass of Japan. The company was founded in 1826 as a family-run business in St Helens. It was at one time the biggest employer in the northwest industrial town. The distinctive blue-glass head office still dominates the town's skyline. Pilkington has turnover of £2.7 billion, with manufacturing operations on five continents, and sales in more than a hundred countries. In 1957 Pilkington invented the Float Process, a revolutionary method of glass production floating molten glass over a bath of molten tin, abolishing the costly need to polish and grind glass to make it clear. Pilkington then sold the Float Process under licence to the rest of the world. In late 2005 the company received a takeover bid from the smaller Japanese company Nippon Sheet Glass. The initial bid and the first revised bid were not accepted, but in February 2006 Nippon increased its offer for the 80% it did not already own to 165 pence per share (£1.8 billion or $3.14 billion in total) and this was accepted. The combined company will compete for global leadership in the glass industry with the leading Japanese glassmaker Asahi Glass, which had around a quarter of the global market at the time of the deal. Pilkington had 19% and Nippon Sheet Glass around half that.

2) Float glass
Float glass is sheet glass made by floating the molten glass on a bed of molten tin. This method gives the glass uniform thickness and very flat surfaces.

In earlier centuries, window glass was made by blowing either large bottles or large disks. The bottles were cut apart and flattened and then window panes were cut from the large surface. Most glass for windows up to the early 19th century was made from rondels, while most window glass during the 19th century was made using the bottle method (these 'bottles' were 6 to 8 feet (2 to 2.5 m) long and 10 to 14 inches (250 to 350 mm) in diameter). 1)Alastair Pilkington has been identified by many sources as the inventor of the float glass process even though it was first patented in 1848 by Henry Bessemer, an English engineer. The float glass process was also patented in the United States in 1902 by W. E. Heal and again in 1925 by Hitchcock (a revised version of Heal's patent). Sir Alastair Pilkington was however the first person who successfully developed and achieved a commercial implementation of this revolutionary flat glass production technology.

Before the development of float glass, larger sheets of plate glass were made by casting a large puddle of glass on an iron surface, and then grinding and polishing both sides to smooth clarity, a very expensive process. Glass of lower quality, sheet glass, was made by drawing upwards from a pool of molten glass a thin sheet, held at the edges by rollers. As it cooled the rising sheet solidified and could then be cut. The two surfaces were less parallel and of lower quality than those of float glass. This process continued for many years after the development of float glass.

Float glass is made by melting raw materials consisting of sand, limestone, soda ash, dolomite, iron oxide and salt cake. The raw materials are mixed together and fed into a large furnace that is natural gas or fuel oil fired. The raw materials, referred to as batch, blend together to form a large pool of molten glass. The molten glass is fed into the float bath (tin bath) through a delivery canal. The amount of glass allowed to pour onto the molten tin is controlled by a refractory gate called a tweel. The tin bath is pressurized by a mixture of nitrogen and hydrogen to prevent oxidation. The glass flattens out, forming a perfectly smooth glossy surface on both sides with an even thickness of approximately 6 mm. Thinner glass is made by stretching the glass ribbon to achieve the proper thickness. Thicker glass is made by not allowing the glass pool to flatten to 6 mm. Machines called attenuators are used in the tin bath to control both the thickness and the width of the glass ribbon. As the glass flows down the tin bath, the temperature is gradually reduced until the sheet can be lifted from the tin onto rollers. It then passes through the lehr where it is further cooled gradually so that it anneals without strain and does not crack from the change in temperature. The glass travels down the rollers in the lehr for about 100 metres and comes out at the "cold end" where it is cut by machines. The nitrogen and hydrogen keeps the tin from oxidizing. Some tin is absorbed into the glass, and with a proper ultraviolet light a sheen can be seen which differentiates the tin from the non-tin side.

3) Surface tension

In physics, surface tension is an effect within the surface layer of a liquid that causes that layer to behave as an elastic sheet. This effect allows insects (such as the water strider) to walk on water, and causes capillary action. Surface tension is caused by the attraction between the molecules of the liquid by various intermolecular forces. In the bulk of the liquid each molecule is pulled equally in all directions by neighboring liquid molecules, resulting in a net force of zero. At the surface of the liquid, the molecules are pulled inwards by other molecules deeper inside the liquid, but there are no liquid molecules on the outside to balance these forces. (There may also be a small outward attraction caused by air molecules, but as air is much less dense than the liquid, this force is negligible.) All of the molecules at the surface are therefore subject to an inward force of molecular attraction which can be balanced only by the resistance of the liquid to compression. Thus the liquid squeezes itself together until it has the locally lowest surface area possible. Surface tension, measured in newtons per meter (N·m-1), is represented by the symbol σ or γ or T and is defined as the force along a line of unit length perpendicular to the surface, or work done per unit area. Dimensional analysis and the work-energy theorem show that the units of surface tension (N·m-1) are equivalent to joules per square metre (J·m-2). This means that surface tension can also be considered as surface energy. If a surface with surface tension σ is expanded by a unit area, then the increase in the surface's stored energy is also equal to σ. A related quantity is the energy of cohesion, which is the energy released when two bodies of the same liquid become joined by a boundary of unit area. Since this process involves the removal of a unit area of surface from each of the two bodies of liquid, the energy of cohesion is equal to twice the surface energy. A similar concept, the energy of adhesion, applies to two bodies of different liquids. Energy of adhesion is linked to the surface tension of an interface between two liquids. Surface tension prevents a coin from sinking: the coin is indisputably denser than water, so cannot be floating due to buoyancy alone.

4) Risk
Risk is a concept which relates to human expectations. It denotes a potential negative impact to an asset or some characteristic of value that may arise from some present process or from some future event. In everyday usage, "risk" is often used synonymously with "probability" of a loss or threat. In professional risk assessments, risk combines the probability of an event occurring with the impact that event would have and with its different circumstances. However, where assets are priced by markets, all probabilities and impacts are reflected in the market price, and risk therefore comes only from the variance of the outcomes; this startling fact is one of the conclusions of Black-Scholes pricing theory.

Defined aspects of risk
There are many definitions of risk, they depend on the specific application and situational contexts. Most general, every risk (indicator) is proportional to the expected losses which can be caused by a risky event and to the probability of this event. A threat and vulnerability are characterised by a risk. Greater the vulnerability and the threat higher the risk. Therefore, the differentiation of risk definitions depends on the losses context, their assessment and measurement, as well as, when the losses are clear and invariable, for example a human life, the risk assessment is focused on the probability of the event, event frequency and its circumstances. We distinguish two types of risk, the first is based on scientific and engineering estimations and the second, called effective risk is dependent on human risk perception. In practice, these two assessments are in continuous conflicts in social and political sciences. Engineering definition of risk, an example: . Financial risk is often defined as the unexpected variability or volatility of returns, and thus includes both potential worse than expected as well as better than expected returns. References to negative risk below should be read as applying to positive impacts or opportunity (e.g. for loss read "loss or gain") unless the context precludes. In statistics, risk is often mapped to the probability of some event which is seen as undesirable. Usually the probability of that event and some assessment of its expected harm must be combined into a believable scenario (an outcome) which combines the set of risk, regret and reward probabilities into an expected value for that outcome. (See also Expected utility) Thus in statistical decision theory, the risk function of an estimator δ(x) for a parameter θ, calculated from some observables x; is defined as the expectation value of the loss function L, where:
• •

δ(x) = estimator θ = the parameter of the estimator

There are many informal methods used to assess or to "measure" risk. Although it is not usually possible to directly measure risk. Formal methods measure the value at risk. In scenario analysis "risk" is distinct from "threat." A threat is a very low-probability but serious event - which some analysts may be unable to assign a probability in a risk

assessment because it has never occurred, and for which no effective preventive measure (a step taken to reduce the probability or impact of a possible future event) is available. The difference is most clearly illustrated by the precautionary principle which seeks to reduce threat by requiring it to be reduced to a set of well-defined risks before an action, project, innovation or experiment is allowed to proceed. In information security a "risk" is defined as a function of three variables:
• • •

the probability that there's a threat the probability that there are any vulnerabilities the potential impact.

If any of these variables approaches zero, the overall risk approaches zero. The management of actuarial risk is called risk management.

Risk vs. Uncertainty
In his seminal work "Risk, Uncertainty, and Profit", Frank Knight (1921) established the distinction between risk and uncertainty. … Uncertainty must be taken in a sense radically distinct from the familiar notion of Risk, from which it has never been properly separated. … The essential fact is that "risk" means in some cases a quantity susceptible of measurement, while at other times it is something distinctly not of this character; and there are far-reaching and crucial differences in the bearings of the phenomena depending on which of the two is really present and operating. … It will appear that a measurable uncertainty, or "risk" proper, as we shall use the term, is so far different from an un-measurable one that it is not in effect an uncertainty at all.

Risk in business
Means of measuring and assessing risk vary widely across different professions--indeed, means of doing so may define different professions, e.g. a doctor manages medical risk, a civil engineer manages risk of structural failure, etc. A professional code of ethics is usually focused on risk assessment and mitigation (by the professional on behalf of client, public, society or life in general).

Risk-sensitive industries
Some industries manage risk in a highly-quantified and numerate way. These include the nuclear power and aircraft industries, where the possible failure of a complex series of engineered systems could result in highly undesirable outcomes. The usual measure of risk for a class of events is then, where P is probability and C is consequence;

The total risk is then the sum of the individual class-risks. In the nuclear industry, 'consequence' is often measured in terms of off-site radiological release, and this is often banded into five or six decade-wide bands. The risks are evaluated using Fault Tree/Event Tree techniques (see safety engineering). Where these risks are low they are normally considered to be 'Broadly Acceptable'. A higher level of risk (typically up to 10 to 100 times what is considered broadly acceptable) has to be justified against the costs of reducing it further and the possible benefits that make it tolerable - these risks are described as 'Tolerable if ALARP'. Risks beyond this level are classified as 'Intolerable'. The level of risk deemed 'Broadly Acceptable' has been considered by Regulatory bodies in various countries - an early attempt by UK government regulator & academic F. R. Farmer used the example of hill-walking and similar activities which have definable risks that people appear to find acceptable. This resulted in the so-called Farmer Curve, of acceptable probability of an event versus its consequence. The technique as a whole is usually referred to as Probabilistic Risk Assessment (PRA), (or Probabilistic Safety Assessment, PSA). See WASH-1400 for an example of this approach.

Risk in finance
Main article: Financial risk "The chance that an investment's actual return will be different than expected. This includes the possibility of losing some or all of the original investment. It is usually measured by calculating the standard deviation of the historical returns or average returns of a specific investment". Risk in finance has no one definition, but some theorists, notably Ron Dembo, have defined quite general methods to assess risk as an expected after-the-fact level of regret. Such methods have been uniquely successful in limiting interest rate risk in financial markets. Financial markets are considered to be a proving ground for general methods of risk assessment. However, these methods are also hard to understand. The mathematical difficulties interfere with other social goods such as disclosure, valuation and transparency. In particular, it is often difficult to tell if such financial instruments are "hedging" (purchasing/selling a financial instrument specifically to reduce or cancel out the risk in another investment) or "gambling" (increasing measurable risk and exposing the investor to catastrophic loss in pursuit of very high windfalls that increase expected value). As regret measures rarely reflect actual human risk-aversion, it is difficult to determine if the outcomes of such transactions will be satisfactory. Risk seeking describes an individual who has a positive second derivative of his/her utility function. Such an individual would willingly (actually pay a premium to) assume all risk in the economy and is hence not likely to exist.

In financial markets one may need to measure credit risk, information timing and source risk, probability model risk, and legal risk if there are regulatory or civil actions taken as a result of some "investor's regret". "A fundamental idea in finance is the relationship between risk and return. The greater the amount of risk that an investor is willing to take on, the greater the potential return. The reason for this is that investors need to be compensated for taking on additional risk". "For example, a U.S. Treasury bond is considered to be one of the safest investments and, when compared to a corporate bond, provides a lower rate of return. The reason for this is that a corporation is much more likely to go bankrupt than the U.S. government. Because the risk of investing in a corporate bond is higher, investors are offered a higher rate of return".

Risk in public works
In a peer reviewed study of risk in public works projects located in 20 nations on five continents, Flyvbjerg, Holm, and Buhl (2002, 2005) documented high risks for such ventures for both costs [1] and demand [2]. Actual costs of projects were typically higher than estimated costs; cost overruns of 50% were common, overruns above 100% not uncommon. Actual demand was often lower than estimated; demand shortfalls of 25% were common, of 50% not uncommon. Due to such cost and demand risks, cost-benefit analyses of public works projects have proved to be highly uncertain. The main causes of cost and demand risks were found to be optimism bias and strategic misrepresentation. Measures identified to mitigate this type of risk are better governance through incentive alignment and the use of reference class forecasting [3].

Psychology of risk
In decision theory, regret (and anticipation of regret) can play a significant part in decision-making, distinct from risk aversion (preferring the status quo in case one becomes worse off).

Framing is a fundamental problem with all forms of risk assessment. In particular, because of bounded rationality (our brains get overloaded, so we take mental shortcuts) the risk of extreme events is discounted because the probability is too low to evaluate intuitively. As an example, one of the leading causes of death is road accidents caused by drunk driving - partly because any given driver frames the problem by largely or totally ignoring the risk of a serious or fatal accident.

The above examples: body, threat, price of life, professional ethics and regret show that the risk adjustor or assessor often faces serious conflict of interest. The assessor also faces cognitive bias and cultural bias, and cannot always be trusted to avoid all moral hazards. This represents a risk in itself, which grows as the assessor is less like the client. For instance, an extremely disturbing event that all participants wish not to happen again may be ignored in analysis despite the fact it has occurred and has a nonzero probability. Or, an event that everyone agrees is inevitable may be ruled out of analysis due to greed or an unwillingness to admit that it is believed to be inevitable. These human tendencies to error and wishful thinking often affect even the most rigorous applications of the scientific method and are a major concern of the philosophy of science. But all decisionmaking under uncertainty must consider cognitive bias, cultural bias, and notational bias: No group of people assessing risk is immune to "groupthink": acceptance of obviouslywrong answers simply because it is socially painful to disagree. One effective way to solve framing problems in risk assessment or measurement (although some argue that risk cannot be measured, only assessed) is to ensure that scenarios, as a strict rule, must include unpopular and perhaps unbelievable (to the group) high-impact low-probability "threat" and/or "vision" events. This permits participants in risk assessment to raise others' fears or personal ideals by way of completeness, without others concluding that they have done so for any reason other than satisfying this formal requirement. For example, an intelligence analyst with a scenario for an attack by hijacking might have been able to insert mitigation for this threat into the U.S. budget. It would be admitted as a formal risk with a nominal low probability. This would permit coping with threats even though the threats were dismissed by the analyst's superiors. Even small investments in diligence on this matter might have disrupted or prevented the attack-- or at least "hedged" against the risk that an Administration might be mistaken.

5) Windshield
The windshield or windscreen of an aircraft, automobile, bus, motorcycle, or tram is the front window. Modern windshields are generally made of laminated safety glass, which consists of two curved sheets of glass with a plastic layer laminated between them for safety, and are glued into the window frame. Earlier windshields were made of toughened glass and were fitted in the frame using a rubber or neoprene seal. The modern, glued-in screens contribute to the vehicle's rigidity. Windshields protect the vehicle's occupants from wind, temperature extremes, and flying debris such as dust, insects, and rocks. Properly installed automobile windshields are also essential to safety; along with the roof of the car, they provide protection in the case of a roll-over accident in the vehicle. Motorcycle windscreens are often made of high-impact acrylic plastic.

In many places, laws restrict the use of heavily tinted glass in vehicle windshields; generally, laws specify the maximum level of tint permitted. There is noticeably more tint in the uppermost part of the windshield of motorvehicles that blocks glare from the sun. In aircraft windscreens, a current is applied through a conducting layer of tin(IV) oxide to generate heat to prevent icing. A similar system for automobile windshields, introduced on Ford vehicles ("Quickclear") in the 1980s, uses very thin heating wires embedded between the two laminations.

The term windshield is used generally throughout North America, although windscreen is often used for motorcycles and similar vehicles. The term windscreen is the usual term in the UK and Australia/New Zealand for all vehicles. In the USA, windscreen refers to the mesh or foam screen placed over a microphone to minimize wind noise, while a windshield refers to the front window of a car. In the UK, the meaning of these terms is reversed. Today’s windshields are a safety device just as your seat belts and air bags. The installation of the auto glass is done with an automotive grade urethane designed specifically for automobiles. The adhesive creates a molecular bond between the glass and the vehicle. If the adhesive bond fails at any point on the glass it can reduce the effectiveness of the air bag and substantially compromise the roofs structural integrity. Raymond Clough Auto windscreens less than 20 cm (8 in) in height are sometimes known as aeroscreens since they only deflect the wind. The twin aeroscreen setup (often called Brooklands) was popular among older sports and modern cars in vintage style. A wiperless windshield is a windshield that uses a mechanism other than wipers to remove snow and rain from the windshield. The concept car Acura TL features a wiperless windshield using a series of jet nozzles in the cowl to blow pressurized air onto the windshield.

Stone chip and crack damage
Many types of stone damage can be successfully repaired. Bulls eyes, cracks, starbreaks or a combination of all three, can be repaired without removing the screen, eliminating the risk of leaking or bonding problems sometimes associated with replacement.

6) Composite material

Composite materials (or composites for short) are engineered materials made from two or more constituent materials with significantly different physical or chemical properties and which remain separate and distinct within the finished structure.

== == There are of constituent materials: matrix and reinforcement. At least one portion (fraction) of each type is required. The matrix material surrounds and supports the reinforcement materials by maintaining their relative positions. The reinforcements impart special physical (mechanical and electrical) properties to enhance the matrix properties. A synergism produces material properties unavailable from naturally occurring materials. Due to the wide variety of matrix and reinforcement materials available, the design potential is incredible. This great variety has resulted in an enormous lexicon that confounds both new and experienced students. Names and descriptors arise from the respective experiences of different perspectives. While different industries use different terms to describe the same things, the same term can be applied in vastly different contexts. There are the so-called natural composites like bone and wood. Both of these are constructed by the processes of nature and are beyond the scope of this text. The emerging field of tissue engineering has several enabling technologies, one of them is composite materials. Much success has been achieved with a composite comprising a bioactive reinforcement material such as hydroxyapatite and a biodegradable matrix such as polylactic acid. == ==

The geometry of a two-phase composite material may have any of the following 10 connectivities: 0-0, 0-1, 0-2, 0-3, 1-1, 1-2, 1-3, 2-2, 2-3, and 3-3, where 0, 1, 2, 3 represent the dimensions of either phase.

Earliest examples
The most primitive composite materials comprised straw and mud in the form of bricks for building construction; the Biblical book of Exodus speaks of the Israelites being oppressed by Pharaoh, by being forced to make bricks without straw.making process can still be seen on Egyptian tomb paintings in the Metropolitan Museum of Art

Modern composites
The most advanced examples are used on spacecraft in demanding environments. The most visible applications pave roadways in the form of either steel and portland cement concrete or asphalt concrete. Engineered composite materials must be formed to shape. This involves strategically placing the reinforcements while manipulating the matrix properties to achieve a melding

event at or near the beginning of the component life cycle. A variety of methods are used according to the end item design requirements. These fabrication methods are commonly named moulding or casting processes, as appropriate, and both have numerous variations. The principle factors impacting the methodology are the natures of the chosen matrix and reinforcement materials. Another important factor is the gross quantity of material to be produced. Large quantities can be used to justify high capital expenditures for rapid and automated manufacturing technology. Small production quantities are accommodated with lower capital expenditures but higher labour costs at a correspondingly slower rate.

Many commercially produced composites use a polymer matrix material often called a resin or resin solution. There are many different polymers available depending upon the starting raw ingredients. There are several broad categories, each with numerous variations. The most common categories are known as polyester, vinyl ester, epoxy, phenolic, polyimide, polyamide, and others. The reinforcement materials are often fibers but also commonly ground minerals. Fibers are often transformed into a textile material such as a felt, fabric, knit or stitched construction. Advanced composite materials constitute a category comprising carbon fiber reinforcement and epoxy or polyimide matrix materials. These are the aerospace grade composites and typically involve laminate molding at high temperature and pressure to achieve high reinforcement volume fractions. These advanced composite materials feature high stiffness and/or strength to weight ratios. One component is often a strong fibre such as fiberglass, quartz, kevlar, Dyneema or carbon fiber that gives the material its tensile strength, while another component (called a matrix) is often a resin such as polyester, or epoxy that binds the fibres together, transferring load from broken fibers to unbroken ones and between fibers that are not oriented along lines of tension. Also, unless the matrix chosen is especially flexible, it prevents the fibers from buckling in compression. Some composites use an aggregate instead of, or in addition to, fibers. In terms of stress, any fibers serve to resist tension, the matrix serves to resist shear, and all materials present serve to resist compression, including any aggregate. Composite materials can be divided into two main categories normally referred to as short fiber reinforced materials and continuous fiber reinforced materials. Continuous reinforced materials will often constitute a layered or laminated structure. Shocks, impact, loadings or repeated cyclic stresses can cause the laminate to separate at the interface between two layers, a condition known as delamination. Individual fibers can separate from the matrix e.g. fiber pull-out.

7) Polyvinyl butyral

Polyvinyl butyral (or PVB) is a resin usually used for applications that require strong binding, optical clarity, adhesion to many surfaces, toughness and flexibility. It is prepared from polyvinyl alcohol by reaction with butanal. The major application is laminated safety glass for automobile windshields. Tradenames for PVB-films are: BUTACITE, SAFLEX, S-Lec, TROSIFOL

8) Bulletproof glass
Bulletproof glass is glass that is capable of stopping all manner of bullets fired at it. Such glass does not currently exist and so within the industry is referred to as Bullet resistant glass. It is usually constructed using a strong but transparent material such as polycarbonate thermoplastic or by using layers of laminated glass. The desired result is a material with an appearance and light-transmitting behaviour of standard glass but offers varying degrees of protection from small arms fire. The polycarbonate layer, usually consisting of products such as Cyrolon, Lexan and Tuffak, is often sandwiched between layers of regular glass. The use of plastic in the laminate provides impact-resistance, such as physical assault with a hammer, an axe, etc. The plastic provides little in the way of bullet-resistance. The glass, which is much harder than plastic, flattens the bullet and thereby prevents penetration. This type of bullet resistant glass is usually 70-75 mm thick. Bullet resistant glass constructed of laminated glass layers is built from glass sheets bonded together with polyvinyl butyral or polyurethane. This type of bullet resistant glass has been in regular use on combat vehicles since World War II; it is typically about 100120 mm thick and is usually extremely heavy.

One-way bullet resistant glass
Advances in bullet resistant glass have led to the invention of one-way bulletproof glass, such as used in some bank armored cars. This glass will resist incoming small arms fire striking the outside of the glass, but will allow those on the other side of the glass, such as guards firing from inside the armored car, to fire through the glass at the exterior threat. One-way bulletproof glass is usually made up of two layers, a brittle layer on the outside and a flexible one on the inside. When a bullet is fired from the outside it hits the brittle layer first, shattering an area of it. This shattering absorbs some of the bullet's kinetic energy, and spreads it on a larger area. When the slowed bullet hits the flexible layer, it is stopped. However, when a bullet is fired from the inside, it hits the flexible layer first. The bullet penetrates the flexible layer because its energy is focused on a smaller area, the brittle layer then shatters outward due to the flexing of the inner layer and does not hinder the bullet's progress.

A glass's ability to withstand shock is also helped by the annealing process of tempering. When treated at the right temperature, the glass remains harder, which means it takes more force to shatter the surface.

Recent advances in bullet resistant glass composition
U.S. military researchers are moving quickly to develop a new class of transparent armour incorporating aluminium oxynitride (Trade name: AlON) as the outside "strike plate" layer (Air Force Research Lab article). It is much lighter and performs much better than traditional glass/polymer laminates. This allows aluminium oxynitride "glass" to defeat threats surpassing .50 caliber armor piercing rounds using material that is not prohibitively heavy.

9) Gas mask
A gas mask is a mask worn on the face to protect the body from airborne pollutants and toxic materials. The mask forms a sealed cover over the nose and mouth, but may also cover the eyes and other vulnerable soft tissues of the face. Some gas masks are also respirators, though 'gas mask' is often used to refer to military equipment (e.g. Field Protective Mask, etc.) Airborne toxic materials may be gaseous (for example the chlorine used in WWI) or particulate (such as many biological agents developed for weapons such as bacteria, viruses and toxins). Many gas masks include protection from both types. Unlike other breathing devices, gas masks do not require the user to carry an air supply as in the use of scuba gear. However, this means that the wearer depends on the air in the atmosphere, the same medium of the toxic materials. Thus, the mask must remove them and relay clean air to the wearer. There are three main ways of achieving this: filtration, absorption and adsorption, and reaction and exchange.


Annealing (glass)

Annealing, in glassblowing and lampworking, is heating a piece of glass until its temperature reaches a stress-relief point, that is, a temperature at which the glass is still too hard to deform, but is soft enough for internal stresses to ease. The piece is then allowed to heat-soak until its temperature is even throughout; the time necessary for this varies depending on the type of glass and thickness of the thickest section. The piece is then slowly cooled at a predetermined rate until its temperature is below a critical point, at which it can no longer generate internal stresses, and then the temperature can safely be dropped to room temperature. This relieves the internal stresses, making the glass much more durable. Glass which has not been annealed will crack or even shatter when subjected to a relatively small temperature change or other shock.

11) Tensile stress
Tensile stress (or tension) is the stress state leading to expansion; that is, the length of a material tends to increase in the tensile direction. The volume of the material stays constant. Therefore in a uniaxial material the length increases in the tensile stress direction and the other two directions will decrease in size (see Poisson's ratio for detail). In the uniaxial manner of tension, tensile stress is induced by pulling forces across a bar, specimen, etc. Tensile stress is the opposite of compressive stress. Structural members in direct tension are ropes, soil anchors and nails, bolts, etc. Beams subjected to bending moments may include tensile stress as well as compressive stress and/or shear stress. Tensile stress may be increased until the reach of tensile strength, namely the limit state of stress. The formula for compute the tensile stress in a rod is: σ=P/A where σ is the tensile stress, P is the tensile force over the rod and A is the cross-sectional area of the rod. Units for tensile stress are newtons per square meter (N/m², also called pascals, Pa)

12) Physical compression
Physical compression is the result of the subjection of a material to compressive stress, resulting in reduction of volume. Compression has many implications in material science, physics and structural engineering, for compression yields noticeable amounts of stress and tension. By inducing compression, mechanical properties such as compressive strength or modulus of elasticity, can be measured. Scientists may utilize press machines to induce compression. In mechanical engineering the term is applied to the arrangement by which the exhaust valve of a steam engine is made to close, shutting a portion of the exhaust steam in the cylinder, before the stroke of the piston is quite complete. This steam being compressed as the stroke is completed, a cushion is formed against which the piston does work while its velocity is being rapidly reduced, and thus the stresses in the mechanism due to the inertia of the reciprocating parts are lessened. This compression, moreover, obviates the shock which would otherwise be caused by the admission of the fresh steam for the return stroke. In internal combustion engines it is a necessary condition of economy to compress the explosive mixture before it is ignited: in the Otto cycle, for instance, the

second stroke of the piston effects the compression of the charge which has been drawn into the cylinder by the first forward stroke.

13) Force
In physics, force is that which changes or tends to change the state of rest or motion of a body. As well as causing acceleration (a change in the body's change of motion) a force may also cause the body to distort (pressure) or turn (moment), thereby changing the body's state of rest.

Types of force
Many forces exist: the Coulomb force (between electrical charges), gravitational force (between masses), magnetic force, frictional forces, centrifugal forces (in rotating reference frames), spring force, magnetic forces, tension, chemical bonding and contact forces to name a few. Only four fundamental forces of nature are known: the strong force, the electromagnetic force, the weak force, and the gravitational force. All other forces can be reduced to these fundamental interactions. The modern quantum mechanical view of the first three fundamental forces (all except gravity) is that particles of matter (fermions) do not directly interact with each other but rather by exchange of virtual particles (bosons) (as, for example, virtual photons in case of interaction of electric charges). In general relativity, gravitation is not strictly viewed as a force. Rather, objects moving free in gravitational fields (say, a basket ball) simply undergo inertial motion along a straight line in the curved space-time (straight line in curved space-time is defined as the shortest space-time path between two points, and it is called geodesic). This straight line in space-time is a curved line in space, and we call it "ballistic trajectory" of object (say, a parabola for a basket ball moving in a uniform gravitational field). The time derivative of changing momentum of that body is what we label as "gravitational force" (=weight). Light (which has no mass but has energy and momentum in its e/m field) also propagates in gravitational field along a straight space-time path (geodesic). (kilogram-force metres) mentioned above, used without properly separating the units for kilogram and metre with either a space or a centered dot.

Below are several conversion factors between various measurements of force:

1 dyne = 10-5 newtons

• • • • •

1 kgf (kilopond kp) = 9.80665 newtons 1 metric slug = 9.80665 kg 1 lbf = 32.174 poundals 1 slug = 32.174 lb 1 kgf = 2.2046 lbf

14) Polarization
From Wikipedia, the free encyclopedia

(Redirected from Polarized light) Jump to: navigation, search This article is about electromagnetic waves; for other senses of this term, see polarization (disambiguation). In electrodynamics, polarization (also spelled polarisation) is the property of electromagnetic waves, such as light, that describes the direction of their transverse electric field. More generally, the polarization of a transverse wave describes the direction of oscillation in the plane perpendicular to the direction of travel. Longitudinal waves such as sound waves do not exhibit polarization, because for these waves the direction of oscillation is along the direction of travel.

15) Low-emissivity
Low-emittance (Low-E) coatings are microscopically thin, virtually invisible, metal or metallic oxide layers deposited on a window or skylight glazing surface primarily to reduce the U-factor by suppressing radiative heat flow. The principal mechanism of heat transfer in multilayer glazing is thermal radiation from warm surfaces to cooler surfaces. Coating a glass surface with a low-emittance material reflects a significant amount of this radiant heat, thus lowering the total heat flow through the window. Low-E coatings are transparent to visible light, and Opaque to Infrared radiation. Different types of Low-E coatings have been designed to allow for high solar gain, moderate solar gain, or low solar gain. To make Low-E glass, certain properties such as the iron content may be controlled. Also, some types of glass have natural Low-e properties, such as borosilicate or "pyrex" (tm). Specially designed coatings, often based on metallic oxides, are applied to one or more surfaces of insulated glass. These coatings reflect radiant infrared energy, thus tending to keep radiant heat on the same side of the glass from which it originated. This often results in more efficient windows because: radiant heat originating from indoors is

reflected back inside, thus keeping heat inside in the winter, and infrared radiation from the sun is reflected away, keeping it cooler inside in the summer.

16) Fighter aircraft
An A-10 Thunderbolt II, F-86 Sabre, P-38 Lightning and P-51 Mustang fly in formation during an air show at Langley Air Force Base, Virginia. The formation displays three generations of Air Force aircraft - three of them fighters A fighter aircraft is a military aircraft designed primarily for attacking other aircraft, as opposed to a bomber, which is designed to attack ground targets, primarily by dropping bombs. Fighters are comparatively small, fast, and maneuverable. They were developed in response to the fledgling use of aircraft and dirigibles in World War I for reconnaissance and ground attack roles. These early fighters were mostly wooden biplanes with light machine guns. As aerial warfare became increasingly important, so did control of the airspace. By World War II, fighters were predominantly metal monoplanes with wing-mounted cannon. Following the war, turbojets replaced piston engines as the means of propulsion, and missiles augmented or replaced guns. For historical purposes, jet fighters are classified by generation. The generation terminology was initiated by Russian defense parlance in referring to the F-35 Lightning II as a "fifth generation" plane. Years are not exact and intended as a guideline. Modern jet fighters are predominantly powered by one or two turbofan engines, armed primarily with missiles (from as few as two on some lightweight day fighters to as many as eight to ten on air superiority fighters like the F/A-18E/F Super Hornet or F-15 Eagle), with a cannon as backup armament (typically between 20 and 30mm in calibre), and equipped with a radar as the primary method of target acquisition. Fighter aircraft are the primary means by which armed forces gain air superiority. At least since World War II, air superiority has a crucial component of victory in most modern warfare, particularly "conventional" warfare between regular armies, and their acquisition and maintenance represent a very substantial proportion of military budgets in militaries that maintain modern fighter forces.

17) Ultraviolet
Ultraviolet (UV) light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than soft X-rays. It can be subdivided into near UV (380–200 nm wavelength), far or vacuum UV (200–10 nm; abbrev. FUV or VUV), and extreme UV (1–31 nm; abbrev. EUV or XUV). When considering the effect of UV radiation on human health and the environment, the range of UV wavelengths is often subdivided into UVA (400–315 nm), also called Long Wave or "blacklight"; UVB (315–280 nm), also called Medium Wave; and UVC (< 280 nm), also called Short Wave or "germicidal". See 1 E-7 m for a list of objects of comparable sizes.

In photolithography, in laser technology, etc., the term deep ultraviolet or DUV refers to wavelengths below 300 nm. The name means "beyond violet" (from Latin ultra, "beyond"), violet being the color of the shortest wavelengths of visible light. Some of the UV wavelengths are colloquially called black light, as it is invisible to the human eye. Some animals, including birds, reptiles, and insects such as bees, can see into the near ultraviolet. Many fruits, flowers, and seeds stand out more strongly from the background in ultraviolet wavelengths as compared to human color vision. Scorpions glow or take on a yellow to green color under UV illumination. Many birds have patterns in their plumage that are invisible at usual wavelengths but observable in ultraviolet, and the urine of some animals is much easier to spot with ultraviolet. The Sun emits ultraviolet radiation in the UVA, UVB, and UVC bands, but because of absorption in the atmosphere's ozone layer, 99% of the ultraviolet radiation that reaches the Earth's surface is UVA. (Some of the UVC light is responsible for the generation of the ozone.) Ordinary glass is partially transparent to UVA but is opaque to shorter wavelengths while Silica or quartz glass, depending on quality, can be transparent even to vacuum UV wavelengths. Ordinary window glass passes about 90% of the light above 350 nm, but blocks over 90% of the light below 300 nm[1][2][3]. The onset of vacuum UV, 200 nm, is defined by the fact that ordinary air is opaque below this wavelength. This opacity is due to the strong absorption of light of these wavelengths by oxygen in the air. Pure nitrogen (less than about 10 ppm oxygen) is transparent to wavelengths in the range of about 150–200 nm. This has wide practical significance now that semiconductor manufacturing processes are using wavelengths shorter than 200 nm. By working in oxygen-free gas, the equipment does not have to be built to withstand the pressure differences required to work in a vacuum. Some other scientific instruments, such as circular dichroism spectrometers, are also commonly nitrogen purged and operate in this spectral region.

Health concerns and protection
ozone depletion and the ozone hole. Protective eyewear is beneficial to those who are working with or those who might be exposed to ultraviolet radiation, particularly short wave UV. Given that light may reach the eye from the sides, full coverage eye protection is usually warranted if there is an increased risk of exposure, as in high altitude mountaineering. Mountaineers are exposed to higher than ordinary levels of UV radiation, both because there is less atmospheric filtering and because of reflection from snow and ice. Ordinary, untreated eyeglasses give some protection. Most plastic lenses give more protection than glass lenses, because, as noted above, glass is transparent to UVA and the common acrlyic plastic used for lenses is less so. Some plastic lens materials, such as

polycarbonate, inherently block most UV. There are protective treatments available for eyeglass lenses that need it which will give better protection. But even a treatment that completely blocks UV will not protect the eye from light that arrives around the lens. To convince yourself of the potential dangers of stray UV light, cover your lenses with something opaque, like aluminum foil, stand next to a bright light, and consider how much light you see, despite the complete blockage of the lenses. Most intraocular lenses help to protect the retina by absorbing UV radiation.

Beneficial effects
A positive effect of UV light is that it induces the production of vitamin D in the skin. [8] claims tens of thousands of premature deaths occur in the US annually from cancer due to insufficient UVB exposures (apparently via vitamin D deficiency). Another effect of vitamin D deficiency is osteomalacia (rickets), which can result in bone pain, difficulty in weight bearing and sometimes fractures. Ultraviolet radiation has other medical applications, in the treatment of skin conditions such as psoriasis and vitiligo. UVB and UVA radiation can be used, in conjunction with psoralens (PUVA treatment). Most effective in case of psoriasis and vitiligo is UV light with wavelength of 311 nm.

Black lights
A bird appears on every Visa credit card when held under a UV light source. A black light is a lamp that emits long wave UV radiation and very little visible light. Fluorescent black lights are typically made in the same fashion as normal fluorescent lights except that only one phosphor is used and the normally clear glass envelope of the bulb is replaced by a deep bluish purple glass called Wood's glass. To thwart counterfeiters, sensitive documents (e.g. credit cards, driver's licenses, passports) may also include a UV watermark that can only be seen when viewed under a UV-emitting light. Passports issued by most countries usually contain UV sensitive inks and security threads. Visa stamps and stickers such as those issued by Ukraine contain large and detailed seals invisible to the naked eye under normal lights, but strongly visible under UV illimunation. Passports issued by the United States have the UV sensitive threads on the last page of the passport along with the barcode.

Fluorescent lamps
Fluorescent lamps produce UV radiation by ionising low-pressure mercury vapour. A phosphorescent coating on the inside of the tubes absorbs the UV and converts it to visible light.

The main mercury emission wavelength is in the UVC range. Unshielded exposure of the skin or eyes to mercury arc lamps that do not have a conversion phosphor is quite dangerous. The light from a mercury lamp is predominantly at discrete wavelengths. Other practical UV sources with more continuous emission spectra include xenon arc lamps (commonly used as sunlight simulators), deuterium arc lamps, mercury-xenon arc lamps, metalhalide arc lamps, and tungsten-halogen incandescent lamps.

Pest control
Ultraviolet fly traps are used for the elimination of various small flying insects. They are attracted to the UV light and are killed using an electrical shock or trapped once they come into contact with the device.

Food Processing
As consumer demand for fresh and "fresh like" food products increases, the demand for nonthermal methods of food processing is likewise on the rise. In addition, public awareness regarding the dangers of food poisoning is also raising demand for improved food processing methods. Ultraviolet radiation is used in several food processes to remove unwanted microorganisms. UV light can be used to pasteurize fruit juices by flowing the juice over a high intensity ultraviolet light source. The effectiveness of such a process depends on the UV absorbance of the juice (see Beer's law).

Fire detection
Ultraviolet detectors generally use either a solid-state device, such as one based on silicon carbide or aluminum nitride, or a gas-filled tube as the sensing element. UV detectors which are sensitive to UV light in any part of the spectrum respond to irradiation by sunlight and artificial light. A burning hydrogen flame, for instance, radiates strongly in the 185 to 260 nanometer range and only very weakly in the IR region, while a coal fire emits very weakly in the UV band yet very strongly at IR wavelengths; thus a fire detector which operates using both UV and IR detectors is more reliable than one with a UV detector alone. Virtually all fires emit some radiation in the UVB band, while the Sun's radiation at this band is absorbed by the Earth's atmosphere. The result is that the UV detector is "solar blind", meaning it will not cause an alarm in response to radiation from the Sun, so it can easily be used both indoors and outdoors. UV detectors are sensitive to most fires, including hydrocarbons, metals, sulfur, hydrogen, hydrazine, and ammonia. Arc welding, electrical arcs, lightning, X-rays used in nondestructive metal testing equipment (though this is highly unlikely), and radioactive materials can produce levels that will activate a UV detection system. The presence of UV-absorbing gases and vapors will attenuate the UV radiation from a fire, adversely affecting the ability of the detector to detect flames. Likewise, the presence of an oil mist in the air or an oil film on the detector window will have the same effect.

18) Dead Air Space
Dead Air Space is a blog kept by members of the British band Radiohead on their website. The blog was first started in August 2005 as a means of communicating with fans while the band work on their as yet untitled seventh studio album. In addition to using the blog to keep fans up to date on the recording process, members have posted pictures, links to various articles and have at times used it to voice their opinions on current political matters. On November 28, 2005, lead singer Thom Yorke posted a scathing article concerning the United States stance on global warming as the United Nations Climate Change Conference was taking place in Montreal. The band prefer this method of communication with their dedicated fan base over the more traditional route where the press act as the middle man. This way, there is no skewed view as to what any member may be trying to get across. This, however, does not prevent members of the music press from reporting about each and every addition made to Dead Air Space.

19) Ozone depletion
The term ozone depletion is used to describe two distinct but related observations: a slow, steady decline, of about 3% per decade, in the total amount of ozone in the earth's stratosphere during the past twenty years and a much larger, but seasonal, decrease in stratospheric ozone over the earth's polar regions during the same period. (The latter phenomenon is commonly referred to as the "ozone hole".) The detailed mechanism by which the polar ozone holes form is different from that for the mid-latitude thinning, but the proximate cause of both trends is believed to be catalytic destruction of ozone by atomic chlorine and bromine. The primary source of these halogen atoms in the stratosphere is photodissociation of Chlorofluorocarbon (CFC) compounds, commonly called freons, and bromofluorocarbon compounds known as Halons, which are transported into the stratosphere after being emitted at the surface. Both ozone depletion mechanisms strengthened as emissions of CFCs and Halons increased. CFCs, Halons and other contributary substances are commonly referred to as "ODS", or "Ozone Depleting Substances." Since the ozone layer prevents most harmful UVB wavelengths (270- 315 nm) of ultraviolet light from passing through the Earth's atmosphere, observed and projected decreases in ozone have generated worldwide concern leading to adoption of the Montreal Protocol banning the production of CFCs and halons as well as related ozone depleting chemicals such as carbon tetrachloride and trichloroethane (also known as methyl chloroform). It is suspected that a variety of biological consequences, including, for example, increases in skin cancer, damage to plants, and reduction of plankton populations in the ocean's photic zone, may result from the increased UV exposure due to ozone depletion.

Ozone cycle overview
Ozone creation
Three forms (or allotropes) of oxygen are involved in the ozone-oxygen cycle: Oxygen atoms (O or atomic oxygen), oxygen gas (O2 or diatomic oxygen), and ozone gas (O3 or triatomic oxygen). Ozone is formed in the stratosphere when oxygen molecules photodissociate after absorbing an ultraviolet photon whose wavelength is shorter than 240 nm. This produces two oxygen atoms. The atomic oxygen then combines with O2 to create O3. Ozone molecules absorb UV light between 310 and 200 nm, following which ozone splits into a molecule of O2 and an oxygen atom. The oxygen atom then joins up with an oxygen molecule to regenerate ozone. This is a continuing process which terminates when an oxygen atom "recombines" with an ozone molecule to make 2 O2 molecules. It is theorized that prior to the beginning of the depletion trend, the amount of ozone in the stratosphere was kept roughly constant by a balance between the rates of creation and destruction of ozone molecules by UV light.

Ozone destruction
Chemical factors Ozone can be destroyed by a number of free radical catalysts, the most important of which are the hydroxyl radical (OH·), the nitric oxide radical (NO·) and atomic chlorine (Cl·) and bromine (Br·). All of these have both natural and anthropogenic (manmade) sources; at the present time, most of the OH· and NO· in the stratosphere is of natural origin, but human activity has dramatically increased the chlorine and bromine. These elements are found in certain stable organic compounds, especially chlorofluorocarbons (CFCs), which may find their way to the stratosphere without being destroyed in the troposphere. Once in the stratosphere, the Cl and Br atoms are liberated from the parent compounds by the action of ultraviolet light, and can destroy ozone molecules in a catalytic cycle. In this cycle, a chlorine atom reacts with an ozone molecule, taking an oxygen atom with it (forming ClO) and leaving a normal oxygen molecule. A free oxygen atom then takes away the oxygen from the ClO, and the final result is an oxygen molecule and a chlorine atom, which then reinitiates the cycle. The chemical shorthand for these reactions are: Cl + O3 ? ClO + O2 ClO + O ? Cl + O2 In sum O3 + O ? O2 + O2 For this mechanism to operate there must be a source of O atoms, which is primarily the photodissociation of O3. A single chlorine atom would keep on destroying ozone for up to two years (the time scale for transport back down to the troposphere) were it not for reactions that remove

them from this cycle by forming reservoir species such as hydrochloric acid (HCl) and chlorine nitrate (ClONO2. On a per atom basis, bromine is even more efficient than chlorine at destroying ozone, but there is much less bromine in the atmosphere at present. As a result, both chlorine and bromine contribute significantly to the overall ozone depletion.

The ozone hole and its causes
Image of the largest Antarctic ozone hole ever recorded in September 2000. Data taken by the Total Ozone Mapping Spectrometer (TOMS) instrument aboard NASA's Earth Probe satellite. The Antarctic ozone hole is an area of the antarctic stratosphere in which the recent ozone levels have dropped to as low as 33% of their pre-1975 values. The ozone hole occurs during the Antarctic spring, from September to early December, as strong westerly winds start to circulate around the continent and create an atmospheric container. Within this "polar vortex", over 50% of the lower stratospheric ozone is destroyed during the antarctic spring.[5] As explained above, the overall cause of ozone depletion is the presence of chlorinecontaining source gases (primarily CFCs and related halocarbons). In the presence of UV light, these gases dissociate, releasing chlorine atoms, which then go on to catalyze ozone destruction. The Cl-catalyzed ozone depletion can take place in the gas phase, but it is dramatically enhanced in the presence of polar stratospheric clouds (PSCs). These polar stratospheric clouds form during winter, in the extreme cold. Polar winters are dark, consisting of 3 months without solar radiation (sunlight). Not only lack of sunlight contributes to a decrease in temperature but also the “polar vortex” traps and chills air. Temperatures hover around or below -80 °C. These low temperatures form cloud particles and are composed of either nitric acid (Type I PSC) or ice (Type II PSC). Both types provide surfaces for chemical reactions that lead to ozone destruction. The photochemical processes involved are complex but well understood. The key observation is that, ordinarily, most of the chlorine in the stratosphere resides in stable "reservoir" compounds, primarily hydrogen chloride (HCl) and chlorine nitrate (ClONO2). During the Antarctic winter and spring, however, reactions on the surface of the polar stratospheric cloud particles convert these "reservoir" compounds into reactive free radicals (Cl and ClO). The clouds can also remove NO2 from the atmosphere by converting it to nitric acid, which prevents the newly formed ClO from being converted back into ClONO2. The role of sunlight in ozone depletion is the reason why the Antarctic ozone depletion is greatest during spring. During winter, even though PSCs are at their most abundant, there is no light over the pole to drive the chemical reactions. During the spring, however, the sun comes out, providing energy to drive photochemical reactions, and melt the polar stratospheric clouds, releasing the trapped compounds.

Most of the ozone that is destroyed is in the lower stratosphere, in contrast to the much smaller ozone depletion through homogeneous gas phase reactions, which occurs primarily in the upper stratosphere. Warming temperatures near the end of spring break up the vortex around mid-December. As warm, ozone-rich air flows in from lower latitudes, the PSCs are destroyed, the ozone depletion process shuts down, and the ozone hole heals.

Consequences of ozone depletion
Since the ozone layer absorbs UVB ultraviolet light from the Sun, ozone layer depletion is expected to increase surface UVB levels, which could lead to damage, including increases in skin cancer. This was the reason for the Montreal Protocol. Although decreases in stratospheric ozone are well-tied to CFCs, and there are good theoretical reasons to believe that decreases in ozone will lead to increases in surface UVB, there is no direct observational evidence linking ozone depletion to higher incidence of skin cancer in human beings. This is partly due to the fact that UVA, which has also been implicated in some forms of skin cancer, is not absorbed by ozone. [edit]

Increased UV due to the ozone hole
Ozone, while a minority constituent in the earth's atmosphere, is responsible for most of the absorption of UVB radiation. The amount of UVB radiation that penetrates through the ozone layer decreases exponentially with the slant-path thickness/density of the layer. Correspondingly, a decrease in atmospheric ozone is expected to give rise to significantly increased levels of UVB near the surface. Increases in surface UVB due to the ozone hole can be partially inferred by radiative transfer model calculations, but cannot be calculated from direct measurements because of the lack of reliable historical (pre-ozone-hole) surface UV data, although more recent surface UV observation measurement programmes exist (e.g. at Lauder, New Zealand [6]). Because it is this same UV radiation that creates ozone in the ozone layer from O2 (regular oxygen) in the first place, a reduction in stratospheric ozone would actually tend to increase photochemical production of ozone at lower levels (in the troposphere), although the overall observed trends in total column ozone still show a decrease, largely because ozone produced lower down has a naturally shorter photochemical lifetime, so it is destroyed before the concentrations could reach a level which would compensate for the ozone reduction higher up.

Curtain wall
From Wikipedia, the free encyclopedia

Jump to: navigation, search Glass curtain wall of the Bauhaus Dessau. Curtain wall is a term used to describe a building façade which does not carry any dead load from the building other than its own dead load. These loads are transferred to the main building structure through connections at floors or columns of the building. A curtain wall is designed to resist air and water infiltration, wind forces acting on the building, seismic forces, and its own dead load forces. Curtain walls are typically designed with extruded aluminum members, although the first curtain walls were made of steel. The aluminum frame is typically infilled with glass, which provides an architecturally pleasing building, as well as benefits such as daylighting and environmental control. Other common infills include: stone veneer, metal panels, louvers, and operable windows or vents. Curtain walls differ from storefront systems in that they are designed to span multiple floors, and take into consideration design requirements such as: thermal expansion and contraction; building sway and movement; water diversion; and thermal efficiency for cost-effective heating, cooling, and lighting in the building.


1 History
o o

1.1 Medieval curtain wall 1.2 Modern curtain wall 1.2.1 Stick systems 1.2.2 Unitized systems 1.2.3 Rainscreen principle

2 Design

2.1 Loads 2.1.1 Dead load 2.1.2 Wind load

2.1.3 Seismic load 2.1.4 Snow load 2.1.5 Thermal load 2.1.6 Blast load
o o o o •

2.2 Infiltration 2.3 Deflection 2.4 Stress 2.5 Thermal criteria

3 Infills
o o o o o

3.1 Glass 3.2 Stone veneer 3.3 Panels 3.4 Louvers 3.5 Windows and vents

• • •

4 Fire safety 5 Maintenance and repair 6 External links



Medieval curtain wall
Curtain wall is used to describe the set of walls that surround and protect the interior (bailey) of a medieval castle. These walls are often connected by a series of towers or mural towers to add strength and provide for better defense of the ground outside the castle, and are connected like a curtain draped between these posts. Additional provisions and buildings were often enclosed by such a construction, designed to help a garrison last longer during a siege by enemy forces. Examples of curtain walls as part of castles are at Muchalls Castle, Scotland and Dunstanburgh Castle, England, the latter of which is in a ruined state. [edit]

Modern curtain wall
Prior to the mid-20th Century, buildings were constructed with the exterior walls of the building supporting the load of the entire structure. With the advent of the structural concept of shear walls and building cores, the exterior walls of buildings no longer had to support high dead loads and could be designed as much lighter and more open than the brick and steel facades of the past. This gave way to increased use of glass as an exterior façade, and the modern day curtain wall was born. The first curtain walls were made with steel mullions, and the glass was attached to the mullions with tape and urethane sealant. An outer cap was installed to hold the glass in place and to protect the integrity of the seals. The first curtain wall installed in New York City was this type of construction (see Lever House). Earlier modernist examples are the Bauhaus in Dessau and the Hallidie Building in San Francisco. The 1970’s began the widespread use of aluminum extrusions for mullions. Aluminum offers the unique advantage of being able to be easily extruded into nearly any shape required for design and aesthetic purposes. Today, the design complexity and shapes available are nearly limitless. Custom shapes can be designed and manufactured with relative ease. Similarly, sealing methods and types have evolved over the years, and as a result, today’s curtain walls are high performance systems which require little maintenance. [edit] Stick systems The vast majority of curtain walls are installed long pieces (referred to as sticks) between floors vertically and between vertical members horizontally. Framing members may be fabricated in a shop environment, but all installation and glazing is typically performed at the jobsite. [edit] Unitized systems Unitized curtain walls entail factory fabrication and assembly of panels and may include factory glazing. These completed units are hung on the building structure to form the building enclosure. Unitized curtain wall has the advantages of: speed; lower field installation costs; and quality control within an interior climate controlled environment. The economic benefits are typically realized on large projects or in areas of high field labor rates. [edit] Rainscreen principle The latest technology in curtain wall includes rainscreen construction. The rainscreen principle theorizes that equilibrium of air pressure between the outside and inside of the building, and the interior of the curtain wall, prevents air and water infiltration into the building structure. [edit]

Curtain wall systems must be designed to handle all loads imposed on it as well as keep air and water from penetrating the building envelope. [edit]

The loads imposed on the curtain wall are transferred to the building structure through the anchors which attach the mullions to the building. The building structure needs to be designed and account for these loads. [edit] Dead load Dead load is defined as the weight of structural elements and the permanent features on the structure. In the case of curtain walls, this load is made up of the weight of the mullions, anchors, and other structural components of the curtain wall, as well as the weight of the infill material. Additional dead loads imposed on the curtain wall, such as sunshades, must be accounted for in the design of the curtain wall components and anchors. [edit] Wind load Wind load acting on the building is the result of wind blowing on the building. This wind pressure must be resisted by the curtain wall system since it envelopes and protects the building. Wind loads vary greatly throughout the world, with the largest wind loads being near the coast in hurricane-prone regions. Building codes are used to determine the required design wind loads for a specific project location. Oftentimes, a wind tunnel study is performed on large or unusually shaped buildings. A scale model of the building and the surrounding vicinity is built and placed in a wind tunnel to determine the wind pressures acting on the structure in question. These studies take into account vortex shedding around corners and the effects of surrounding buildings. [edit] Seismic load Seismic loads need to be addressed in the design of curtain wall components and anchors. In most situations, the curtain wall is able to naturally withstand seismic and wind induced building sway because of the space provided between the glazing infill and the mullion. In tests, standard curtain wall systems are able to withstand three inches (75 mm) of relative floor movement without glass breakage or water leakage. Anchor design needs to be reviewed, however, since a large floor-to-floor displacement can place high forces on anchors. [edit] Snow load

Snow loads and live loads are not typically an issue in curtain walls, since curtain walls are designed to be vertical or slightly inclined. If the slope of a wall exceeds 20 degrees or so, these loads may need to be considered. [edit] Thermal load Thermal loads are induced in a curtain wall system because aluminum has a relatively high coefficient of thermal expansion. This means that over the span of a couple of floors, the curtain wall will expand and contract some distance, relative to its length and the temperature differential. This expansion and contraction is accounted for by cutting horizontal mullions slightly short and allowing a space between the horizontal and vertical mullions. In unitized curtain wall, a gap is left between units, which is sealed from air and water penetration by wiper gaskets. Vertically, anchors carrying wind load only (not dead load) are slotted to account for movement. Incidentally, this slot also accounts for live load deflection and creep in the floor slabs of the building structure. [edit] Blast load Accidental explosions and terrorist threats have brought on increased concern for the fragility of a curtain wall system in relation to blast loads. The bombing of the Alfred P. Murrah Federal Building in Oklahoma City, Oklahoma, has spawned much of the current research and mandates in regards to building response to blast loads. Currently, all new federal buildings in the U.S., and all U.S. embassies built on foreign soil, must have some provision for resistance to bomb blasts. Since the curtain wall is at the exterior of the building, it becomes the first line of defense in a bomb attack. As such, blast resistant curtain walls must be designed to withstand such forces without compromising the interior of the building to protect its occupants. Since blast loads are very high loads with short durations, the curtain wall response should be analyzed in a dynamic load analysis, with full-scale mock-up testing performed prior to design completion and installation. Blast resistant glazing consists of laminated glass, which is meant to break but not separate from the mullions. Similar technology is used in hurricane-prone areas for the protection from wind-borne debris. [edit]

Air infiltration is the air which passes through the curtain wall from the exterior to the interior of the building. The air is infiltrated through the gaskets, through imperfect joinery between the horizontal and vertical mullions, through weep holes, and through imperfect sealing. The American Architectural Manufacturers Association (AAMA) is the governing body in the U.S. which sets the acceptable levels of air infiltration through a curtain wall. This limit is expressed in cubic feet per minute per square foot of wall area (currently, most standards cite less than 0.6 CFM/sq ft as acceptable).

Water penetration is defined as any water passing from the exterior of the building through to the interior of the curtain wall system. Sometimes, depending on the building specifications, a small amount of controlled water on the interior is deemed acceptable. To test the ability of a curtain wall to withstand water penetration, a water rack is placed in front a mock-up of the wall with a positive air pressure applied to the wall. This represents a wind driven heavy rain on the wall. Field tests are also performed on installed curtain walls, in which a water hose is sprayed on the wall for a specified time. [edit]

One of the disadvantages of using aluminum for mullions is that its modulus of elasticity is about one-third that of steel. This translates to three times more deflection in an aluminum mullion compared to the same steel section. Building specifications set deflection limits for perpendicular (wind-induced) and in-plane (dead load-induced) deflections. It is important to note that these deflection limits are not imposed due to strength capacities of the mullions. Rather, they are designed to limit deflection of the glass (which may break under excessive deflection), and to ensure that the glass does not come out of its pocket in the mullion. Deflection limits are also necessary to control movement at the interior of the curtain wall. Building construction may be such that there is a wall located near the mullion, and excessive deflection can cause the mullion to contact the wall and cause damage. Also, if deflection of a wall is quite noticeable, public perception may raise undue concern that the wall is not strong enough. Deflection limits are typically expressed as the distance between anchor points divided by a constant number. A deflection limit of L/175 is common in curtain wall specifications. Say a given curtain wall is anchored at 12 foot (144 in) floor heights. The allowable deflection would then be 144/175 = 0.823 inches, which means the wall is allowed to deflect inward or outward a maximum of 0.823 inches at the maximum wind pressure. Deflection in mullions is controlled by different shapes and depths of curtain wall members. The depth of a given curtain wall system is usually controlled by the area moment of inertia required to keep deflection limits under the specification. Another way to limit deflections in a given section is to add steel reinforcement to the inside tube of the mullion. Since steel deflects at 1/3 the rate of aluminum, the steel will absorb much of the system’s deflection at a lower cost or smaller depth. [edit]

Contrary to popular belief, stress is not related to deflection; it is a separate criterion in curtain wall design and analysis. For example, the advantage of some curtain wall designs is the ability to span more than one floor (commonly known as twin-span or multi-span, as opposed to single or simple span). Multiple floor spans significantly reduce the required area moment of inertia for a mullion. The stresses in the mullion,

however, are significantly increased in a multiple span, giving way for a higher required section modulus (S, expressed in cubic inches) in the mullion. As mentioned above, the deflection of aluminum is three times greater than an equivalent steel shape under the same load. However, the allowable stress in that same aluminum member may be roughly equivalent to or higher than its steel counterpart. This means that aluminum mullions can be as strong as or stronger than steel members. [edit]

Thermal criteria
Relative to other building components, aluminum has a high heat transfer coefficient, meaning that aluminum is a very good conductor of heat. This translates into high heat loss through aluminum curtain wall mullions. There are several ways to compensate for this heat loss, the most common way being the addition of thermal breaks. Thermal breaks are barriers between exterior metal and interior metal, usually made of polyvinyl chloride (PVC). These breaks provide a significant decrease in the thermal conductivity of the curtain wall. However, since the thermal break interrupts the aluminum mullion, the overall moment of inertia of the mullion is reduced and must be accounted for in the structural analysis of the system. Thermal conductivity of the curtain wall system is important because of heat loss through the wall, which affects the heating and cooling costs of the building. On a poorly performing curtain wall, condensation may form on the interior of the mullions. This could cause damage to adjacent interior trim and walls. Rigid insulation is provided in spandrel areas to provide a higher R-value at these locations. [edit]

Infill refers to the large panels that are inserted into the curtain wall between mullions. Infills are typically glass but may be made up of nearly any exterior building element. Regardless of the material, infills are typically referred to as glazing, and the installer of the infill is referred to as a glazier. [edit]

By far the most common glazing type, glass can be of an almost infinite combination of color, thickness, and opacity. For commercial construction, the two most common thicknesses are 1/4 inch (6 mm) monolithic and 1 inch (25 mm) insulating glass. Presently, 1/4 inch glass is typically used only in spandrel areas, while insulating glass is used for the rest of the building (sometimes spandrel glass is specified as insulating glass as well). The 1 inch insulation glass is typically made up of two 1/4-inch lites of glass with a 1/2 inch (12 mm) airspace. The air inside is usually atmospheric air, but some inert

gases, such as argon, may be used to offer better thermal transmittance values. In residential construction, thicknesses commonly used are 1/8 inch (3 mm) monolithic and 5/8 inch (16 mm) insulating glass. Larger thicknesses are typically employed for buildings or areas with higher thermal, relative humidity, or sound transmission requirements, such as laboratory areas or recording studios. Glass may be used which is transparent, translucent, or opaque, or in varying degrees thereof. Transparent glass usually refers to vision glass in a curtain wall. Spandrel or vision glass may also contain translucent glass, which could be for security or aesthetic purposes. Opaque glass is used in areas to hide a column or spandrel beam or shear wall behind the curtain wall. Another method of hiding spandrel areas is through shadow box construction (providing a dark enclosed space behind the transparent or translucent glass). Shadow box construction creates a perception of depth behind the glass that is sometimes desired. [edit]

Stone veneer
Thin blocks (3 to 4 inches (75-100 mm)) of stone can be inset within a curtain wall system to provide architectural flavor. The type of stone used is limited only by the strength of the stone and the ability to manufacture it in the proper shape and size. Common stone types used are: granite; marble; travertine; and limestone. The stone may come in several different finishes, which adds many more options for architects and building owners. [edit]

Metal panels can take various forms including aluminum plate; thin composite panels consisting of two thin aluminum sheets sandwiching a thin plastic interlayer; and panels consisting of metal sheets bonded to rigid insulation, with or without an inner metal sheet to create a sandwich panel. Other opaque panel materials include FRP (fiber-reinforced plastic) and stainless steel. [edit]

A louver is provided in an area where mechanical equipment located inside the building requires ventilation or fresh air to operate. Curtain wall systems can be adapted to accept most types of louver systems to maintain the same architectural site lines and style while providing the necessary functionality. [edit]

Windows and vents
Most curtain wall glazing is fixed, meaning there is no access to the exterior of the building except through doors. However, windows or vents can be glazed into the curtain

wall system as well, to provide required ventilation or operable windows. Nearly any window type can be made to fit into a curtain wall system. [edit]

Fire safety
Fire safing and smoke seal at gaps between the floors and the back of the curtain wall are essential to slow the passage of fire and combustion gases between floors. Spandrel areas must have non-combustible insulation at the interior face of the curtain wall. Some building codes require the mullion to be wrapped in heat-retarding insulation near the ceiling to prevent the mullions from melting and spreading the fire to the floor above. Fireman knock-out glazing panels are often required for venting and emergency access from the exterior. Knock-out panels are generally fully tempered glass to allow full fracturing of the panel into small pieces and relatively safe removal from the opening. [edit]

Maintenance and repair
Curtain walls and perimeter sealants require maintenance to maximize service life. Perimeter sealants, properly designed and installed, have a typical service life of 10 to 15 years. Removal and replacement of perimeter sealants require meticulous surface preparation and proper detailing. Aluminum frames are generally painted or anodized. Factory applied fluoropolymer thermoset coatings have good resistance to environmental degradation and require only periodic cleaning. Recoating with an air-dry fluoropolymer coating is possible but requires special surface preparation and is not as durable as the baked-on original coating. Anodized aluminum frames cannot be "re-anodized" in place, but can be cleaned and protected by proprietary clear coatings to improve appearance and durability. Exposed glazing seals and gaskets require inspection and maintenance to minimize water penetration, and to limit exposure of frame seals and insulating glass seals to wetting.

A) Shear wall
From Wikipedia, the free encyclopedia

Jump to: navigation, search In structural engineering, a shear wall is a wall composed of braced panels (also known as shear panels) to counter the effects of lateral loads acting on a structure. Wind and earthquake loads are the most common loads braced wall lines are designed to

counteract. Under several building codes, including the International Building Code (where it is called a braced wall line) and Uniform Building Code, all exterior wall lines in wood or steel frame construction must be braced. Depending on the size of the building some interior walls must be braced as well. A common method of constructing a braced wall line in wood frames is to create braced panels in the wall line using structural plywood sheathing with specific nailing at the edges and supporting framing of the panel. A more traditional method is to use let-in diagonal bracing throughout the wall line, but this method isn't viable for buildings with many openings for doors, windows, etc. Such walls can be either "load bearing" or "non-load bearing". Shear walls are a type of structural system that provides lateral resistance to a building or structure. They resist "in-plane" loads that are applied along its height. The applied load is generally transferred to the wall by a diaphragm or Collector or drag member. They are built in wood, concrete, CMU (masonry) or steel.

B) Mullion
From Wikipedia, the free encyclopedia

(Redirected from Mullions) Jump to: navigation, search For the village in Cornwall, see Mullion, Cornwall. A mullion is a framing element which divides adjacent window, door, or glass units. Mullions may be made of any material, but wood and aluminum are most common, although stone is also used in windows. Mullions are vertical and are often confused with transoms, which lay horizontally. A mullion acts as a structural member, and carries the dead load of the glass and the wind load acting on the glass to the anchor point and back to the building structure. Mullions vary from small members in doors or small glass areas, to very large and deep structural members in many curtain wall systems. In the past, mullions were necessary because it was not possible to produce sufficiently large panes of glass. In double-glazed windows, a grid resembling mullions is sometimes sandwiched between the panes as decoration. This is called a false mullion or muntin. A false mullion may also be attached to the front face of the glass for an aesthetic division of the glass because it is more costly to produce and install large quantities of smaller panes of glass.

C) Sealant
From Wikipedia, the free encyclopedia

Jump to: navigation, search Please expand this article. Further information might be found in a section of the talk page or at Requests for expansion. A sealant is a material used to prevent some form of fluid from escaping its container. Desirable properties of sealants include insolubility, corrosion resistance, and adhesion. Uses of sealants vary widely and sealants are used in many industries, for example, automotive and aerospace industries. An example of a sealant is silicone. This material-related article is a stub. You can help Wikipedia by expanding it. [edit]

Types of Sealants
• • • • •

Acryl Sealants Polysulfide Sealants Polyurethane Sealants Silicone Sealants WKT Sealants

Retrieved from ""

D) Dead and live loads
From Wikipedia, the free encyclopedia

Jump to: navigation, search Dead and Live loads are terms used in engineering, specifically mechanical and structural disciplines and especially where analysis of real world objects are required. A 'load' refers to any type of force exerted on an object, which may be in the form of a 'weight' (gravitational force), a pressure, or anything which affects the object in question.

• • •

1 Dead loads 2 Live loads 3 Real world usage

• • •

4 Calculating combined loads 5 See also 6 External links


Dead loads
Typically dead loads are considered those which are static, i.e. do not change over the course of normal operations of the object. For example, in the analysis of a staircase where the handrails are attached to the main structure but are not the subject of the analysis and so not included in the model the dead load would be considered to be • •

Self-weight of the staircase (9.8 m/s² x mass in kilograms for force in newtons) Force exerted by the weight of the handrails, applied to the point of attachment to the staircase (again calculated as 9.8 m/s² x mass)


Live loads
Live loads, more commonly referred to as dynamic loads include all the forces that are variable within the objects normal operation cycle. Using the Staircase example the live load would be considered to be • •

Pressure of feet on the stair treads (variable depending on usage and size) Wind load (if the staircase happens to be outside)


Real world usage
The reason for splitting loads into these categories is not always apparent, and in terms of the actual load on the object there is no difference between dead or live loading. For the most part, the split occurs for use in safety calculations or ease of analysis on complex models. When considering the feasibility of a structure, safety always takes precedent and because of this Governing bodies around the world have regulations to which structures have to adhere. Using the example of the staircase, if it was intended for use in the UK it would have to follow British and European Standards
• • •

BS 4592 - Industrial type flooring and stair treads BS 5395 - Code of practice for the design of straight stairs Other standards specific to the application (eg BS 14122-3:2001 - Permanent means of access to machinery. Stairways, stepladders and guard-rails)

Within these standards a safety factor is usually determined where the structure should be able to withstand a certain force above the maximum expected load. Once again using the Staircase example, assuming it is an indoor medium-usage industrial staircase the current safety factor would be 1.4 times the maximum stress imposed by the dead load and 1.6 times the maximum stress imposed by the live load. The reason for the disparity between values, and thus the reason the loads are initially categorised as dead or live is because while it is not unreasonable to expect a large number of people ascending the staircase at once (or the wind speed increasing, snowfall or any other live load increase), it is less likely that the structure will experience much change in its permanent load. The same can be said of many structures and so it is convenient to assess loading based on its application. [edit]

Calculating combined loads
It is worth noting that on first inspection it seems you should find the maximum stress for each of dead and live, factor them and add them together. This will give you a massively overestimated stress result. The combination needs to be applied with great care and almost exclusively programmatically because you may only combine two stress results at the same point. Since the maximum stress is very rarely at the same place in a structure for dead and live it may well be the case that the overall increase is a fraction of the addition of the two maximum stresses and in a completely different position two either of the two original maximums. To clarify, take the staircase analysis. The maximum stress under dead load appears at the foot of a support beam and it is 80 N mm-2 or 80 MPa, at this point the stress from the live load is 5 N mm-2. The maximum stress under live load is 60 N mm-2 and appears at the corner of the second stairtread where the dead load stress is 30 N mm-2. At a third point the stresses from both dead and live are 50 N mm-2. Given these figures you can see that the combined load cases for each point would be: 1. 80 x 1.4 + 5 x 1.6 = 120 N 2. 30 x 1.4 + 60 x 1.6 = 138 N 3. 50 x 1.4 + 50 x 1.6 = 150 N As you can see the maximum combined stress appears away from both the original maxima but is still well under the 275 N mm-2 yield point of the structural steel this staircase is made of so in this case we could say that the structure is safe.

E) Polyvinyl chloride
From Wikipedia, the free encyclopedia

Jump to: navigation, search Polyvinyl chloride Density 1380 kg/m3 Young's modulus (E) 2900-3400 MPa Tensile strength(σt) 50-80 MPa

Elongation @ break Notch test Glass temperature Melting point Vicat B1 Heat Transfer Coefficient (λ) Linear Expansion Coefficient (α) Specific heat (c) Water absorption (ASTM) Price

20-40% 2-5 kJ/m2 87 °C 212 °C 85 °C 0.16 W/m.K 8 10-5 /K 0.9 kJ/(kg·K) 0.04-0.4 0.5-1.25 €/kg

Deformation temperature at 10 kN needle load source: [1]

Polyvinyl chloride Polyvinyl chloride, (IUPAC Polychloroethene) commonly abbreviated PVC, is a widely-used plastic. In terms of revenue generated, it is one of the most valuable products of the chemical industry. Globally, over 50% of PVC manufactured is used in construction. As a building material, PVC is cheap and easy to assemble. In recent years, PVC has been replacing traditional building materials such as wood, concrete and clay in many areas. Despite appearing to be an ideal building material, concerns have been raised about the costs of PVC to the natural environment and human health. There are many uses for PVC. As a hard plastic, it is used as vinyl siding, magnetic stripe cards, window profiles, gramophone records (which is the source of the name for vinyl records), pipe, plumbing and conduit fixtures. It can be made softer and more flexible by the addition of plasticizers, the most widely used being phthalates. In this form, it is used in clothing and upholstery, and to make flexible hoses and tubing, flooring, roofing membranes, and electrical cable insulation. The material is often used for pipelines in the water and sewer industries because of its inexpensive nature and flexibility.

• • •

1 Preparation 2 History 3 Applications
o o o

3.1 Electric wires 3.2 Pipes 3.3 Unplasticized polyvinyl chloride (uPVC)

4 Health and safety

o o o • • • • •

4.1 Phthalate plasticizers 4.2 Vinyl chloride monomer 4.3 Dioxins

5 Resin identification code 6 References 7 Films 8 See also 9 External links


Polyvinyl chloride is produced by polymerization of the monomer vinyl chloride, as shown. [edit]

Polyvinyl chloride was accidentally discovered on at least two different occasions in the 19th century, first in 1835 by Henri Victor Regnault and in 1872 by Eugen Baumann. On both occasions, the polymer appeared as a white solid inside flasks of vinyl chloride that had been left exposed to sunlight. In the early 20th century, the Russian chemist Ivan Ostromislensky and Fritz Klatte of the German chemical company Griesheim-Elektron both attempted to use PVC in commercial products, but difficulties in processing the rigid, sometimes brittle polymer blocked their efforts. In 1926, Waldo Semon of B.F. Goodrich developed a method to plasticize PVC by blending it with various additives. The result was a more flexible and more easily processed material that soon achieved widespread commercial use. [edit]


Electric wires
PVC is commonly used as for the insulation on electric wires; the plastic used for this purpose needs to be plasticized. In a fire, PVC-coated wires can form HCl fumes; the chlorine serves to scavenge free radicals and is the source of the material's fire retardance. However, these (intentional) fumes can also pose a health hazard in their own

right. Frequently in applications where smoke is a major hazard (notably in tunnels) PVC-free LSOH (low smoke, zero halogen) cable insulation is used. [edit]

Polyvinylchloride is also widely used for producing pipes. About 90% of all PVC pipes are used for drainage and for protecting/containing cables in buildings. [edit]

Unplasticized polyvinyl chloride (uPVC)
Modern "Tudorbethan" house with uPVC gutters and downpipes, fascia, decorative imitation "half-timbering", windows and doors. uPVC is often used in the building industry as a low maintenance material, particularly in the UK, and in the USA where it is known as vinyl.[2][3]. The material comes in a range of colours and finishes, including a photo-effect wood finish, and is used as a substitute for painted wood, most obviously for window frames and sills when installing double glazing in new buildings or to replace older single glazed windows. It has many other uses including fascia, and siding or weatherboarding. The same material has almost entirely replaced the use of cast iron for plumbing and drainage, being used for waste pipes, drainpipes, gutters and downpipes,[4] Due to environmental concerns[5] use of PVC is discouraged by some local authorities[6] and in countries such as Germany and The Netherlands. [edit]

Health and safety
This section does not cite its references or sources. You can help Wikipedia by introducing appropriate citations. [edit]

Phthalate plasticizers
Many Vinyl products contain additional chemicals to change the chemical consistency of the product. Some of these additional chemicals called additives can leach out of vinyl products. Plasticizers which must be added to make PVC flexible have been an additive of particular concern. Because soft PVC toys have been made for babies for years, there are concerns that these additives leach out of soft toys into the mouths of the children chewing on them. Vinyl IV bags used in neo-natal intensive care units have also been shown to leach DEHP (Bis(2-ethylhexyl) phthalate), a phthalate additive. In January 2006, the European Union placed a ban on six types of phthalate softeners in toys (See directive 2005/84/EC). In 2003, the US Consumer Product Safety Commission (CPSC) denied a petition for a similar ban in the United States[1]; however, in the USA most companies have

voluntarily stopped manufacturing PVC toys for this age group or have eliminated the phthalates. In a draft guidance paper published in September 2002, the US FDA recognizes that many medical devices with PVC containing DEHP are not used in ways that result in significant human exposure to the chemical[2]. However, FDA is suggesting that manufacturers consider eliminating the use of DEHP in certain devices that can result in high aggregate exposures for sensitive patient populations such as neonates. However, alternative softeners have not been properly tested to determine whether they are more or less safe. Other vinyl products, including car interiors, shower curtains, flooring, etc., initially release chemical gases into the air. Some studies indicate that this outgassing of additives may contribute to health complications, but this information is preliminary and further study is needed. PVC comes under many terms as Petro Vinyl Common (another scientific name) According to some medical studies, the plasticizers added to PVC may cause chronic conditions such as scleroderma, cholangiocarcinoma, angiosarcoma, brain cancer, and acrosteolysis. PVC has been used in many products for many years and still there is not proof of significant harmful effects from exposure. There have been studies, some cited in this article, that indicate "links" with certain medical problems and exposure to PVC products. These links deserve additional study. In 2004, a joint Swedish-Danish research team found a very strong link between allergies in children and the phthalates DEHP and BBzP, commonly used in PVC[7]. Alternative plasticisers are being developed but in many cases these alternatives remain significantly more expensive and their technical performance varies. It is also worth noting that some, though not all, of the alternatives pose significant health risks. One hospital network called the Catholic Healthcare West network, the 8th largest hospital network in the country, recently signed a contract with B.Braun for Vinyl free Intravenous(IV) bags and tubing. [edit]

Vinyl chloride monomer
In the late 1960s, Dr. John Creech and Dr. Maurice Johnson were the first to clearly link and recognize the carcinogenicity of vinyl chloride monomer to humans when workers in the polyvinyl chloride polymerization section of a B.F. Goodrich plant near Louisville, Kentucky, were diagnosed with liver angiosarcoma, a rare disease.[8] Since that time, studies of PVC workers in Australia, Italy, Germany, and the UK have all associated certain types of occupational cancers with exposure to vinyl chloride. The link between angiosarcoma of the liver and long-term exposure to vinyl chloride is the only one which has been confirmed by the International Agency for Research on Cancer. All the cases of angiosarcoma developed from exposure to vinyl chloride monomer, were in workers who were exposed to very high VCM levels, routinely, for many years. According to the EPA, "vinyl chloride emissions from polyvinyl chloride (PVC), ethylene dichloride (EDC), and vinyl chloride monomer (VCM) plants cause or contribute to air pollution that may reasonably be anticipated to result in an increase in

mortality or an increase in serious irreversible, or incapacitating reversible illness. Vinyl chloride is a known human carcinogen which causes a rare cancer of the liver."[9] A front-page series in the Houston Chronicle claimed the vinyl industry has manipulated vinyl chloride studies to avoid liability for worker exposure and to hide extensive and severe chemical spills into local communities.[10]

F) Thermal insulation
From Wikipedia, the free encyclopedia

Jump to: navigation, search See Building insulation for How-to sections. For electrical insulation see electrical insulation, and for sound insulation see soundproofing. Insulation must not be confused with insolation (incoming solar radiation). The term Thermal Insulation can refer to materials used to reduce the rate of heat transfer, or the methods and processes used to reduce heat transfer. The major types of insulation are associated with the major types of heat transfer:
• •

Reflectors are used to reduce radiative heat transfer. Foams or fibrous materials are used to reduce conductive heat transfer by reducing physical contact between objects Foams, fibrous materials or evacuated spaces are used to reduce convective heat transfer by stopping or retarding the movement of fluids (liquids or gases) around the insulated object.

Combinations of some of these methods are often used, for example the combination of reflective surfaces and vacuum in a vacuum flask. Understanding heat transfer is important when planning how to insulate an object or a person from heat or cold, for example with correct choice of insulated clothing, or laying insulating materials beneath in-floor heat cables or pipes in order to direct as much heat as possible upwards into the floor surface and reduce heat loss to the ground underneath.


1 Materials used for thermal insulation
o o

1.1 Trapped air insulators 1.2 Solid insulators

2 Choice of insulation
o o

2.1 Heat Bridging 2.2 Optimum insulation thickness

3 Reasons for insulation
o o

3.1 Domestic insulation 3.2 Industrial insulation

• • • •

4 External links 5 References 6 See also 7 External links


Materials used for thermal insulation
Many different materials can be used as insulators. Many organic insulators are made from petrochemicals and recycled plastic. Many inorganic insulators are made from recycled materials such as glass and furnace slag. [edit]

Trapped air insulators
Most insulators in common use rely on the principle of trapping air to reduce convective heat transfer. These insulators can be fibrous (e.g. down feathers and asbestos), cellular (e.g. cork or plastic foam), or granular (e.g. sintered refractory materials). The quality of such an insulator depends on:

The degree to which air flow is eliminated (large cells of trapped air will have internal convection currents) The amount of solid material surrounding the air (large percentages of air are better, as this reduces thermal bridging within the insulator) The degree to which the properties of the insulator are appropriate to its use:

Stability at the temperatures encountered (e.g. refractory materials used in kilns) Mechanical properties (e.g. softness and flexibility for clothes, hardness and toughness for steam pipe insulation)



Service lifetime (due to thermal breakdown, water resistance or resistance to microbial decomposition)


Solid insulators
Any material with low thermal conductivity can be used to reduce conductive heat transfer, for example the use of a ceramic block or tile to keep a kitchen counter from being damaged by a hot pot. For a list of good and bad insulators, see thermal conductivity. [edit]

Choice of insulation
Often, one mode of heat transfer predominates, leading to a specific choice of insulation. Some materials are good insulators against only one of the heat-transfer mechanisms, but poor insulators against another. For example, metals are good radiative insulators, but poor conductive insulators, so their use as thermal reflective insulators in buildings is limited to wherever they can be installed in contact with air and not with solid material, such as in roofs or in cavity walls when trapped air (as air pockets, bubbles or foam) is next to the layer of metal. [edit]

Heat Bridging
Comparatively more heat flows through a path of least resistance than through insulated paths. This is known as a "thermal bridge" or "heat leak". Insulation around a bridge is of little help in preventing heat loss or gain due to thermal bridging; the bridging has to be rebuilt with smaller or more insulative materials. When a thermal bridge is desired, it can be a heat source, heat sink or a heat pipe. [edit]

Optimum insulation thickness
For practical and economic reasons, it is undesirable to use too much insulation. Although it is sufficient to choose the thickness of an insulator intuitively in household situations, specifications of industrial insulation is usually done following a heat-transfer analysis. As the rate of heat transfer depends on the surface area of the object being insulated, adding a thin layer of poor quality insulation material to a small object can actually increase heat transfer. It can be shown that for some systems, there is a minimum insulation thickness required for an improvement to be realized. [1] [edit]

Reasons for insulation


Domestic insulation
Maintaining acceptable temperatures for daily life is the main reason for domestic and personal insulation. Clothing is chosen to maintain the temperature of the human body by matching the degree of insulation to the environmental temperature and rate of heat production: We chose light clothes when we anticipate high temperatures and physical exertion. Maintaining acceptable temperatures in buildings (by heating and cooling) uses a large proportion of total energy consumption worldwide[citation needed]. When well insulated, a building:
• •

being more energy-efficient, saves the owner money. absorbs noise and vibration, both coming from the outside and from other rooms inside the house. It is quieter. is more comfortable, temperatures being more uniform throughout the house. There is less temperature gradient between exterior shell (walls, ceiling, ground floor) and the interior. does not need extra effort and expense. Insulation is permanent and does not require maintenance, upkeep, or adjustment.

See also weatherization and thermal mass; both describe important methods of saving energy and creating comfort. [edit]

Industrial insulation
In industry, energy has to be expended to raise, lower, or maintain the temperature of objects or process fluids. If these are not insulated, this increases the energy requirements of a process, and therefore the cost and environmental impact.

G) Spandrel
From Wikipedia, the free encyclopedia

Jump to: navigation, search For the anti-tank missile, see AT-5 Spandrel. A spandrel is originally a term from Architecture, but has more recently been given an analogous meaning in Evolutionary biology. Architecturally, a spandrel (less often spandril or splaundrel) is the space between two arches or between an arch and a

rectangular enclosure. In evolutionary biology, a spandrel is a phenotypic characteristic that evolved as a side effect of a true adaptation. [edit]

Illustration of spandrel There are four or five accepted and cognate meanings of spandrel in architectural and art history, all relating to the space between a curved figure and a rectangular boundary such as the space between the curve of an arch and a rectilinear bounding moulding, or the wallspace bounded by adjacent arches in an arcade and the stringcourse or moulding above them, or the space between the central medallion of a carpet and its rectangular corners, or the space between the circular face of a clock and the corners of the square revealed by its hood. Also included is the space under a flight of stairs, if it is not occupied by another flight of stairs. This is a common location to find storage space in residential structures. The spandrels over doorways in Perpendicular work are generally richly decorated. At Magdalen College, Oxford, is one which is perforated, and has a most beautiful effect. The spandrel of doors is sometimes ornamented in the Decorated period, but seldom forms part of the composition of the doorway itself, being generally over the label. Because arches are commonly used in bridge construction, spandrels may also appear in those structures. Most arch spans up until the advent of steel and reinforced concrete in the 19th and 20th centuries were solid-spandrel, meaning that the areas between arches were completely filled in—usually with stone. Open-spandrel bridges later became fairly common, where thin ribs were used to connect the upper deck to the bridge arches, resulting in a significant savings in material, weight, and therefore cost. Reinforcedconcrete open-spandrel bridges were fairly common for crossing large distances in the 1920s and 1930s. Spandrels can also occur in the construction of domes, and are typical in grand architecture from the medieaval period onwards. Where a dome needed to rest on a square or rectangular base, the dome was raised above the level of the supporting pillars, with three-dimensional spandrels called pendentives taking the weight of the dome and concentrating it onto the pillars. See also: Cathedral architecture [edit]

This section is a stub. You can help by adding to it. Another usage of the word spandrel was popularized by Stephen Jay Gould and Richard Lewontin in their influential paper "The Spandrels of San Marco and the Panglossian

Paradigm: A Critique of the Adaptationist Programme". In the context of evolution, they introduced the term spandrel as a metaphor for characteristics that are or were originally side effects and not true adaptations to the environment. They are analogous to misbugs in hacker jargon. Critics such as Dennett argue that the spandrels (pendentives, to be precise) of San Marco are not the undesigned gaps between design features that Gould and Lewontin describe, but that they are intentionally designed features themselves; deliberate solutions to an architectural problem where alternative solutions available to the designers included the use of corbels or squinches. The critics also argue that this misidentification of a design feature as an accident is an illustration, in parallel, of Lewontin's and Gould's underestimation of the adaptedness of evolved lifeforms.

H) Insulated glazing
From Wikipedia, the free encyclopedia

Jump to: navigation, search For other uses, see Glaze. Insulated glazing unit (commonly referred to as IGU) is a piece of glazing consisting of two or more layers (lites) of glazing separated by a spacer along the edge and sealed to create a hermetically sealed air space between the layers. This provides better heat and sound insulation than standard single-glazed windows. Insulating windows are usually double paned and are also referred to as "double glazing" but windows with triple panes or more, "triple glazing" are sometimes seen in very cold areas. Insulated glazing is framed in a sash or frame as if it were a very thick piece of glass.

• • • •

1 Insulated glass 2 Glass coatings 3 See also 4 External links


Insulated glass

IGU made of glass is called insulated glass (which refers to heat insulation, not sound[1]). A more technically correct term, though, is insulating glass, since the the glass itself has no insultive properties. It is actually the air space between the glass layers (lites) that provides the insulative qualities. The air space between the lites may be filled with air or an inert gas like argon or krypton which would provide better insulating performance. Argon (Ar) has an atomic mass of 39.9, which is much more than nitrogen (N2) and oxygen (O2) molecules, which have a molecular mass of 28.0 and 32.0 respectively. As a result, argon atoms move significantly slower than nitrogen and oxygen molecules at the same temperature. This reduces convection and decreases the energy transfer between one side of the glass and the other.[2] [citation needed] Typically the spacer is filled with desiccant to prevent condensation and improve insulating performance. Less commonly, the air is removed, leaving a vacuum, which has no convection at all. This is called evacuated glazing. Often the insulating quality is used in reference to heat flow where the gap between glazed sheets is optimum at about one centimetre. A larger gap allows for convection currents and negates the dead air space. However, in some situations the insulation is in reference to noise mitigation. In these circumstances a large gap improves the noise insulation quality or Sound transmission class. Insulated glass may not be cut to size in the field like plate glass but must be manufactured to the proper size in a shop equipped with special equipment. [edit]

Glass coatings
The heat and sound insulation of glazing may also be improved through the use of a film or coating applied to its surface. This film is typically made of polyester or metal, and may give a reflective appearance and one-way mirror effect to the window, and may improve both heat and sound insulation. This may be used on single-glazed windows as an alternative to insulated glazing, or on the outside layer of insulated glazing to further improve its effectiveness.[3] Such coatings may reduce fading of fabric and improve safety in the case of breakage.[4] "Secondary glazing" is sometimes used as a cheaper alternative. This consists of a layer of glazing placed retrofitted inside the window, to provide sound and heat insulation. Plastic sheet may be used for heat insulation, but may only last for one season.[5] I)

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