Materials

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Factors affecting selection of Materials


Availability
Construction materials are used in large quantities and therefore need to be readily
available. In some parts of the world, other locally available materials are excellent
for construction. Also, the local labour force is likely to be familiar with the use of
locally available materials.

Stiffness
Stiffness, or rigidity, is not to be confused with strength: some strong materials are
not stiff (e.g. rope) and some stiff materials are not particularly strong (e.g. glass).
The stiffer a material, the less it will deflect. The stiffness of a material is
proportional to its Young’s Modulus value. Typical Young’s Modulus values for the
following materials are:
• Steel: 210 kN/mm2
• Aluminium: 71 kN/mm2
• Concrete: 14 kN/mm2
• Timber: 5–10 kN/mm2
It can be seen from the above that steel is by far the stiffest of the common
structural materials – for a given cross-section steel is three times as stiff as
aluminium, 15 times as stiff as concrete and over 20 times as stiff as timber. But
remember, this is for a constant cross-section, so these relative stiffnesses will vary
according to the cross- section used.

Strength
Materials need to be strong enough (in tension and/or compression) for their
intended purpose. Clearly, some materials are stronger than others. Selection of too
weak a material for a particular application is an obvious mistake, but selection of a
needlessly strong material is also undesirable.

Speed of construction
Some building types can be constructed more quickly than others. For example, a
steel- framed structure can be completed far more quickly than a masonry one. But
speed of construction is not always critical and there may well be a trade-off
between speed and cost. Being told that a building could be built twice as fast for
twice the cost greatly concentrates the mind!

Cost/economics
A complex issue. Architects and engineers are always looking to minimise cost.
There is an old saying that an engineer can do for a penny what anyone can do for
two pence. We have to consider the cost of the raw materials, the cost of conversion
of the material into its usable form, transportation costs and associated labour costs.



Ability to accommodate movement
All buildings tend to move. Some materials can accommodate this better than
others. For
example, brickwork can cope with movement more readily than a steel-framed
structure can.

Durability
Some materials rot, decompose, corrode or spall, etc. over time. Some materials do
this more readily than others; in other words, some materials are less durable than
others. Maintenance costs and programmes need to be taken into account. For
example, it is well-known that the Forth Rail Bridge in Scotland is repainted on a
three- to five-year cycle to control corrosion
of the steel structure.

Disposal
Nothing lasts for ever. How is the building going to be disposed of at the end of its
life? Can the material be re-used or converted into some other usable form? What
are the costs associated with this?

Fire protection
There is an unfortunate possibility that any structure may catch fire. Some materials
have better fire resistance properties than others.

Size and nature of the site
The location of the site may influence the choice of materials. Traffic congestion
problems, local by-laws and physical obstructions may limit the size of deliveries to
the
site and the times of day that deliveries can take place













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