Introduction to Computer Aided Design & Manufacture

STEEL CONSTRUCTION:COMPUTER AIDED DESIGN & MANUFACTURE 917
__________________________________________________________________________

STEEL CONSTRUCTION:
COMPUTER AIDED DESIGN & MANUFACTURE

Lecture 5.1: Introduction to
Computer Aided Design & Manufacture
OBJECTIVE/SCOPE
To review briefly the developments in computing generally and to describe the various
ways in which computers can be used in the context of steel construction, with particular
emphasis on design, drafting and modelling.
PREREQUISITES
None.
RELATED LECTURES
Lecture 5.2: The Future Development of Information Systems for Steel Construction
SUMMARY
The reduced cost of relatively powerful computing facilities has led to many activities
traditionally performed by hand being performed with the aid of a computer. The
improvements in computing which have largely enabled this development are reviewed.
The potential for using computers within the whole process associated with steelwork
construction, from client brief through to construction on site, is described. General
applications such as the use of wordprocessing, spreadsheets and databases are included,
but the emphasis is on analytical and design calculations, and computer aided design
(CAD). The distinction between 2-D drafting systems and solid modelling is discussed
and the potential for transferring the data from a solid modelling system onto numerically
controlled fabrication machinery is considered.

1. INTRODUCTION
The ways in which computers have affected the various activities involved in steel
construction have been led by developments in computing hardware, user environments,
software and systems for data exchange. These developments in themselves have been
interlinked, typically by advances in hardware allowing new possibilities for software
development. However, not all advances for the end-user have followed this sequence; to

918 STEEL CONSTRUCTION:COMPUTER AIDED DESIGN & MANUFACTURE
__________________________________________________________________________
a very large extent the development of user-friendly interfaces has gone on in anticipation
of suitable computing facilities becoming available.
The computerised processes involved in computer aided design and manufacture
(CAD/CAM) have to be integrated within the normal sequence of events involved in the
inception, design and construction of structures (Figure 1). The process may be handled by
a group of individual consultants on various aspects, together with a fabricator and
contractor(s). Alternatively, it may be a "design-and-build" process in which one large
organisation takes responsibility for the whole operation, even if specialist aspects are
contracted-out of the parent company. In either case, problems of communication exist,
and the degree of success in overcoming them is crucial to the success of the project.

STEEL CONSTRUCTION:COMPUTER AIDED DESIGN & MANUFACTURE 919
__________________________________________________________________________
Information Technology (IT) is largely concerned with efficient exchange of data and can
be used to maximise the efficiency of all stages of the project. Although structural aspects
are of primary concern here, it is assumed that all the specialist groups associated with a
project consider themselves to be part of an integrated team. In this case, the facilities
afforded by computerised systems for sharing data will be used, for example, to ensure
that services can be fitted into the structure without any problems arising at a later stage in
the contract, particularly on site. From the architectural point of view, it is also important
that structural members do not obscure natural light from windows or the free flow of
occupants within the building. Even in the structural steelwork context, there are areas
where problems commonly arise; a typical one is where a consulting engineer has selected
individual optimum-sized members throughout a building, giving the fabricator the
problem of having to order small quantities of a large number of different sections and to
design and fabricate different connections. Alternatively, consultants may themselves
design connections which, although efficient in their use of material, cause extra
fabrication cost which could have been saved by standardisation on a system which suits
the fabricator's capabilities. These problems should, of course, never arise in any case, and
the fact that they commonly do is essentially the result of inefficient communication
between members of the design team.
During the initial tendering phase, the structural designers have to:
•
•
•
•
•
•

Interact with the client, architect and other specialists, possibly including a
fabricator.
Conceive, agree and rationalise a structural form.
Perform rapid structural design calculations.
Produce a limited range of drawings
Decide on material requirements and construction processes.
Use these for estimating a tender price and producing tender documents.

This stage clearly involves a great deal of work which may, after the contract is awarded,
have been fruitless. From this point of view, therefore, there is a need to minimise the
effort expended in a very risky endeavour. On the other hand, in the event of winning the
contract, it is essential to reduce the amount of eventual variation from the tender
specification, so this process must be carried out in a conscientious fashion. There is
obvious scope at this stage for a relatively crude computerised approach to save a larger
amount of employee-time in preliminary sizing of members, in production of tender
drawings and in cost-estimating.
When the contract has been awarded, the successful design team is then faced with the
need to:
•
•
•
•

Produce detailed design calculations.
Produce a range of drawings for fabrication, construction and building control and
for interaction between the structural, architectural and services specialists.
Produce a detailed bill of quantities and contract documents.
Identify an efficient fabrication and construction sequence, which ensures that
components arrive on site shortly before they are needed and that the unexpected
does not happen on site.

920 STEEL CONSTRUCTION:COMPUTER AIDED DESIGN & MANUFACTURE
__________________________________________________________________________
In each of these processes the use of computers directly, using software, and to share data
is an important aspect of ensuring that the building is constructed efficiently and works
well.
Although it is natural in a lecture such as this to concentrate on the technical input of
computerisation to the design and fabrication processes, it must be borne in mind that a
significant part of the potential gain in efficiency in any complex multi-stage process can
come from a suitable integration of normal office-automation software such as
wordprocessors, spreadsheets and databases. Decisions about how data is shared and
communicated, and how the total process is organised, can also make significant
differences to its efficiency.
In this lecture it is assumed that the reader has only a general awareness of computers and
their uses, and of the applications of automatic control to fabrication and manufacturing
operations. The lecture gives, therefore, a general review of current computing and the
routes by which computing has developed over the past 40 years or so. It is necessary to
introduce and use some computer jargon, which is initially printed in italics.
Computing developments are subject to rapid advancement and, therefore, all such
descriptions are valid only for a short period of time after they are written.

2. COMPUTER HARDWARE
Mechanically-based digital 'computers' were first developed by mathematicians in the 19th
Century. They were developed further only as far as the 'adding machines' and electromechanical calculators (sometimes analogue rather than digital) used in commercial,
industrial and military applications until the mid-20th Century. They performed numerical
computations much faster than could be done manually, but were limited by their large
numbers of precision-made moving parts to fairly simple general arithmetic, or to unique
tasks such as range finding for artillery.
The first electronic computers began to be developed in the mid-20th century, using radio
valves as their basic processing components. These components were accommodated on
racks and the computers thus acquired the title of mainframes. They generated large
amounts of heat and efficient cooling and air-conditioning systems were always required.
Early computers were unreliable because of the limited life of the thermionic valves and as
the size of installations grew so did the probability of failure. The natural limit to the size
of such computers arrived when a design was considered which employed so many valves
that it was estimated by normal probability theory that it would average 57 minutes of
'down-time' out of every hour. Maintenance and operation of a computer required a large
number of specialised personnel. Compared with the previous generation of mechanical
devices, these computers were extremely powerful. Within industry they tended to be
installed mainly for payroll and financial management, but in the research environment
their development allowed the field of numerical analysis to begin to grow.
The development in the 1950's of transistors and in the 1960's and 70's of miniaturised
integrated circuits (microchips) led to progressive improvements in the size, energy
consumption, computing power, reliability and cost of computer hardware. This enabled a

STEEL CONSTRUCTION:COMPUTER AIDED DESIGN & MANUFACTURE 921
__________________________________________________________________________
great diversification in the applications of computing and the machines which do it. The
first of these developments was of mini-computers - relatively portable computers with
sufficient processing power to perform tasks which had previously only been possible on
mainframes. The central processor unit was typically accommodated within a cabinet
which could be mounted on a trolley with the required peripherals and used within a
normal office or laboratory environment. In comparison with mainframes, mini-computers
had only modest technical support requirements. Their size reduced dramatically during
the 1980's to the extent that their current descendants, usually known as workstations, are
very similar visually to personal computers. Mainframes themselves developed into
supercomputers, with the emphasis being on massive memory and data storage together
with extremely fast processing. Supercomputers are now used to run huge database
applications and numerical simulations of complex systems.
By the mid-1970's microchip technology had developed to the extent that significant
computing power could be fitted within very small units - variously referred to as micro,
desktop, personal or home computers. Initially, they had very low on-board memory, but
were directly programmable from the keyboard in BASIC and could load programs from
audio cassettes. The early microcomputer manufacturers each had their own operating
system (or control program) and there was no possibility of transferring programs or data
directly from one type of machine to another. There were also several types of processor
chip in use, each with its own instruction set, so that even programming language
compilers had to be rewritten for each type. A considerable step forward came when a
common operating system (CP/M) was written for one family of processor. This system
spurred the production of a large range of microcomputers between which programs were
interchangeable. This process of standardisation has continued to the extent that at the
time of writing there are only two major groups of personal computers used in business
and professional environments; the IBM PC-compatibles and the Apple Macintosh. In the
case of PC-compatibles, little more than the basic specifications are set by IBM itself and
a huge worldwide industry exists to produce the hardware and software. No such
'compatible' manufacturing industry exists in the case of the Macintosh which, however,
has a very strong software base in some areas, especially in graphic design and publishing.

3. PRINTERS AND PLOTTERS
Despite the current multiplicity of ways for presenting and storing information, a facility
for obtaining hard (paper) copy of input data, program listing, results of analyses, graphics
and documents is still very important. For alphanumeric output hard copy is most
conveniently obtained using a printer. In this area also, there is now a considerable range
of options, but the principal change in recent years has been from hard-formed character
printers to raster (or matrix) printers of various types. The great majority of modern
printers belong to the latter group, in which the output is formed from a matrix of dots
which covers the print area in similar fashion to the pixels which form screen images. In
black-and-white printing each of these dots is simply turned on or off to form the character
shapes or graphical images, and the fineness of the printed output depends on how densely
the dots are spaced. The method by which the dots are printed on the paper constitutes the
main technical difference between one printer type and another.

922 STEEL CONSTRUCTION:COMPUTER AIDED DESIGN & MANUFACTURE
__________________________________________________________________________
The original mainframe lineprinters were based on similar principles to the typewriter,
with hard-formed characters being struck via an inked ribbon onto the paper. These line
printers can achieve high-volume text output at high speed, but are very limited in their
ability to print graphics. Their smaller derivatives include daisywheel and thimble printers
which suffer from the same limitation, and also from rather slow printing, although their
text output is generally of a high quality.
Impact dot-matrix printers have been in use for many years and provide a relatively cheap
system for producing output of reasonable quality for both text and graphics. A moving
print head contains one or more vertical rows of pins each of which can be fired at the
paper producing a single dot. Typical systems offer 9 pins in a single column or 24 pins in
three offset columns. Draft output is produced rapidly by printing dots which do not
overlap at all, while near letter quality (NLQ) is produced by simulating publishers'
character fonts with arrays of overlapped dots. In simple 9-pin printers this is achieved by
the print head making two passes over a line with a slight shift in position to give a denser,
more precise image. Various fonts may be provided and a wide range of characters
incorporated. Given the ability to control each pin of the print-head as it passes across the
paper, it is also possible to print graphical images. These may be defined as bitmaps in
which the image is stored as a continuous array of dots covering the whole print area and
which may be sent to the printer as a simple screen dump which converts a screen pixel
directly to one or more printer dots. Alternatively, vector images (such as engineering
drawings) may be converted to bitmaps by software either at the computer or embodied in
the printer.
Much more dense bitmaps can be achieved with laser printers, which deposit their dots
electrostatically, in similar fashion to photocopiers. Although expensive, they offer
excellent print quality, speed and flexibility (in terms of range of characters, fonts and
print sizes). The high density of the matrix makes laser printers capable of printing highquality graphical images as well as text. The cheaper inkjet printers, which project tiny
individual droplets of ink at the paper from a moving print head produce output of almost
comparable quality, but are less flexible and are much slower.
Most engineering drawings produced by CAD systems are stored as vector data (or
drawing instructions). The pen-plotters which have been in use for many years basically
have used pens to obey these instructions, acting very much as a mechanised draughtsman.
The manufacturing technology of these plotters has developed to the extent that at the time
of writing they still represent an economical way of producing large drawings at a
reasonable speed, in multiple colours and with a variety of pen thicknesses. Since they are
based on servo-motors there is no great penalty to be paid for increasing the physical size
of the drawing space and the amount of plotting data sent and stored is merely
proportional to the number of vector instructions on the plot. However, a dependence on
moving parts limits their speed and precision of plotting. These plotters cover the
complete range of paper sizes in use, from A4 to A0. Since their whole method of working
is to move the pen in vectors across the paper (sometimes by moving the paper as well as
the pen) their most economical use of text is to draw "simplex" characters rather than to
attempt to simulate character fonts. For the same reasons they do not perform well when
used to produce blocks of solid colour, for which they simply have to "shade" the area
with huge numbers of strokes. Continuous, or automatic, paper feed is usually available on
higher-priced models.

STEEL CONSTRUCTION:COMPUTER AIDED DESIGN & MANUFACTURE 923
__________________________________________________________________________
Electrostatic plotters, which derive from laser printers are increasing in use at the expense
of pen-plotters. Since a high-quality dot-matrix image requires massive amounts of
memory at the plotter to hold it, the penalties for requiring large paper size are at present
considerable and these plotters can be very expensive. They are, however, very fast and
accurate. It has already been mentioned that laser printers produce very high-quality
plotted output and these represent a very much cheaper solution for a large amount of
technical material for which the smaller paper sizes (A4-A3) are considered suitable.
Inkjet plotters are also available at much cheaper prices than electrostatic and provide an
economical way towards accurate colour plotting.

4. INPUT/OUTPUT AND STORAGE
In batch-processing systems all information, including program code and input data, is
supplied by the user before any processing begins. It can be done in a number of different
ways. Early mainframe systems used punched paper tape or cards, which were
cumbersome to edit and conducive to errors. They were superseded during the 1970s by
magnetic tape and disk storage. In the case of early microcomputers the tape often took the
form of audio cassette tapes which have now largely been replaced by the much more
controllable floppy disks. They provide portable storage for a relatively large amount of
data and, having been through several phases of development, have now settled for the
present in the 3,5 inch format which is robust enough to be almost self-protecting against
reasonable physical abuse. The so-called hard disks found on many current personal
computers provide both quicker access and very much greater storage capacity than floppy
disks, but are usually not portable between machines. Tape cassette systems (often known
as streamers) are now largely used for making compressed backup copies of material
normally stored on hard disks.
A form of data storage rather different from the magnetic systems mentioned above is
compact disk (CD-ROM) storage. This is very much the same product as the CD's used for
sound or video reproduction, and allows huge amounts of data to be held and rapidly
retrieved, compared with the magnetic systems. CD-ROM is often included with personal
computers used for training and information retrieval, because it provides a facility for
mixing software, large information bases and video-quality graphics interactively. In some
cases it is possible to write to CD as portable storage, but it is not possible to re-use the
space on the disk once it has been written to, so that CD is considered as a write once read
many (WORM) storage medium. However, where there is a need to produce, store and
retrieve huge amounts of data, it is the obvious choice.
It is now fairly common to use scanners to enter text and pictures directly into a computer
from paper copy. The key to this technology is not so much in the ability of the scanner
device to input a picture of the sheet placed upon it, but in the character-recognition
software which resolves individual character bit-images into normal printer font
characters. For graphics, the production of a bitmap of a photograph or a line-drawing is
fairly straightforward. Software which produces vector plot files from bitmaps of
engineering drawings exists, although at the time of writing it is still under development.
In either case, scanned input can still be fairly unreliable, given the problems which can be
encountered with the original paper documents.

924 STEEL CONSTRUCTION:COMPUTER AIDED DESIGN & MANUFACTURE
__________________________________________________________________________

5. INTERACTION
Direct interactive use of computers was not possible on the early mainframes, but it has
progressively become the most effective method of use in most cases. Initially, dumb
terminals were used so that users could type and send to the computer directly the kind of
batch programming commands which had previously been read from punched cards.
However, with mainframes two-way communication was slow since a large number of
users might be sharing time on the central processor and data transmission rates were
rather low in any case. It was only when communication and processing speeds had
increased that interactive programs became possible. At this point, an executing program
could be made to pause and request additional data or decisions from the user at the
remote terminal, and to resume execution when this data had been entered. Results could
be shown on the terminal or printed as a hard copy.
The use of dumb terminals has now largely been superseded by distributed computing.
The personal computer itself has enough processing power and memory for most
applications, so that communication with the central processor is not subject to timesharing and truly interactive software is possible. Where access to software or data needs
to be shared between numbers of users, computers tend to be attached to a network. In a
network a number of computers, each of which uses its own processing power, is linked
together (Figure 2) so that each has access to the others and, more importantly, each has
access to a very large central filestore on which data and software is stored. This filestore
is controlled by a "slave" computer known as the file server which generally runs the
network. When a computer in the ring needs to use a particular program it loads the
program from the filestore and runs it locally. Data produced by one computer can be held
in a common database on the central filestore and accessed by others. Such networks are
often provided with gateways to larger, national or international networks so that
information can be shared by a large group of people. Even with a home computer the use
of a modem allows a user to access the network via an ordinary telephone connection, thus
providing a dial-in facility. This possibility obviously carries the implication that data
needs protection against being corrupted by unauthorised users and, in some cases,
confidentiality must be maintained. Various systems of password protection are used to
attempt to ensure that network users do not have access beyond the areas in which they
have a legitimate interest.

STEEL CONSTRUCTION:COMPUTER AIDED DESIGN & MANUFACTURE 925
__________________________________________________________________________

Computers are not the only devices which can be attached to a network. Most of the
common types of peripheral (such as printers, plotters, scanners and other input/output
devices) can also be attached. In the case, say, of a plotter the file server will control
access to the device by queuing the output to it so that control is maintained. This queuing
system can be applied to any peripheral device which can be attached to the network; in
the context of a fabrication plant, it can be applied to a numerically controlled workshop
machine for which a number of jobs may be waiting at any one time.

6. THE USER INTERFACE
The term user interface refers to the way in which the user and the computer exchange
information. In the most basic sense it might refer to how the user gives instructions when
the computer is first accessed or switched on, and to how the computer responds.
It is controlled by the computer's operating system, which is loaded from its hard disk
when it is started, and includes a series of utility functions which can be initiated by
appropriate (shorthand) commands issued by the user. As many of these functions are
concerned with file operations on a disk (deleting, running, renaming, etc.), the operating
system is usually referred to as a disk operating system, or DOS.
In the days of dumb terminals the only two functions of a user interface were:
•

•

To show on the VDU screen the line of characters which was being typed at the
keyboard and eventually to send them to the remote computer (typically when the
"Enter" key was pressed).
To show on the screen any characters sent to the terminal from the computer.

926 STEEL CONSTRUCTION:COMPUTER AIDED DESIGN & MANUFACTURE
__________________________________________________________________________
The nature of this interaction was very sequential. Lines of text would progress from top
to bottom of the screen and thenceforward the display would progressively scroll up the
screen as more lines were added to the bottom.
With the very fast data transfer rates which are now possible, and because a screen is
controlled by just a single computer, communication between computer and screen is
virtually instantaneous as far as the user is concerned. This has enabled a very rapid
development of the user interface to take place, with the objective of making the use of
computers a more "natural" and less specialised human activity. A recognition has grown
that normal thought processes are largely based on pictorial images rather than verbalised
logic. Opening up the use of computers to the majority of people depends on removing the
necessity to learn even high-level programming languages, including the specialised
commands of an operating system or of a piece of software.
The current generation of windowing user-interfaces (Figure 3) has attempted to minimise
the amount of specialist knowledge needed by users and to address the non-verbal nature
of human decision-making. Their basic context is a computer screen, considered as a
desktop on which a number of ledgers (windows) are placed. These ledgers contain
collections of tools (programs) and documents (data files). The ledgers may be put into the
background or brought forward and their contents displayed, and one ledger may be
partially overlaid by another. The tools are each represented by an icon - a small picture and a title. A pointer directly controlled by a mouse is used to select a program simply by
pointing at it and clicking a button on the mouse. Once a piece of software is running it
obeys the common standards of the windows interface, so that there is no new working
method to be learned by the user on coming to a new software tool. The working principle
is usually to minimise the use of the keyboard for decision-making (it is obviously the best
tool for direct text or data entry) by using the pointer to select options using a large but
standard range of visual devices on the screen. These options include pull-down menus
and dialogue boxes, both of which are small screen overlays on which selections can be
made, which remove themselves after the action has been taken. It is currently fashionable
to make major selections by "pushing buttons" with the pointer. It is possible, while
running one program in a window, to pause operation and use another application in
another window. This is not true multi-tasking, since there is only one program running
actively at a time, but it is possible to mix a range of tasks in a given period without
completely closing down any one of them. For example, in writing a technical report it
might be appropriate to keep a word-processor, a spreadsheet, a specific design or analysis
program and a CAD program all open simultaneously, so that the final document can be
produced as new figures, calculation results and tabular information or graphs are
generated or modified. Real multi-tasking, in which a large finite element analysis, for
example, could be running while more routine interactive tasks are being performed, is
only available in practice on the most powerful types of workstation.

STEEL CONSTRUCTION:COMPUTER AIDED DESIGN & MANUFACTURE 927
__________________________________________________________________________

Although window interfaces make computers accessible to a very wide range of potential
users, they present some difficulties for developers of software. The requirement for onboard memory is high, as is that for hard disk storage. Development of original software
for windows environments is usually rather slow and time-consuming and, therefore, the
economics of writing original technical programs for a restricted market is not always
favourable. Conversion of well-established software running in the normal operating
system environment, in such a way that it keeps its full functionality and retains the
working methods which have made it popular whilst taking advantage of the common
user-interface, is an even more difficult task. It is, therefore, often necessary to work
within the normal keyboard-based operating system environment. On PCs this is usually
MSDOS and on workstations Unix. Using a computer in these environments requires much
more understanding of the functions of the operating system and how data is stored on
disk. Visually the user sees a blank screen, or part of a screen, with a flashing cursor to the
right of a brief prompt. In order to make the computer perform any useful task it is
necessary to type in a command in the operating system's high-level language. This is less
daunting than it sounds - with only a few commands in one's vocabulary and a working
knowledge of the directory structuring of hard disks it is possible to work very effectively
with either a personal computer or a workstation.

7. PROGRAMMING COMPUTERS
At the level of the processor chip very large numbers of very simple instructions are
executed in order to perform even the simplest of computing tasks. The task of
programming a computer in such terms is a very tedious process and is only attempted
when execution speed is the very highest priority for an item of software. High-level
programming languages provide an alternative means of presenting a sequence of more

928 STEEL CONSTRUCTION:COMPUTER AIDED DESIGN & MANUFACTURE
__________________________________________________________________________
advanced instructions to a computer in a form reasonably comparable with ordinary
language. The set of instructions (the computer program) are then translated (compiled)
into machine code form comprehensible to the processor.
Any programming language has a vocabulary of functional commands and a syntax of
rules. In addition, there are numerous arithmetic operators, including many of those used
in conventional mathematics and the ability to use variables of many different types. The
programmer prepares a list of such instructions which represents the flow of control within
the program. There are numerous programming languages, nearly all of which are capable
of performing most programming tasks, but each of which has a unique basic philosophy
which makes it efficient in a specific field. For engineering applications FORTRAN
(originally used on mainframes for batch processing) is still very widely used on account
of its mathematical efficiency and its huge library of mathematical subroutines. The
world's most popular language for general programming is BASIC which exists in many
different forms, from the almost unstructured interpreted versions generally bundled with
any type of personal computer to very advanced compiled languages with very large
libraries of functions. Perhaps the most versatile and powerful general-purpose language
used by professional programmers is C which includes operators which allow very easy
direct access to computer memory. Other languages are used mainly in specific types of
application with their own functional requirements, and it is not necessary to go into their
detail here. At this time computer users do not formally need to write programs in any
case, but will use software produced by professional developers over many man-years. A
particular exception to this is in the context of spreadsheets, and occasionally databases, in
which it may be convenient to write applications in the high-level languages which are
included in these types of software.

8. STRUCTURAL ANALYSIS AND DESIGN
SOFTWARE
Largely because of its direct links with computational research in universities, structural
analysis software has been available for a long time, initially on mainframe computers but
more recently on all types of hardware. Except in the most complex analytical processes
the power of modern personal computers is adequate for even the more specialised tasks
needed for structural engineering. In the case of statically determinate analysis of
structural components, the analysis is normally contained within the detail design
software. Elastic analysis of plain frames or grillages is probably the most useful general
tool for the structural designer. It now exists on personal computers in a multiplicity of
different forms. The important differences between these programs tend to be more in
their ease of use than in their technical capabilities; all tend to have graphical
rationalisation capabilities (Figure 4), so that geometry and results can be viewed
conveniently, but the processes for editing geometry and loads vary widely, as do their
capabilities of interacting with design and CAD software. Nonlinear, elasto-plastic and
three-dimensional frame analyses are now routinely available on personal computer,
usually within general-purpose finite-element packages which derive from mainframe
software developed in academic research. These packages, although useful for checking
stresses, deflections and dynamic motions in very complex cases, tend to be over-specified
for most structural design problems, require very large amounts of data to be defined and
often produce far more output than is necessary. Their use is more appropriate as a final

STEEL CONSTRUCTION:COMPUTER AIDED DESIGN & MANUFACTURE 929
__________________________________________________________________________
validation of a design than in the earlier stages when the analysis is being used often as
part of the member selection process.

Structural design software is a much more recent phenomenon, since it relies very heavily
on interaction with the design engineer and only started to become widespread when
microcomputers began to flourish in the early 1980s. Much structural design involves
relatively simple calculations - standard loading calculations, analysis and element sizing
based on rules embodied in codes of practice. These calculations have traditionally been
performed by hand, but interactive computing now enables designers to take advantage of
the power of the computer without relinquishing control over design decisions. Design
software relieves the designer of the tedium of laborious manual calculations - in many
cases a degree of 'optimisation' is incorporated within the program, but decisions about
selecting the most appropriate individual member sizes remain with the designer. Design
software now reaches into nearly all areas, but is very variable in its nature, style and
quality. The best allows considerable flexibility in use, making revisions to existing
designs easy and allowing data to be exchanged with software for analysis, CAD and
modelling and for estimating quantities.
In the context of steel structure design, the material available starts with "free disks"
provided by manufacturers of cold-formed products such as sheeting, composite decking
and purlins, which effectively provide quick look-up tables for safe working loads and
spans against key dimensions. Element design to various codes includes beams (both steel
and composite), columns and beam-columns, and connections of various kinds. Whilst
element design usually takes the form of free-standing executable programs the power of
present-day spreadsheet software is such that applications for standard spreadsheets can
provide a very flexible way of automating these fairly straightforward design processes,
with good links to other standard software. Plastic design of steel frames, particularly lowrise frames such as portals, is available in different degrees of sophistication in terms of its
convenience in use, links to downstream software and CAD, and in the order of analysis it
offers. Plastic design is one area where different degrees of analytical capability provide
different orders of realism in results; the more non-linear analysis, which allows
development of plastic zones, can produce distinctly lower load resistances than the rigidplastic and elastic-plastic versions.

930 STEEL CONSTRUCTION:COMPUTER AIDED DESIGN & MANUFACTURE
__________________________________________________________________________
Perhaps the most important thing to appreciate about design software is that different ways
of working will be convenient for different design environments. A steel fabricator with a
large commitment to design-and-build will really need an integrated system, preferably
based on a 3-D modeller, in which it is easy to handle large numbers of members, to
standardise sizes and connections, to make rapid revisions, and to produce accurate
costing and fabrication data. A small firm of general consulting engineers, on the other
hand, may find it more convenient to keep a fairly extensive library of free-standing
design programs with an easily understood user-interface, so that basic member sizing and
presentation calculations for building control approval can be done reliably and without a
significant re-learning process when the software is occasionally used.

9. COMPUTER-AIDED DESIGN: TWODIMENSIONAL DRAUGHTING
The development of interactive graphs at about the beginning of the 1970s provided the
opportunity for using computers for draughting. These early systems used mainframe
computers with graphics terminals ("green" screens) and provided three-dimensional
draughting capabilities. Initially this was limited in use to heavy manufacturing industry,
particularly in the production of aircraft, ships and motor cars, where the benefits of 'mass'
production justified the enormous investment then required for CAD. Even in those
pioneering days, the output from the CAD systems was providing automatic bills of
quantities and also being linked into numerically controlled (NC) machines, thus
improving manufacturing efficiency.
In the late 1970s the development of 'super mini' computers was a significant factor in a
very large growth in the use of CAD. They provided a single-user facility and can be
referred to as 'personal designers'. Application was still concentrated in the productionbased industries, but with increasing use of relatively cheap, unsophisticated, twodimensional systems in the construction industry. These personal designers were difficult
to learn and use, largely because they were not developed with the end-user in mind. User
interfaces, which were not standardised, generally took the form of a command line with
complex syntax. The capabilities typically replicated those of conventional draughting
processes and often provided little additional intelligence. For instance, it was often
possible to change the numerical value of a dimension without the drawn length changing,
and without appropriate warning messages. Some simple systems still allow this. The
advantages of this type of CAD are very limited - essentially the ease of revising a
drawing and replotting. Time to produce the original drawing might often be as much or
more than producing the same drawing at a conventional drawing-board.
More sophisticated features have rapidly been introduced, offering greater advantages.
The advantages start with improved geometrical constructions such as:
•
•
•

Snapping, for instance onto the end or mid-points of lines, grid points, tangents,
etc.
Automatic grid generation.
Rubber-band shapes, including lines, rectangles, circles and other shapes, allowing
them to be replaced, dragged, stretched and distorted.

STEEL CONSTRUCTION:COMPUTER AIDED DESIGN & MANUFACTURE 931
__________________________________________________________________________
•
•

•

•

Associative dimensioning, in which a dimension arrow is tied to two points on an
object and changes its printed value if the object is stretched or distorted.
Layering, enabling different groups of information, for instance those relating to
architectural detail, foundations, structural details, and various building services, to
be superimposed on a basic common plan, see Figure 5.
Objects which can be defined so that they can be scaled and placed anywhere
within the drawing. Thus, changing information relating to a particular dimension
of a defined object influences other dimensions dependent upon it.
Symbol libraries of standard geometric forms, e.g. architectural or structural
details. These libraries not only facilitate the drawing process, but can also provide
data for use elsewhere, for example, in the production of bills of quantities.
Additional symbol libraries for specialised purposes can be created or purchased.

These facilities are now fairly typical in professional personal computer CAD tools.
Increased intelligence has been introduced into the way elements are represented, for
instance, in according specific relationships between drawn elements. There is, however, a
penalty to be paid for storing data in an intelligent form, since:
•
•
•

Additional data must often be specified by the user.
It requires a sophisticated database system and increased computer memory.
In order to take advantage of the intelligence, a significant amount of processing is
needed.

932 STEEL CONSTRUCTION:COMPUTER AIDED DESIGN & MANUFACTURE
__________________________________________________________________________
Two-dimensional draughting systems still have a role in the production of general
arrangement drawings, traditionally the responsibility of the consulting engineer. Unless
the system is to be used subsequently to produce detail drawings which are normally the
fabricator's responsibility, there is no real advantage to be gained for this kind of user by
using the three-dimensional structural modelling approach. A standard 2D system also
allows easy interaction with architects and building services engineers. It also enables the
integration of different parts of the civil and structural engineering design work via simple
layering. Drawings, or parts of drawings, are easily copied directly into word processing
packages for report writing. It may also be possible, in future, for the 2D system to act as a
partial pre-processor for full structural modelling.

10. THREE-DIMENSIONAL STRUCTURAL
MODELLING
Three-dimensional CAD systems can vary from a simple wire-frame model which
operates on lines only, through surface modelling to complete solid modelling which
requires comprehensive data definition and relationships but offers enormous potential.
Simple three-dimensional systems offer little advantage over 2D CAD for the construction
industry. However, the development of specialised forms of modelling system provides
enormous power with direct relevance to steelwork fabrication (including detail design).
In this context, the 3D solid model is a means of representing the complete structure, as
distinct from conventional CAD where individual elements are merely drawn as flat
shapes. This provides a complete description of the steelwork, including connections from
which all necessary fabrication and erection information can be extracted automatically.
The model is typically created in a manner similar to the design sequence itself, coarsely
defined at the start, with progressively more detail added as appropriate.
Initially the structural layout is defined using a wire frame model (Figure 6a). This can be
done with the aid of a 3D framework of grid lines and datum levels and corresponds to the
general arrangement produced by the architect or consulting engineer. With 3D modelling,
it is also possible at this stage to generate more detailed engineering drawings, including
isometric views (Figure 6b). Information regarding section sizes, geometric offsets and
additional data such as end reactions from design calculations can all be entered very
easily. The fabricator's next responsibility is to design connection details. Detail design is
facilitated by using a library of standard connection types (which can be tailored to suit the
needs of individual companies or clients) which will scale automatically to account for the
members to be connected (Figure 7). Appropriate detailed calculations can also be
performed according to accepted design rules and based on the end reactions prescribed
when setting up the wire frame model. Where non-standard connections are required,
interactive modelling facilities exist for constructing the appropriate details, either from
first principles or by modifying standard forms. These can be added subsequently to the
library for future use.

STEEL CONSTRUCTION:COMPUTER AIDED DESIGN & MANUFACTURE 933
__________________________________________________________________________

934 STEEL CONSTRUCTION:COMPUTER AIDED DESIGN & MANUFACTURE
__________________________________________________________________________

The definition of a 3D model in this way contains a complete geometrical and topological
description of the structure, including all vertices, edges and surfaces of each physical
piece of steel. As a result all element dimensions are automatically tested for
compatibility, and clashes which can easily carry through in the traditional processes are
removed. The model allows the efficient generation of conventional drawing information,
including general arrangement drawings (plans, elevations, sections, foundations,
isometric views - Figure 8), full shop fabrication details for all members, assemblies and
fittings (Figures 9a and 9b), and calculation of surface areas and volumes for all steelwork.
Further benefits of such systems are related to the links which can be established with
other parts of the production process. Full size templates can be drawn, e.g. for gusset
plates, and wrap-around templates for tubes. Erection drawings can be output and material
lists (including details of cutting, assembly, parts, bolts, etc. produced automatically. An
interface to a management information system can also facilitate stock control, estimating,
accounting, etc. Potentially of greatest importance is the possibility of downloading data
directly to Numerically Controlled (NC) fabrication machinery, automating much of the
fabricating work itself. At this level, 3D modelling is the central controlling tool for an
integrated steel fabrication works in which the total design-and-build package is offered.

STEEL CONSTRUCTION:COMPUTER AIDED DESIGN & MANUFACTURE 935
__________________________________________________________________________

936 STEEL CONSTRUCTION:COMPUTER AIDED DESIGN & MANUFACTURE
__________________________________________________________________________

In more general terms, surface modelling provides additional information about a 3D
model. At its simplest, but probably most cumbersome, this can take the form of defining
boundaries within which there is a surface with specified characteristics. More
sophisticated surface modelling techniques, such as rubber surfacing which allows a
surface to be stretched and squeezed into shape, are not directly relevant to most
construction work, but are particularly valuable where shell forms are being developed,
e.g. for motor car body design and manufacture. It may be that developments in steelwork
modelling of the type described above will allow a convenient way of integrating the
skeletal models with surface models of the building envelope and architectural
visualisation models, but at the time of writing this is not yet a reality.
The general arrangement drawings have typically provided the basis for a Bill of
Quantities used for tendering. Preparation of a Bill requires the weight of steelwork in
different parts of the structure to be calculated, including an allowance for attachments and
connections, and a brief description of the operations required for fabrication and erection.
The specification, which may be in a largely standardised form, provides additional
information, e.g. regarding the corrosion protection system to be applied. The Bill of
Quantities is traditionally prepared by hand. However, if a suitable 3D modeller is used,
the output can form the basis of the Bill, with quantities called off automatically. This
technique not only avoids time spent on tedious calculation, but also minimises the
potential for errors in the quantities. As part of the steelwork detail drawings, each item is
given a unique reference number. This number is used to identify each workpiece in the
subsequent fabrication and erection operations and also serves as the basis for a materials
list which is issued for ordering stock and planning production.
In a design-and-build contract, a formal Bill of Quantities is not used. Instead the
steelwork contractor must estimate a lump sum on the basis of experience and preliminary

STEEL CONSTRUCTION:COMPUTER AIDED DESIGN & MANUFACTURE 937
__________________________________________________________________________
calculations. When the contract is awarded, the fabricator produces the design calculations
and general arrangement of drawings. Preliminary buying lists for purchasing stock from
the steel mills or stockholders are then required and the sequence of operations follows a
similar route to the more traditional method of procurement. In this environment also, it is
clear that use of a suitable 3D modeller can enhance the accuracy of estimation of
quantities, even before a complete detailed solid model exists.

11. NUMERICAL CONTROL IN FABRICATION
Traditional methods of preparing steelwork elements for construction - cutting to length,
drilling, making attachments (cleats, brackets, etc.) and assembling sub-frames (e.g.
trusses) were labour-intensive, and based on precise information on the steelwork detail
drawings. Measurements and marking were performed manually using templates, typically
of timber construction, for repetitive or complicated details. Appropriate machine tools
(saws, drills, etc.) would be aligned visually and each operation performed in sequence,
with the workpiece being transported between individual items of equipment. Subframes
were typically put together on a laying-out floor on which the form of the geometry had
been marked using traditional setting-out methods.
The introduction of NC machines has enabled preparation details such as overall length
and position of holes to be defined numerically via a computer console. Handling
equipment automatically positions the workpiece in relation to the machine tool, which
performs the necessary operations. In this way, the labour-intensive operations of marking,
positioning and preparation are integrated into a single process which leads to major
improvements in fabrication efficiency, especially where fairly standard or repetitive
operations are concerned. Even greater efficiency can be achieved by transferring the
necessary information on machining directly from the steelwork modeller into the NC
machines rather than by transcribing it manually from drawings or paper specifications.
This process requires a computer modeller which is capable of providing the machining
operations data in a suitable form. The data can then be transferred either by writing to a
floppy disk which can then be read by the NC machine, or via a direct network connection
between the machine and the CAD workstation. At the time of writing only a minority of
fabrication plants have complete computer-integration in this way because of
incompatibilities between computing hardware and machine tools, but this integration is
clearly capable of providing much greater efficiency and higher quality than the present
semi-manual process.

12. THE FUTURE
Predicting future developments in computing is notoriously hazardous. However, the trend
of increasing power of computers with little or no increase in cost shows no sign of
slowing down, suggesting that the application of computing is likely to spread even
further. Applications, which currently require excessive amounts of processing making
them impractical, will become feasible. The evolution of graphical user interfaces appears
to have reached a plateau, but the application of graphics may well become much wider,
with "virtual reality" applications, for instance, allowing the structural designer as well as
the architect a realistic visualisation of new developments. This application has already
been used in demonstration form for a small number of new constructions.

938 STEEL CONSTRUCTION:COMPUTER AIDED DESIGN & MANUFACTURE
__________________________________________________________________________
Routine design calculations may become more sophisticated, possibly allowing more
adventurous design solutions, but there is a danger that the designer may become overreliant on the processing power of the computer. A simple understanding of general
structural behaviour is still essential. There may be a temptation to use over-elaborate
methods of analysis and design, and the engineer should always consider whether these
are appropriate, particularly bearing in mind unavoidable uncertainties regarding design
loads, material strengths, etc. There is also a danger of refining designs to an excessive
degree in an attempt to optimise structural efficiency. For example, a structure in which
every steelwork element has been designed for minimum weight will result in the lowest
overall tonnage, but almost certainly at the expense of increased fabrication and erection
costs.
Some aspects of steel design, for instance fire resistance, have traditionally been treated in
an over-simplified fashion and increased computer usage will rightly allow more rational
approaches to become more commonly considered as part of the design calculations.
Other aspects of structural behaviour have often simply been ignored. Dynamic analysis,
for instance, is a specialist topic which the designer may be called on increasingly to look
at in detail, and again the integrated computer model could enable this to be done
painlessly as far the design is concerned. Increasingly, the designer will create an
intelligent model of the structure and expose it to a number of design scenarios, observing
and interpreting the responses. In this respect graphics is again likely to become
prominent, with visualisation of behaviour rather than the presentation of lists of
numerical results requiring careful interpretation.
In building forms where complex geometry is involved, such as the International Terminal
at Waterloo, the use of conventional draughting methods would have been almost
impossible. In this case a 3D modeller was used to set up the geometry of a single bay of
the three-pinned trussed-arch system. This acted as the starting point for the whole roof
and also facilitated the setting-out on site, with a number of targets attached to each arch
which could then be positioned on site using precise three-dimensional co-ordinates and
conventional electronic distance measuring equipment.
Integration of computers into each of the different stages of design and construction will
not only lead to improved efficiency with data automatically carried forward, but it will
also extend computing into areas which might be regarded as trivial. If the scheme design
involves the creation of a simple 3D wire-frame model of the structure, then loading
calculations become almost automatic. Whilst this is not a difficult part of engineering
design calculations, it is somewhat tedious and automatic load assessment would result in
valuable time-savings. It is possible that eventually expert systems, which have so far had
limited success in structural engineering, may be of use at the concept stage and in
integrating the structural form with services and building-use requirements.
These developments, which all depend on the establishment of a common, universal
database structure, will allow information about a structure to be shared between different
applications, so that a change in data as a result of one process automatically feeds through
to other dependent processes to ensure consistency. Object-orientated programming
concepts and relational databases provide the vehicles for these developments. It has been
seen that the 3D modeller is already being linked to fabrication machinery and to other
aspects of the whole building. This linking is likely to become more common as standard

STEEL CONSTRUCTION:COMPUTER AIDED DESIGN & MANUFACTURE 939
__________________________________________________________________________
data structures are established and fabricators exploit the improvements in efficiency
which integration offers. The linking can be extended through to site planning, allowing
more precise control over component delivery and operations, where even greater
improvements in efficiency could be realised. Integration is also likely to be extended to
non-structural areas with, for instance, analysis of energy requirements, day lighting, etc.
all being integrated and making use of a central database.

13. CONCLUDING SUMMARY
•
•
•
•

Computing facilities continue to improve dramatically and their use is now highly
cost effective for a wide range of activities within steel construction.
Interactive graphical user interfaces have become standard, making it easier for
non-specialists to use computers.
Different facilities are required by different organisations within the design and
construction process.
The greater the degree of automatic data transfer between different applications,
the more efficient the overall process will be.