Civil Engineering Reference
In-Depth Information
produce them almost automatically, without struggling to understand manuals, sign
conventions etc. One must at all costs not engage one's brain into an 'analysis mode',
or one's creativity will be swamped by one's intellect.
It is very important that, at this early stage, all calculations are kept conservative, so
that one is not deluding oneself about the feasibility of a favoured option. These initial
calculations allow the designer to develop his understanding of how the structure
works, of how the forces fl ow. They also enable him to put sizes to members, and so to
gain a fi rst insight into the appearance of a structure. The aesthetics of structures are
critically dependent on member sizes, and how size varies along a member.
The diagrams of normal forces, bending moments, shear forces and torques may be
drawn along the members, and stresses calculated. As the structure is better understood,
the logic of how it works becomes apparent. Member sizes may be refi ned to improve
economy, to provide reserves of strength and to affect the appearance. However, one
must not defy the basic logic of the structure; one cannot make a member excessively
slender in defi ance of the structural logic, just because it looks better; there must be
a concordance between function and appearance. This does not mean that the size
of members is dictated by their stress levels, but that one must not act contrary to
the structural logic. This usually leaves a considerable margin for discretion in sizing
members. For instance, two members that are equally stressed may be given different
sizes for the sake of appearance.
At this stage, it may become clear that the structure is evolving in a way that is
not satisfactory, either technically or aesthetically. When one embarks on this design
process, it is frequently not clear what the nature of the destination will be. An essential
part of design is the readiness to tear up what one has done and start again. An engineer
who does not have the courage, or the time, to recognise that he is engaged in a dead
end and to start again cannot pretend to be a creative designer.
1.3 Teamwork in design
Design is inevitably a team exercise. At its simplest, the bridge designer will have another
engineer and one or two draftsmen working directly for him, while on large bridge
projects the core team may include ten or more people. Generally, other specialist
disciplines will also be involved for part of the design period, such as geotechnical
engineers, quantity surveyors and the suppliers of proprietary products such as the
bearings, expansion joints etc. An architect may also be involved, either as a partner in
the concept or as a specialist involved in the design of fi nishes, handrails, lighting and
other decorative aspects. Depending on the form of contract, some decisions are likely
to require input from the client or the contractor.
If the design is to be anything other than banal, the team needs a leader who makes
the project his own. There is no aspect of a bridge design that is not capable of more
than one solution, whether it is the overall concept or the type of bridge bearing.
There is thus great potential for diverging views and for indecision. This multitude
of design decisions must be welded into a coherent project, and this can only be done
successfully through one mind.
This need for a 'chief designer' is sometimes challenged by professionals, who claim
that design is the result of collective decision making, with no one dominating the
process. However, it is usually only necessary to consider what the effect on the design
would be if each member of the team were to be substituted in turn. For most of
