Civil Engineering Reference
In-Depth Information
mediocre is the quality of the conceptual design. This requires in addition to technical
knowledge and skill, imagination, aesthetic judgement and an appreciation of the
context of the structure. These skills are much more diffi cult to teach.
Thus engineering designers have to rely on any innate talent for the vital aesthetic
component of their practice, or alternatively seek input from architectural specialists.
Enlisting the help of architects in the design of bridges is far better than simply ignoring
the aesthetic component of design, but is much inferior to both aesthetic and technical
components being in the mind of one person. An engineering designer should have an
education that is reasonably balanced between the technical and the aesthetic.
1.6 Economy and beauty in design
An engineer designing a bridge has twin obligations, to his client to use his money
wisely, and to society to produce a structure that will enhance the built environment.
In fact, beauty in engineering design has its roots in the tension that exists between
designing for economy and designing for appearance.
Economy in this context is not simply saving money; it is a concept of rationality
and frugality. It is fundamental to engineering design that the designer is constantly
planning how he can save materials, and how he can make the construction process
simpler, even if many of these design decisions in isolation would not register on the
overall balance sheet of a project.
An example of this tension between appearance and economy is given by the design
of an access ramp to a high level bridge, Figure 1.1. The main bridge consists of a
trapezoidal box section, 2.4 m deep, allowing it to span 60 m or more. The access
ramp must climb from ground level to merge with the main structure. At the point
of merger, the ramp has the same depth and shape as the main bridge. However, the
2.4 m depth would be out of scale for a deck close to ground level. Consequently, the
ramp is given a depth that gradually reduces to 0.7 m as it approaches the ground, with
the spans shortening correspondingly. This is clearly not the most economical choice,
as the formwork for the downstand webs of the ramp will be continually changing. In
order to mitigate this additional cost of formwork, the geometry of the ramp deck may
be defi ned by keeping the length of the web shutters constant and equal to those of the
main bridge, but changing their angle. Thus if the ramp is built span-by-span, the side
shutters of the webs may be re-used for each span. This is an intellectual concept based
on an attempt to rationalise the construction method and save cost, which gives rise to
a distinctive appearance. Finally this appearance must be judged on its own merits.
When an engineer designs, whether it is the overall concept of a bridge or an
individual member, he fi rst must understand the structural behaviour, and then seek
rationality and economy. The search will usually leave him many options, which allows
him to make choices concerning the appearance of the structure.
A very simple example is the design of the bridge pier carrying a single bearing,
Figure 1.2. The pier is subjected to a vertical load and to a horizontal load at the top
which produces a bending moment that increases linearly to a maximum at the base
of the pier, Figure 1.2 (a). The size of the pier at the top will be limited by the size of
the bridge bearing, while at the bottom it will be governed by the combined effect of
the compression force and the bending moment. The engineer has a choice between,
for instance:
