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 defined 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 first 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:
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• a prismatic column of a generous size that allows minimum reinforcement to be used throughout, Figure 1.2 (b);
• a smaller prismatic column that needs minimum reinforcement at the top, but heavy reinforcement at the base, Figure 1.2 (c);
• a column that is as small as possible at the top and tapers uniformly to the bottom, Figure 1.2 (d);
• a column which is as small as possible at the top and whose width then varies such that the minimum reinforcement may be used throughout, Figure 1.2 (e);
• some combination of any of these.
His choice will be informed by other aspects of the project, for instance:
• the number of similar columns in the project;
• the range of heights of such columns;
• the need for variations on the basic column size to cater, for instance, for bridge expansion joints, anchor piers or different length spans;
• the need for a family of columns to cater for other bridges forming part of the same project;
• the architectural context of the bridge.
As the engineer considers the economy of the various choices to be made, he will most probably find that several options have costs that are within the margin of estimating error. Consequently, although the search for economy is at the heart of his design, it cannot be used as an alternative to aesthetic judgement; the engineer must choose the shape he considers is best in all the circumstances.
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Figure 1.3 Options for flared column
Once he has made his basic choices, he then has to refine his design, both for economy and appearance; small changes of shape can greatly affect the appearance, as may be seen in comparing the options for a column of varying width shown in Figure 1.3. Reinforced concrete detailing considerations may also suggest minor dimensional changes, to give a rational arrangement of bars, or to make best use of standard bar lengths and minimise waste.
What an engineering designer cannot do and retain the integrity of his design is to fly in the face of rationality and economy, and design a heavily loaded column that, for instance, tapers towards the bottom, Figure 1.2 (f), creating an artificial problem that then needs to be solved by misdirected engineering ingenuity. This is true even if the additional cost as compared with a rational design is negligible.
There is no reason that the column should not be decorated, with corners cut off, the sides faceted, Figure 1.4, or with ribs or other decorative finish, Figure 1.5 (7.15.4), as long as the cost of this decoration is reasonable in the context of the project. Some aspects of such decoration may be functional, for instance to reduce the apparent bulk of the column by changing the way light reflects off it or to control water runs to improve its weathering, while some may be just to make it more attractive.
Engineering design is thus driven by the simultaneous consideration of rationality, economy and appearance. Designing economically alone is not enough. There is no automatic linkage between economy and beauty; aesthetic judgement is required at every step of a design.
Engineers have been known to put their faith in the idea that if they design honestly, and reflect in their structure the flow of forces, the result will inevitably be aesthetically satisfactory, or even beautiful: the idea that 'form follows function'. Unfortunately, this is not sufficient. Within the confines of honesty and economy, the engineer is left with a wide choice, which requires aesthetic judgement. A useful analogy is to consider the design of the human face, which is well defined by its function, but which gives rise to an infinite number of outcomes.
If bridge designers are not confident of their aesthetic ability, they should request the assistance of an architect, who should be involved from the earliest stages of the design. If they are lucky, they will find one who understands the special quality of engineering design, and who does not take over the project with his own, nonengineering taste. Such collaboration can be very creative, but success depends firstly on the engineer being skilled and confident in the technical domain, and secondly in the architect having a genuine interest and feeling for engineering structures. Even engineers who have confidence in their aesthetic judgement can find collaboration with a talented architect very creative, with the architect questioning the engineer's choices, and proposing different ways of seeing the design.
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