Bridge aesthetics

Therefore, although function and utility are paramount, aesthetic considerations should also weigh heavily in their design and execution. Sadly this is by no means always the case.

Apart from personal taste there are some specific aesthetic factors that may need to be considered in the design of bridges and these have to satisfy several different points of view, for example, that of motorists, passengers on buses and trains, pedestrians and cyclists, as well as those living and working in the vicinity of the bridge. As a minimum the following need to be investigated:

• External appearance (i.e. looking towards the bridge from a distance) which should account for surrounding context, background and foreground, and different angles of view
• Internal view (i.e. looking from the bridge while crossing over or under it), which is influenced most strongly by material, structure and detailing
• Sequential view (i.e. changing aspect during approach)

It is generally recognised that the aesthetic of bridges with an odd number of spans is superior to that of bridges with an even number of spans (i.e. 1, 3, 5 etc is better than 2, 4, 6 etc.) and that the proportion of those spans also has a considerable influence on the a bridge's appearance. A larger central span is generally more pleasing to the eye.

Bridges are structures designed to traverse space and where there is an odd number of spans the eye is drawn to the clear opening at the centre, emphasising the function. On the other hand, with an even number of spans, the eye is drawn to the central pier blocking the view - the negation of the bridge's function. This is rather unfortunate for designers of motorway bridges where there are usually two carriageways of equal width. Once the expensive single span solution is ruled out, if solid abutments are used with a pier on the central reservation, a 2-span bridge is inevitable. With side piers and bankseats replacing the solid abutments, a 4-span bridge results. Neither is aesthetically very pleasing.

Where an even number of spans cannot be avoided it is better, if possible, to resist the symmetrical solution. Where a non-symmetrical arrangement is used the eye is drawn towards the larger span rather than a central pier.

A major influence in the appearance of a bridge is the depth of the deck. For simple bridges with girders supporting a deck (usually of reinforced concrete that is acting compositely with the steel girders), some "rule-of-thumb" span to depth ratios can be given.