The client may not know exactly what they want only that it must satisfy certain important visual and other subjective criteria. For example, a simple beam footbridge may be functional safe and economic, but austere and inappropriate for the site. This brings in a fourth consideration: The appearance, or aesthetics of the bridge.
In the case for economy, it is necessary to note carefully the client's requirement, as the cheapest construction is not always the best solution or what the client wants. The site, ground conditions and surroundings also need to be studied and considered carefully. Formal adherence to these quality requirements is essential. Maintenance and durability are additional key considerations. The three classic overriding considerations in structural design are function, safety and economy. Effortlessness in visual form is made possible only by completely hidden, highly complex prestressing and reinforcement layouts, which through much iteration have been designed to satisfy prescribed stress limits under a multitude of load cases. A large arched prestressed concrete box girder bridge is a good example. The simplicity displayed is often deceptive. The joy of good bridge design to casual and interested observers alike is the visual purity of the structural form and the logic of the load transfer and functionality.
Their preliminary designs will almost certainly be subjected to the most critical scrutiny including vetting by the Design Council Commission for Architecture and the Built Environment (Cabe) as a prerequisite to the formal planning consent process. On the other hand, large scale crossings usually have a major impact on the landscape and are highly visible, landmark structures. Their size allows considerable scope for engineering inventiveness and architectural expression. These bridges are less constrained by the prescriptive demands of codes of practice than highway and rail bridges, but their appearance can transform a locality and form focal points in addition to fulfilling a prime function. Footbridges have become a means of celebration and decoration at many sites across the country and the design may be procured through a design competition. It is the bridges at either end of the scale that often demand the most work. The amount of work involved in the preliminary design of a bridge is usually proportional to its size. It is during the former that the key decisions are made, which determine how the bridge will satisfy the design objectives, what it will look like, how it might be built and possibly how much a contractor will bid to build it. Preliminary design is broadly defined as that preceding detailed design, which is possibly more accurately described as detailed analysis and elemental design.
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