Overview

Ancient building works included roads, fortifications and harbours; even on a small scale, the builder needed a good knowledge of the properties and use of appropriate materials. At a larger scale, when the works may be thought of as civil engineering rather than building, rules were needed as well as practical knowledge. The design of a cathedral required both scientia and ars. In the same way, a tree-trunk thrown across a small stream will provide a serviceable bridge, but bridges of larger span must be designed and require the use of specialised theory.

Perhaps surprisingly, two sophisticated types of bridge have been in use for at least 6000 years. In the suspension bridge, a deck is hung from cables , and the form has hardly changed through the millennia, although the materials are no longer vegetable. Originally, cables were made from creepers or lianas, or from ropes of twisted strands, made in turn from fibrous plants, and the walkway (if it existed at all) was made from wood. The structural action of such a bridge is straightforward and, so to speak, visible in the form of the bridge; the loads on the walkway are transferred to the cables by hangers, and the cables act in tension, finally pulling on their anchorages at either end of the span.

The other early type of bridge is the arch. Originally, perhaps in Mesopotamia in 4000 BC, the material used was brick, although the bricks were sun-dried rather than baked. Similar use...