BRIDGES

Five types of bridge are illustrated in diagrammatic form in Figure 8.1. In 'a' a simple beam bridge is shown. Almost certainly this was the earliest type of bridge: in its most primitive form it was simply a log or a stone slab laid across a ditch or a small stream. From these beginnings came the first real bridges, with prepared abutments and spans consisting of several beams on top of which timber planking was laid. Handrails would often be provided. The problem with beam bridges, at a time when the only available materials were stone or timber, was that span lengths were limited to about 15 metres for timber, including abutments, and usually about 5 metres for stone. There were, however, some remarkable multi-span stone bridges built in China from the eleventh to the thirteenth century. These had spans up to 70 feet (21.3 metres) involving the handling of stones weighing up to 200 tonnes. This type of construction was, however, extremely wasteful in materials and labour. Also, a heavy load was placed upon the foundations, which often failed. Such structures were unknown in the Islamic world where multi-span beam bridges were generally built of timber.

The origins of the ancient cantilever bridge and its modern counterpart are quite different and demonstrate one of the dangers to which the historian of technology is exposed, namely that of assuming that because two structures look similar the older must be the ancestor of the younger. The modern cantilever bridge is a development of the continuous bridge: the introduction of hinges at certain points in a continuous bridge produces a statically determinate structure. In medieval times continuous spans were not possible, since structural steel and reinforced concrete were unknown. The ancient cantilever bridge was not based upon any analytical consideration. Rather it was a development from the beam bridge.