span required, the materials at hand, and the type of load anticipated - pedestrian, vehicular, railroad, or a channel of water as in aqueducts. Primitive bridges Other than the clapper bridges of England and similar spans surviving in other countries, bridges dating from prehistoric periods are rare. Bridges of twisted vines and creepers found in India, Africa, and South America, the ancient cantilevers of China, Kashmir, and Japan, if any survive, or the wooden arches of Japan may be candidates for World Heritage listing because they perpetuate primitive ingenuity and craft technology that is important to recognize. Since some of their materials cannot be original, these structures will have to pass the test of authenticity. In 51 BC, during the Gallic War, Caesar attested to the construction of narrow wooden bridges by
All lateral loads are resisted by latticelike steel bracing on the exterior walls; some additional bracing runs through the interior. To avoid the angled walls from deflecting during the construction they had to be shored. The challenge was to determine which geometry to use so that when the gravity loads were imposed on the structure it would deflect to the desired architectural geometry (CE 06, Dec). Due to the severely inclined walls and large cantilevers, it was decided early on that the dead weight of the building would need to be minimized, thus making structural steel the material of choice. The walls of the building generally lean outwards, so to some extent, the lateral loads balance each other. Thus the floors act as tension ties for the inclined walls, with the steel beams helping with tension and compression. However, where these forces are not balanced they must be taken to ground through the building’s lateral
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