Bridging the Gaps
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HISTORICAL NOTE
Bridging the Gaps “Cast iron will resist a greater crushing force than any other substance whose cost will admit of its being used as a building material,” the U.S. engineer Squire Whipple wrote in his 1847 treatise, A Work on Bridge Building. “Steel has a greater power of resistance, but its cost precludes its use as a material for building. Wrought iron resists nearly equally with cast iron, but its cost is twice as great, which gives cast iron the advantage. On the other hand, wrought iron resists a tensile force nearly four times as well as cast iron, and twelve to fifteen times as well as wood, bulk for bulk.” This brief analysis summarizes the choices faced by bridge designers throughout the ages: the tradeoff between materials properties and cost, with availability surely implied in the cost factor. Since the three basic bridge structures— beam, arch, and suspension—were invented in ancient times, virtually every structural material provided by nature or invented has been used in bridge building. Wood, stone, and plant fibers, readily available and workable with simple tools, predominated early on; iron, steel, concrete, and composite structures, requiring costly fabrication, have been prominent in the last several centuries. Each basic bridge form places different strength demands on its components. When a beam suspended between two supports is subjected to a vertical load, the underside of the beam is stretched in tension, while the top side is compressed. These forces present no problem to a stone or wooden beam spanning a short distance. However, as the distance between the supports increases, the load that the beam can withstand decreases; clearly, a single beam is not a solution for long span distances. The earliest bridgebuilding efforts were confined to lengths readily available in standard monoliths or timbers. In the Dartmoor district of England (River Dart), slabs ~15 ft long transported from a nearby quarry by Neolithic humans span boulders in a streambed to this day. It is known as the Post Bridge. Suspension bridges transfer their vertical loads to curved cables in tension, and eventually to abutments in the surrounding rock structures. In The Conquest of Peru, William H. Prescott describes suspension bridges “made of the tough fibers of the maguey, or of the osier of the country, which has an extraordinary degree of tenacity and strength. These osiers were woven into cables of the thickness of a person’s body. The huge 412
ropes, then stretched across the water, were conducted through rings or holes cut in immense buttresses of stone raised on the opposite banks of the river, and there secured to heavy pieces of timber.” These early suspension bridges were constructed by the ancient Incas, who apparently had insight into, or perhaps catastrophic practical experience of, the tensile failure of materials. Arch bridges support vertical loads through compressive forces extending through the curve of the arch to place both vertical and horizontal forces on its foundation and abutm
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