【正文】 was caused by the bends . When extra strength is necessary in the piers, they sometimes keyed into the bedrockthat is ,they are extended down into the bedrock .This method was used to build the piers for the Golden Gate Bridge in San Francisco ,which is subject to strong tidies and high winds ,and is located in an earthquake zone .The drilling was carried out under water by deepsea divers . Where bedrock cannot be reached ,piles are driven into the water bed .Today ,the piles in construction are usually made of prestressed concrete beams .One ingenious technique ,used for the Tappan Zee Bridge across the Hudson River in New York ,is to rest a hollow concrete box on top of a layer of piles .When the box is pumped dry ,it bees buoyant enough to support a large proportion of the weight of the bridge . Each type of bridge indeed each individual bridge presents special construction problems. With some truss bridges , the span is floated into position after the piers have been erected and then raised into place by means of jacks or cranes .Arch bridges can be constructed over a false work ,or temporary scaffolding. This method is usually employed with reinforced concrete arch bridges .With steel arches ,however ,a technique has been developed whereby the finished sections are held in place by wires that supply a cantilever support .Cranes move along the top of the arch to place new sections of steel while the tension in the cables increases . With suspension bridges ,the foundations and the towers are built first .Then a cable is run from the anchorageconcrete block in which the cable is fastenedup to the tower and across to the opposite tower and anchorage .A wheel that unwinds wire from a reel puns along this cable .When the reel reaches the other side ,another wire is placed on it ,and the wheel returns to its original position .When all the wires have been put in place ,another machine moves along the cable to pact and to bind them .Construction begins on the deck when the cables are in place ,with work progressing toward the middle from each end o f the structure . 1 The loads to be considered in the design of substructures and bridge foundations include loads and forces transmitted from the superstructure, and those acting directly on the substructure and foundation. AASHTO loads .Section 3 of AASHTO specifications summarizes the loads and forces to be considered in the design of bridges (superstructure and substructure). Briefly , these are dead load ,live load , impact or dynamic effect of live load , wind load , and other forces such as longitudinal forces , centrifugal force ,thermal forces , earth pressure , buoyancy , shrinkage and long term creep , rib shortening , erection stresses , ice and current pressure , collision force , and earthquake stresses .Besides these conventional loads that are generally quantified , AASHTO also recognizes indirect load effects such as friction at expansion bearings and stresses associated with differential settlement of bridge ponents .The LRFD specifications divide loads into two distinct categories : permanent and transient . 1 大 跨度 橋梁 1． 懸索橋 懸索橋是現(xiàn)行的跨徑超過 600m 大橋的唯一解決方案，而且對跨徑在 30