桥梁结构说明.docVIP

附录Ⅰ Bridge Design and Construction Planning. The first step leading to the construction of a modern major bridge is a comprehensive study to determine whether a bridge is needed. If it is to be a highway bridge, in the United States for example, a planning study is initiated by a state bridge authority, possibly in cooperation with local governments or the federal government. Studies are made to estimate the amount of bridge traffic, the relief of aimed traffic in nearby highway networks, the effects on the regional economy studied, and the cost of the bridge. The means for financing the project, such as public taxes or sale of revenue bonds repaid by toll charges, are considered. If the studies lead to a decision to go ahead with the project, the land needed for the bridge and its approaches is acquired at the selected site. At this point, field engineering work is started, Accurate land surveys are made. Tides, flood conditions, currents, and other characteristics of the waterway are carefully studied, Boring samples of soil and rock are taken at possible foundation locations, both on land and under the water. Selection of bridge design. The chief factors in deciding whether a bridge will be built as a girder, cantilever, truss, arch, suspension, of some other type are : (1) location; for example, across a river; (2) purposes; for example, a bridge for carrying motor vehicles; (3) span length ; (4) strength of available materials ; (5) cost ;(6) beauty and harmony with the location. Each type of bridge is most effective and economical only within a certain range of span lengths, as shown in the following table : Bridge Type Best span Range (feet) (meters) Girder Rigid Frame Arch Truss Cantilever Suspension 20 to 1,000 80 to 300 200 to 1,000 200 to 1,400 500 to 1,800 1,000 to 5,000 6.1 to 304.8 24,4 to 91.4 61.0 to 304.8 61.0 to 426.7 152.4 to 548.6 304.8 to 1,524.0 As indicated in the table , there is a considerable overlap in the range of applicability of