【正文】 hose of simple girder bridges. The junction of the pier and the girder can be difficult to fabricate and requires accuracy and attention to detail. Though there are many possible shapes, the styles used almost exclusively these days are the pishaped frame, the batter post frame, and the V shaped frame. The batter post rigid frame bridge is particularly well suited for river and valley crossings because piers tilted at an angle can straddle the crossing more effectively without requiring the construction of foundations in the middle of the river or piers in deep parts of a valley (illustration 1). V shaped frames make effective use of foundations. Each Vshaped pier provides two supports to the girder, reducing the number of foundations and creating a less cluttered profile (illustration 3.) Pi shaped rigid frame structures are used frequently as the piers and supports for inner city highways. The frame supports the raised highway and at the same time allows traffic to run directly under the bridge (illustration 2.) Arch After girders, arches are the second oldest bridge type and a classic structure. Unlike simple girder bridges, arches are well suited to the use of stone. Many ancient and well know examples of stone arches still stand to this day. Arches are good choices for crossing valleys and rivers since the arch doesn39。s longest center span of any bridge at 1,991 meters. A typical suspension bridge (illustration 1) is a continuous girder with one or more towers erected above piers in the middle of the span. The girder itself it usually a truss or box girder though in shorter spans, plate girders are not unmon. At both ends of the bridge large anchors or counter weights are placed to hold the ends of the cables. The main cables are stretched from one anchor over the tops of the tower(s) and attached to the opposite anchor. The cables pass over a special structure known as a saddle (illustration 2.) The saddle allows the cables to slide as loads pull from one side or the other and to smoothly transfer the load from the cables to the tower. From the main cables, smaller cables known as hanger cables or hanger ropes are hung down and attached to the girder. Some suspension bridges do not use anchors, but instead attach the main cables to the ends of the girder. These selfanchoring suspension bridges rely on the weight of the end spans to balance the center span and anchor the cable. Thus, unlike normal bridges which rest on piers and abutments, the girder or roadway is actually hanging suspended from the main cables. The majority of the weight of the bridge and any vehicles on it are suspended from the cables. In turn the cables are held up only by the tower(s), there is an incredible amount of weight that the towers must be able to support. As explained in the cable stayed bridge section, steel cables are extremely strong yet flexible. Like a very strong piece of string, it is good for hanging or pulling something, but it is useless for trying to push something. Long span suspension bridges, though strong under normal traffic loads, are vulnerable to the forces of winds. Special measures are taken to assure that the bridge does not vibrate or sway excessively under heavy winds. The most famous example of an aerodynamically unstable bridge is the Taa Narrows Bridge in Washington state, USA. This page on the Taa Narrows Bridge Disaster at the University of Bristol has some excellent photos and short movies showing why aerodynamic stability is important.