Why are the longest spans all in suspension bridges?
The longest component of a suspension bridge is the set of main cables, which are all in tension, a stable state. In an arch, the main members are all in compression, which means that they are liable to buckling. Hence the thickness of the members and the large amount of bracing that you see in structures such as Sydney Harbour bridge. Suspension bridges do have to include anchorages and towers, which are massive constructions, but they are relatively simple compared with the trussed arch. Worse still are beams, which suffer both tension and compression, unlike the suspension bridge and the arch, which have mainly the one type of stress. In a sense, a beam is like a very shallow tied arch, or a very shallow self-anchored suspension bridge. The cantilever bridge can just about compete with the arch, but the Quebec bridge remains the longest span attempted. With road traffic dominating, the cable-stayed bridge has taken over much of what cantilever bridges used to do. Click here for a mo
