The use of cantilever construction methods saves material and therefore expense, but it is very complex, as great care must be taken not to unbalance the structure during construction. Most arch bridges, and of course cantilever bridges, are built using cantilever methods.
The large towers and cable anchorages of suspension bridges are built without the use of falsework, and then the suspension cables are spun. Many individual wires are draped over the towers and are then squeezed together into a circular shape and clamped at intervals to create a main cable. Suspension wires are dropped from the cables to support the roadway, and the roadway is completed.
For all bridge types, underwater foundations require unique construction methods. Builders use cofferdams and caissons to obtain access to ground that is normally under water. A cofferdam is a temporary watertight enclosure constructed on the spot where a pier is to be built. A cofferdam usually consists of sheets of steel driven into the ground to create a walled chamber. The cofferdam is then pumped dry to expose the riverbed. Soil can be excavated to bedrock, or piles can be driven to create the pier foundation. The cofferdam is removed after the foundation and pier are constructed. A caisson is a large cylinder or box chamber that is sunk into the riverbed. The excavation and foundation work takes place within the submerged caisson. Some caissons are removed after construction, while others are left in place, filled with concrete, and used as part of a permanent foundation.
C Safety
In bridge design, engineers strive to plan an economical structure that will safely transmit loads to the ground without collapsing or deforming excessively. Since it is difficult to predict the exact loading and circumstances that a bridge must withstand, all bridge designs include a substantial margin of safety. Design standards vary throughout the world, but all aim at ensuring that new bridges will provide many years of service and will maintain an adequate margin of safety against failure. Of course, the safety of a structure when it is first erected does not ensure that it will remain safe for all time. All structures require both periodic inspection and proper maintenance to keep them safe.