Good designs minimize total cost.
B1 Bridge Type
The bridge type (such as beam, arch, truss, and others) depends largely on the required dimensions for the bridge and the type of traffic to be carried. The required length and clearances needed by traffic are major considerations in bridge design. Many bridges are long enough to require several intermediate supports, or piers. The location of piers is usually a crucial factor, whether in water or on land.
B2 Materials
Materials historically used for bridge building include rope and other fibers, wood, stone and masonry, iron, steel, and concrete. Fiber, timber, stone, and masonry are still used occasionally, but steel and concrete are the materials used for most modern bridge building. Fiber rope is occasionally used for short pedestrian bridges. Timber is perceived as a rustic material and is sometimes used in public parks, on private property, or in other situations in
which a natural or historic appearance is desirable. The strength and durability of timber are quite limited compared to those of steel and concrete. Therefore, timber is suitable only for short spans that carry minimal traffic loads. Stone and masonry are sometimes used as facing materials on concrete and steel bridges, if appearance is important enough to justify the additional expense.
When deciding between steel and concrete, designers evaluate the tradeoffs among weight, strength, and expense to determine which material is best for a particular bridge. Concrete is heavier than steel, but steel is much stronger. The major advantages of concrete are that it is considerably cheaper than steel and can be formed into a greater variety of shapes. For short bridges, the weight of material is not an important concern, and so concrete is an economical choice. However, as span increases, the weight of the structure grows substantially, and greater strength is needed to support the overall structure. Steel tends to be preferred for large bridges because less material has to be handled and supported during the construction process.
The distinction between steel and concrete is not absolute, as most steel bridges have concrete decks, and all concrete is reinforced with steel to provide greater tensile strength (resistance to pulling). Reinforced concrete is made by pouring concrete mix over steel bars or mesh. The concrete and metal bond as the mix hardens, producing a material in which the high tensile strength of steel is combined with the great compressive strength (ability to resist pushing or squeezing) of concrete. An alternative method of reinforcing concrete is prestressing.