New bridges are built either to replace old structures that no longer meet the demands of modern traffic or to cross obstacles on a new transportation route. Old bridges are replaced when repairs cannot be made economically or when traffic becomes too heavy for the old bridge. New transportation routes are built when traffic levels have outgrown the capacity of existing routes or simply to make it faster to get from one busy place to another. Often, new transportation routes are part of government programs to promote regional economic development.

In the United States, state and local transportation agencies determine where new bridges are needed and pay a small portion of the cost. The federal government usually pays for most of the construction expense, using money generated from taxes. Bridges funded by tax dollars are used free of charge. The few bridges for which a toll is charged to drivers for use are funded through the sale of bonds to raise money for construction. The money collected from the toll is used to pay back the bonds. The use of tolls and borrowing to finance bridge construction was more widespread in the past than it is today.

Engineers must consider several factors when designing a bridge. They consider the distance to be crossed and the feature, such as a river, bay, or canyon, to be crossed. Engineers must anticipate the type of traffic and the amount of load the bridge will have to carry and the minimum span and height required for traffic traveling across and under the bridge. Temperature, environmental conditions, and the physical nature of the building site (such as the geometry of the approaches, the strength of the ground, and the depth to firm bedrock) also determine the best bridge design for a particular situation.

Once engineers have the data they need in order to design a bridge, they create a work plan for constructing it. Factors to be considered include availability of materials, equipment, and trained labor; availability of workshop facilities; and local transportation to the site. These factors, in combination with the funding and time available for bridge design and construction, are the major requirements and constraints on design decisions for a particular site.

There are four basic categories of design decisions: the type of bridge, the materials of which it will be made, the type of foundations that will support the structure, and the construction method to be used. Typically, several feasible choices exist in each category, and each option is evaluated in terms of convenience, appearance, endurance, and cost. Bridges must be convenient to build, use, and maintain. Appearance is important in gaining public approval, which is particularly critical for taxpayer-funded projects. Bridges must be designed to endure, as most structures can be expected to provide service for at least 50 to 100 years. Durability of materials and maintenance requirements are important considerations, as the true cost of a bridge is not simply the initial construction expense but the total cost of constructing and maintaining the structure throughout its service life. Good designs minimize total cost.

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.

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. Prestressed concrete is made by pouring concrete over stretched and anchored steel strands. After the concrete has set, the anchors are released. As the steel tries to return to its original length, it compresses the concrete, resulting in a relatively lightweight, extremely strong material.

All bridge piers rest on foundations that transfer loads from the bridge structure into the ground. The foundations support the bridge, and their design is critical. Difficult conditions, such as deep water or soft ground, can make foundation construction complicated and expensive. In such circumstances designers may choose to decrease the number of piers by increasing span length. Of course, greater span lengths often require a more expensive bridge type, and therefore the tradeoffs must be evaluated carefully.

If the ground is very strong at a bridge site, a foundation is formed by pouring a simple concrete mat beneath each of the piers. If the soil is weak, it may be excavated down to bedrock, and the piers can then be built directly on the solid rock. Alternatively, a group of vertical posts, or piles, can be driven through the soil to bedrock, and piers can be built on top of the piles.

Bridges are erected using a variety of construction methods. Some techniques are associated with a particular bridge type, and care must be taken not to select a design that requires construction methods unsuitable for the site. Concrete and steel bridges are generally built using similar techniques, although concrete bridges are built in shorter sections than are steel bridges because of the greater weight of the material.

One of the simplest construction methods for bridges is to assemble a span away from the bridge site and then transport it to the site. The span can then be lifted into position as one piece. This method is most often suitable for small truss bridges or for the suspended span of a cantilever truss. Another approach is to use false work, or temporary scaffolding, to support the incomplete parts of a bridge before they are joined and able to support themselves. The use of false work is not always possible, owing to strong river currents, interference with river traffic, or great distances to the ground. If false work is impractical, bridges can be constructed by the cantilever method. With this technique, a bridge is built piece by piece, with the entire structure supported from the section previously completed. Thus, the structure is self-supported throughout the entire construction process. 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 false work, 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.

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. Notable bridge failures include the collapse of the Firth of Tay Bridge in Scotland in 1879, the collapse of the Québec Bridge in Canada while under construction in 1907, and the collapse of the Tacoma Narrows Bridge, nicknamed Galloping Gertie, in Washington State in 1940.