At one time, the only test of a bridge design was actual use. Engineers could incorporate all the scientific knowledge and technological craftsmanship they had to produce a sound design. But how well the bridge would stand up under actual use and dynamic loads could only be discovered in the real world.
Today, engineers have two powerful tools with which to test their ideas: wind tunnels and computeraided design (CAD). Wind tunnels have long been used by aeronautical engineers for the testing of aircraft designs. Now they are routinely used also for the testing of bridge designs. A wind tunnel is an enclosed space in which rapidly moving air from giant fans is made to pass over the model of a bridge. Possible structural and design problems can be detected by photographing and studying patterns of air movement over the model.
As in so many other fields, bridge design has benefitted greatly from the growth and development of computer programs. Such programs can incorporate huge amounts of information about various ways in which bridges and the materials of which they are made will react to various kinds of stresses. CAD can be used to actually draw and test bridge designs on screen without even making the models needed for wind tunnel testing.