As strong as suspension bridges are, they have their limits. These limits are defined in part by cost. A bridge that is resistant to greater force is almost invariably more costly to build than a weaker bridge. Because of this, structural engineers try to base their designs on the forces a bridge will realistically need to be able to withstand.

Engineers call the internal and external forces a bridge experiences loads. One of these loads is the bridge's own weight, or the dead load. Another is the live load, the weight of the people and vehicles traveling over it. In addition, a bridge must also resist many types of environmental conditions, including wind, temperature fluctuations, precipitation, seismic events, shifting soil, and so on. These are known as dynamic loads. All of these loads affect a bridge and cause its various parts to be squeezed, stretched, bent, and/or twisted.

Structural engineers consider all of these load types when designing bridges. Their challenge is to design a reasonably strong, stable, and safe bridge without wasting material and money. In the process of working toward a solution that meets these goals, the engineer may first try to predict the worst-case scenario in terms of load. It might be important for an engineer to consider, for example, the possibility of 100 or more cars backed up in a traffic jam on the bridge. The bridge, as a whole, should be strong enough to endure the weight of 100 cars.

However, it is also important to recognize that these cars, even in the worst holiday traffic, would be evenly distributed along the bridge deck and not stacked on top of one another. It would therefore be unnecessary and wasteful to construct individual columns and beams strong enough to withstand the weight of all 100 cars in the same spot, as this is an unrealistic scenario. The structural engineer's job is to weigh the cost of safety features against all of the realistic risks -- and to create a safe design that is cost-efficient.