Structural engineers use load combinations to account for the fact that structures often experience several different types of at the same time. For example, a bridge must simultaneously carry its own weight, plus the weight of the traffic and pedestrians crossing it. But the bridge might also have to carry the loads caused by high winds, snow, ice, or even an earthquake. It is highly unlikely that all of these extreme loads could occur at exactly the same time. For this reason, most structural design codes specify a number of different load combinations, each of which corresponds to a particular extreme event--a really heavy truck loading, or a really strong earthquake, for example. For each load combination, the extreme loading is combined with average levels of other loads that might be present at the same time the extreme event occurs.
The 1994 Bridge Design Specification requires bridge engineers to check eleven different load combinations for every bridge they design. The West Point Bridge Designer 2010 uses only one of these eleven. It is:
Total Load = 1.25Ws + 1.5Ww + 1.75T (1+ DLA)
where:
Ws = weight of the structure, including the and all structural components
Ww = weight of the
T = weight of the
DLA =
The numbers 1.25, 1.5, and 1.75 are .
The fact that the West Point Bridge Designer 2010 considers only one of eleven different code-specified load combinations is one important reason why this software is for educational use only. For other reasons, see What is Not Realistic about WPBD.