Weight distribution in structural members subject to bending stresses, like bridges, is very important. A structure designed to resist bending will respond differently to a 100,000 lb load applied at one point than that same weight distributed evenly over the span of the structure. The location of the applied load with respect to the supports determines the bending stress in the structure at that point. Consider the example of a 2x4 stud 8' long supported at both ends. Stack bricks flat across the board until it bends just short of breaking. Then take the bricks off and stack them in the center of the board. The board will break before all the bricks are stacked. A bridge has to be built to handle dead load (the weight of the bridge itself), service load (the weight of the maximum expected number of heavy vehicles), and other loads such as wind and seismic motion. Each type of load has a different distribution of stress, and a different safety factor.

M. Harris
CE License #27924 CA