Efficiency of Beams

Among the different structural form beam is the least efficient. Commonly, only one cross section of a beam is subjected to the maximum design moment, and consequently, if the member is prismatic, only one cross section of the beam is working at the maximum allowable stress at design load. What is worse, at design load only the extreme fiber of that unique cross section is stressed to the maximum; all the other fibers are understressed.

It is acknowledged that this situation is somewhat improved by improved by continuity, by virtue of which two sections may be subjected to the maximum moment. The situation is further improved by the plasticity of concrete and steel, which may permit a redistribution of moments to occur within the member which will more fully utilize the prismatic beam. But it is still impossible to compete for efficiency with the simple compression struts, for example, in which every cross section along the length is stressed to the full allowed value over width and depth.