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.
