When a beam is bent, the forces at any point tend to pull the fibers apart in the upper part and push them together in the lower part, while the portion between the two is subject to less stress. The nearer the center the force acts, the less becomes the stress, until finally the beam or neutral axis is reached. At this point the bending stress is zero. Accordingly structural steel beams are made with flanges (reinforcements) at the top and bottom to take care of the bending stresses. These flanges are connected by a plate called a web. The material of the web is subject to a shearing stress - the maximum of which occurs at the support and the minimum where the bending is greatest.

Wood offers the greatest resistance when placed in an upright position. A short post is stronger than a long one of the same section, since the stress in the short post is due merely to compression, while in the long post there is apt to be bending. By applying a stay or projection to the part about to bend, firmness may be given to the support.

A fluted column offers a greater resistance to a bending force than a smooth one; therefore it is stronger. When a beam is supported at both ends, it is twice as strong as one-half its length supported only at one end. Of two beams with the same cross-section area, the longer beam is the weaker.