Camber is a curvature built into a member or structure so that when it is loaded, it deflects to a desired shape. Camber, when required, might be for dead load only, dead load and partial live load, or dead load and full live load. The decision to camber and how much to camber is one made by the designer.
Rolled beams are generally cambered cold in a machine designed for the purpose, in a large press, known as a bulldozer or gag press, through the use of heat, or a combination of mechanically applied stress and heat. In a cambering machine, the beam is run through a multiple set of hydraulically controlled rollers and the curvature is induced in a continuous operation. In a gag press, the beam is inched along and given an incremental bend at many points.
There are a variety of specific techniques used to heat-camber beams but in all of them, the side to be shortened is heated with an oxygen-fed torch. As the part is heated, it tries to elongate. But because it is restrained by unheated material, the heated part with reduced yield stress is forced to upset (increase inelastically in thickness) to relieve its compressive stress. Since the increase in thickness is inelastic, the part will not return to its original thickness on cooling. When the part is allowed to cool, therefore, it must shorten to return to its original volume. The heated flange therefore experiences a net shortening that produces the camber. Heat cambering is generally slow and expensive and is typically used in sections larger than the capacity of available equipment. Heat can also be used to straighten or eliminate warping from parts. Some of these procedures are quite complex and intuitive, demanding experience on the part of the operator.
Experience has shown that the residual stresses remaining in a beam after cambering are little different from those due to differential cooling rates of the elements of the shape after it has been produced by hot rolling. Note that allowable design stresses are based to some extent on the fact that residual stresses virtually always exist.
Plate girders usually are cambered by cutting the web plate to the cambered shape before the flanges are attached.
Large bridge and roof trusses are cambered by fabricating the members to lengths that will yield the desired camber when the trusses are assembled. For example, each compression member is fabricated to its geometric (loaded) length plus the calculated axial deformation under load. Similarly, each tension member is fabricated to its geometric length minus the axial deformation.