What Are Cambers?
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.
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