Page 45 - Structural Steel Designers Handbook AISC, AASHTO, AISI, ASTM, and ASCE-07 Design Standards

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FABRICATION AND ERECTION*

FABRICATION AND ERECTION 2.7

reaches a place where the remaining material fractures. A punched hole is slightly conical in shape,
with dimensions of the punch on one side and of the die on the other.
On an individual basis, drilling is much slower than punching but leaves a smooth surface and is
not limited by thickness. Another advantage of drilling is that it can be done on multiple thicknesses
of material. This is done when there are many pieces with the same hole pattern, or to assure uni-
formity of the hole pattern between mating surfaces. In high-production environments, both drills
and punches exist that form many holes at once. Automated lines feed shapes and drill or punch holes
in both flanges and the web at the same time. Punches can be set up to form many holes at once when
there are many pieces with the same pattern. Thermal cutting of holes was not permitted for many
years but is now allowed when the hole is formed accurately and with an appropriate surface quality.
Holes in base plates for anchor rods are commonly cut thermally because base plates are thick and
holes are larger than those for bolts.
Camber is a curvature of a piece in its strong direction or the direction in which the primary load
is applied. Sweep is a curvature in the weak direction. The term camber refers to the curvature of a
piece as it is delivered, or to the curvature induced to compensate for deflection under applied loads.
Hot-rolled shapes are air cooled without physical restraint. When a hot-rolled shape cools, it bends in
both the strong and weak direction, twists, and distorts locally. ASTM A6 limits the amount of sweep
and camber acceptable in a rolled shape, and producers straighten the product to meet those tolerances.
Straightening is done with a rotary straightener or in a press. The delivered product usually has some
camber and sweep within the A6 tolerances. When no camber is specified in rolled shapes fabricated
into beams, the shape is fitted in fabrication so that the natural camber is up in the piece as erected. In
long-span floors typical of commercial buildings, camber is a design parameter and is intentionally
induced in the beam. Camber is designed as a percentage of the dead-load deflection or a percentage of
the dead- and live-load deflection of the beam. Natural mill camber is acceptable if the design camber
3
is / 4 in or less. The decision to camber and how much to camber is made by the designer.
Where camber is required, the fabrication shop cambers the shape after it is cut and punched or
drilled, and before detail material is attached. Camber can be achieved with local application of heat,
and this is typically done for heavy sections. For most beams, however, camber is induced by cold
bending with hydraulic jacks in a cambering machine. Cold camber is achieved by inducing plastic
tensile and compressive strain of elements of the shape. The curvature should be limited so that the
induced strain is limited to a reasonable percentage, based on the minimum elongation requirements
of the material specification. Also, the strain should be induced uniformly over a length of the section
using multiple jacks.
In girders, webs may be cut thermally with a curve calculated to achieve the required camber
when the flanges have been welded to the web. Variations do occur, and camber adjustments are
made using local application of heat after the piece is assembled. Large bridge and roof trusses are
cambered by detailing and fabricating the elements to calculated lengths such that the desired cam-
ber is achieved when the trusses are assembled. In other words, each member is fabricated to its geo-
metric length in the cambered position.
Heat is used to induce camber or sweep in some cases and to adjust camber or straighten pieces
in other cases. 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 a torch. As the part is heated, it tries to elongate. 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 upon cooling. When the part is allowed
to cool, therefore, it must shorten to return to its original volume. The heated flange thus 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 intu-
itive, demanding experience on the part of the operator.
Research has shown that the residual stresses remaining in a beam after cambering are little dif-
ferent from those due to differential cooling rates of the elements of the shape after it has been pro-
duced by hot rolling. Strength limit states in design specifications include the effect of residual
stresses where relevant.

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