Page 48 - Modern Steel Construction (April 2019)

above: Pyramid roll bending a steel channel.
right: Curved steel tops a concourse at Norman Y. Mineta San Jose
International Airport.
below: Heat induction bending.

(b) Channel

Sherman Takata
toward one another. These flange forces induce web compression with the leading end following the arc set by the pivot arm. After
stresses, potentially causing web buckling distortion. This can be passing through the coil, the material adjacent to the heated sec-
controlled with supplementary rolls providing a restraining tension tion is usually sprayed with a coolant (usually water) or cooled with
force on the inner surface of the tension flange on both sides of forced air, or the member is sometimes allowed to cool slowly in
the web. In some cases, the web is restrained against buckling by still air.
compression rollers on each side of the web. To provide support Although induction bending usually costs more than cold bend-
during the bending operation, the rolls are contoured to match the ing, there are several advantages that can make it the most appro-
cross-sectional shape of the workpiece. Contoured rolls can also priate bending method for some structural members. Because the
be used for other cross-sectional profiles, and special rolls can be inelastic bending strains are confined to the narrow heat band,
used to stabilize the cross-sectional elements and reduce distortion small-radius bends are possible with high dimensional accuracy and
in common rolled shapes. low cross-sectional distortion. Also, heavy shapes that exceed the
Incremental step bending. Incremental step bending is a capacity of cold bending machines can often be bent with induc-
cold-bending method that uses hydraulic rams to apply bend- tion bending machines. Hollow shapes with wall thicknesses up to 6
ing forces at several discrete, closely-spaced locations along the in. have been successfully bent with induction bending equipment;
member. Cross-sectional elements can be supported mechani- shapes with 12-in.-thick to 2-in.-thick walls are commonly bent.
cally or hydraulically to reduce distortion during the bending Induction bending may provide a viable method when the
operation, resulting in the potential for small-radius bends with dimensional requirements cannot be met with cold bending. For
minimal distortion. example, induction bending may be the only method with the capa-
Induction bending. Induction bending is a hot-bending bility to bend a multi-sided hollow shape to a small R/D ratio with
method that uses an electric induction coil to heat a narrow limited cross-sectional distortion. Because bending special shapes
band—typically between 2 in. and 6 in.—around the member cir- requires a significant investment in tooling, duplicate member
cumference to between 1,500 °F and 1,950 °F before it is curved quantities are required to make this method economically feasible.
Fig. 2-12. Pyramid roll bending (courtesy of AISC Bender/Roller Committee). Rotary draw bending. Rotary draw bending is a cold bending
by force. Equal wall thickness around the perimeter of the cross
section is necessary for uniform heating throughout the section. method where the member is clamped to a rotating bend die and
As the member moves through an induction heating coil, it can drawn around the bend die. The tailing tangent is held against the
be bent incrementally (similar to incremental step bending) but is bend die by a pressure die, and the bend die rotates until the desired
usually rotated around a fixed-radius pivot arm. When a pivot-arm geometry is formed. A mandrel is often placed inside the member
is used, a hydraulic ram pushes the straight section of the member to restrain cross-sectional distortion during the bending process.
through the coil at a constant rate (typically 1 in. to 2 in./min), Bends can also be formed with special draw-bending equipment

48 | APRIL 2019