Page 46 - Modern Steel Construction (April 2019)

by bending two straight members and splicing them together at Curved steel forms the rotunda of the Transportation Museum in
the tangent point. Bellefontaine, Ohio.

Bending Processes
So how, exactly, is steel bent? There are multiple ways to curve and another may be able to bend hollow shapes to a tight radius
steel, which are listed in Section 2.3 of Design Guide 33. These with minimal distortion. It is advantageous to involve a bender-
include pyramid roll bending, incremental step bending, induction roller early in the design process to provide information on the
bending, rotary draw bending and other processes. Some methods results that can be expected from each bending process.
are more common in the steel construction industry, while others When a member is bent to form a permanent curvature, it must
are used more in the forming of parts for automobile, piping and be strained beyond its yield point, inevitably causing some level of
other industries. Members of almost any shape can be curved by cross-sectional distortion. The tendency for cross-sectional distor-
bending, including rolled open shapes, welded built-up shapes and tion during the bending process can be controlled using various
closed shapes. techniques. HSS wall distortion is often limited by using an inter-
Cold bending, where the member is bent at room temperature, nal support mechanism such as a mandrel, a smaller HSS member
is usually more economical than hot bending or induction bend- or filling the member with a supporting material like sand. Many
ing; however, there are cases where the required geometry cannot bending machines control the distortion of open shapes with spe-
be formed by cold bending. The primary advantage of hot bending cialized rolls and various forms of mechanical restraint. Bender-
is that the material yield strength is lower compared to the room roller companies may have hundreds of specialized rolls, mandrels,
temperature value, requiring smaller forces to be exerted by the die sets and other tooling to bend various HSS and open shapes
bending machine. with minimal distortion.
Each bending method has advantages and disadvantages, and Pyramid roll bending. Pyramid roll bending is a cold-bending
each bender-roller company has developed unique bending meth- method where a member is bent progressively by repeatedly
ods and often use one-of-a-kind, patented machines. Developing passing it through a set of three adjustable rolls in a pyramid
proper bending techniques that ensure accurate and consistent arrangement. Force is applied by opposing rolls, and the distance
dimensions requires significant judgment and experience. Due to between rolls is manipulated before each pass, bending the member
differences in equipment, technique and personnel, the capabilities into successively smaller radii. This process is repeated until the
of each bender-roller vary significantly. proper curvature is formed. Pyramid roll bending can be used
For example, one shop may have the capability to bend heavy to provide curvature up to a 360° angle. When curving I-shaped
members the hard way, another shop may specialize in spiral bends sections, the tension and compression flanges tend to bend locally

46 | APRIL 2019