Bend it Like Gehry
Curved steel hollow sections are widely used in the construction industry, often with very appealing aesthetic results.
Millenium Park, Chicago – Pritzker Pavilion Trellis, by Frank Gehry
The possible curvature of such members is limited by the magnitude of distortion of the cross-section allowed during the bending process. A 1998 study of this topic deduced minimum bending radii for cold bending HSS, based on a series of experiments using three-roller machines and deformation limits of 1% and 2% of the wall dimensions. These recommendations, for square and rectangular HSS, are given in the 1997 CISC book by Packer and Henderson. By using more specialized techniques, modern benders (typically a specialty subcontractor of the fabricator) can achieve even smaller radii than these. A list of AISC member bender-rollers across North America is obtainable from www.aisc.org/benders.
AISC cites a number of available bending methods, listed below. Some are common to the steel construction industry, but some are more common to the automobile or manufacturing industries.
Pyramid Rolling or Cold Bending
Cold bending using “pyramid rollers” is a very common curving technique used to create HSS members with a constant radius. The method involves passing the member between three rollers, oriented in a “pyramid shape”, varying the distance between the rolls with successive passes.
Three-roll bending apparatus for curving round HSS
- Gag Pressing or Point Bending
- Hot Bending
- Rotary-Draw Bending
- Induction Bending
- Synchronized Incremental Cold Bending
Based on the websites of various benders, the most commonly used methods in industry at present appear to be pyramid rolling, gag pressing, rotary-draw bending and induction bending.
Minimum Bending Radii
- The minimum bending radius achievable for HSS depends on many factors including:
- The bending method – note that cold bending methods are the most common.
- Whether the side walls are properly supported during bending (by using rolls that fit the section shape); i.e. the equipment used is influential, as is the operator.
- The shape of the HSS being curved – round, square, or rectangular (and the ratio of wall heights for the latter). Pipe or round HSS is easier to curve than square/rectangular HSS.
- The wall thickness, or the slenderness D/t, H/t or B/t. A lower bend radius is achievable with stockier (thicker) walls, without wall distortions.
- The bending axis, if rectangular HSS (“strong” versus “weak” axis bending). Bending about the strong axis (“the hard way”) requires more force but makes a rectangular HSS less susceptible to deformation problems, relative to bending about the weak axis (“the easy way”).
- The yield strength of the steel.
- The allowable distortions of the HSS walls, and any permitted surface scratching, etc. This may be particularly important for Architecturally Exposed Structural Steel (AESS).
Examples of bent rectangular HSS (Albina Pipe Bending Co.)
(a) Bent “the easy way” (b) Bent “the hard way”
Since benders tend to consider their process both a “science” and an “art”, as well as “intellectual property”, published data for HSS bending radii are usually only given as a guide. Thus, it is very important to contact steel benders at an early stage of the project, to both determine their capabilities and the associated costs.