Question: I am using ASTM A500 tubing in a design and I would like to know if you have any standards for bending it. In particular, what are the minimum bend radii and where should I dimension it to – the centerline or the inside radius? If you do not have any standards, can you direct me to a place that does?

Answer: HSS can be bent or rolled into curved lengths using either cold or hot processes. Cold working is relatively inexpensive and readily available, but smaller bend radii are obtainable with hot bending techniques. Kennedy [1] performed experimental research on cold bending of HSS and found that, at the (tight) bending limit, two distinct forms of HSS deformation were possible: (i) inward bowing of the compression face of square/rectangular HSS, and (ii) outward bulging of a side face of square/rectangular HSS. Relatively thin-walled HSS exhibit early compression face bowing, while thicker-walled HSS tend towards earlier side wall bulging. Minimum cold-bending radii that limit the amount of wall distortion (to 1% or 2%) are given in Section 8.1.3.1 of Packer and Henderson [2], for both square and rectangular HSS with the latter bent about both axes. Such recommendations are still tentative, however, as bending results do show some dependence upon the rolling machines used, the actions of the operator, and the level of HSS wall support during bending. Since the HSS has been cold-worked during (cold) bending, caution should be exercised upon galvanizing unless the material has been stress-relieved.

[1] Kennedy, J.B. 1988. “Minimum Bending Radii for Square & Rectangular Hollow Sections (3-Roller Cold-Bending)”, CIDECT Report 11C-88/14-E, University of Windsor, Windsor, Canada.

[2] Packer, J.A. and Henderson, J.E. 1977. “Hollow Structural Section Connection and Trusses – A Design Guide”, 2nd Edition, Canadian Institute of Steel Construction, Toronto, Canada.