Frequently Asked Questions

Hollow Structural Sections

My query is about the design of a 5m long HSS arched member of a canopy with a 1m vertical. Can we design it simply as if it was a straight member, i.e. a beam column? Are there other methods to design this type of vertical curved member? Any references you can point me to? – S.M.
10/1/2004

I thought I would have a quick answer for you. However, to my knowledge, there is nothing specifically on curved members in S16-01, or other Canadian design references. The American Steel Mills' Technical Committee on Structural Shapes is currently sponsoring the writing of a guide entitled "Bending of steel shapes for construction – A guide for architects, engineers and contractors". The guide will focus on straightening, cambering and curving structural steel shapes of common structural grades. It will briefly highlight bending equipment and applications, but get into more depth on curving mechanics and analysis, including practical criteria for shape curving (bending), such as sample limits of curving using basic curving criterion developed by Reidar Bjorhovde. Some answers to questions about the effect of residual stresses due to the bending process can be found in Question 4.4.7 of AISC's FAQ.

In the UK, the Steel Construction Institute has developed, as part of their Specialist Design Guides, a document called "Design of Curved Steel", intended to assist structural design engineers. There are several worked examples covering common applications and structural arrangements in buildings. Technical aspects regarding the curving process and the metallurgical issues are also briefly addressed. The document can be purchased through their web site.

That's for the documentation. As you know, architects specify curved steel more and more for its undoubted aesthetic appeal, which means that engineers ARE designing them. So what do real engineers do in the real world? They apply basic principles and S16-01. A common practice for a first analysis is to divide the curved member into many discrete elements, and then refine. Design considerations will be very different if the curvature is in elevation or in plan. Depending on the axial load, Babak Seyed, from Yolles Engineering in Toronto, put much of his effort into the P-∆ analysis to design the Niagara Casino curved (in elevation) dome members. A grey zone seems to exist for determining which load factor to apply to the P-∆ loads and what effective length K to assume. He used a finite element analysis program. Cintube, who curved the members for the Casino project, said that a radius as low as 3 times the diameter of the tube can be applied without difficulty provided the wall thickness is not too thin.

Andrew Metten, from Bush Bohlman Partners in Vancouver, said that for one of the curved (in plan) peripheral beams at the Vancouver Airport, much of the engineering effort went into taking the torsional forces away from the member and especially the connections, to reduce deflections. For that reason, he generally encourages architects to use a closed section, such as an HSS. A space frame program was judged sufficient, and subsequent tweaking of the discrete elements was applied. Depending on the radius and type of curvature sought, Andrew Metten likes to discuss with a bending specialist, such as Advanced Bending Technology or Marks Metal Technology, early on in the process. 

This question appeared in the "Ask Dr. Sylvie" column of Advantage Steel no. 21, Fall 2004.