Question: We are trying to find design criteria for using HSS webbing with flattened ends in long-span trusses. We need to flatten the ends to fit between two angles, at the top and bottom chords of the truss. I know that this is done, but I can’t seem to find design criteria for it. I used to use criteria dating from 1991 by CIDECT in their “Design Guide for Circular Hollow Section (CHS) Joints under Predominantly Static Loadings.” Our trusses range from 90’ to 125’ long. We are estimating about 1000 trusses to be built per year. So this will be a lot of material working, and we would like to be as efficient as possible. Do you know of any additional resources related to flattened ends?

Answer: Flattening the ends of round HSS (or pipes) is used in lightly-loaded trusses, and can be used in conjunction with pairs of separated angles for each chord, as you have described. The flattened web member end is inserted between two angle legs and welded to them. Most guidance that is available pertains to the flattening process itself, or the connections when flattened ends are welded to round or square HSS chord members.

The CIDECT Design Guide (No.1) which you have accessed has some discussion on this topic. The most recent (2nd. Edition, 2008) version of this CIDECT Design Guide covers the topic on pages 68 to 70. This information is in accord with the book, “Hollow Structural Section Connections and Trusses” by J.A. Packer and J.E. Henderson, 2nd. Edition, 1997, published by the Canadian Institute of Steel Construction, Toronto, ON (www.cisc-icca.ca).

This latter publication (not available in electronic format) does give broader treatment (pages 148 to 155), but still related to HSS as the chord members. CIDECT Design Guide No.7 (dealing with “Fabrication, Assembly and Erection of Hollow Section Structures”) has some information on the flattening process itself.