Question and Answers about HSS
Fabrication/Erection
Thank you for your interest in steel hollow structural sections (HSS).
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Question #1: Bending of HSS
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 #1: 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.
Circular HSS (see below) are generally easier to bend. Circular HSS can be cold-rolled to a radius of about six times the outside diameter, depending on thickness. As with square/rectangular HSS, the thicker the wall, the smaller the achievable bend radius (without wall distortion).
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| (a) 3-roll cold-bending apparatus for bending round HSS |
(b) cold-bent round HSS |
Question #2: Flattening of Ends of Round HSS or Pipes
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 criteria, or people to contact, concerning this?
Answer #2: 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 legsand 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 which is available in the Resource Library of this website.
Question # 3: Rehabilitation of Concrete Columns using HSS Encasing
I would like to inquire about the feasibility of using a 12”x12” structural tube to encase an existing concrete column. Is it feasible to use 2 halves of a tubular section and weld in the field? If so, what are the pros and cons?
Answer #3 It is possible to encase a reinforced concrete column with a square hollow structural section (HSS). Presumably your existing concrete column is also square, and somewhat smaller in dimensions. You could not expect the HSS casing to fit snugly onto the existing concrete column because the inside hole of an HSS is not square, due to the rounded corners, which have an inside radius of approximately t (the design thickness of the HSS). Thus, the HSS will need to be “oversize”. The HSS will then have to be cut longitudinally into two C-shaped sections and backing shop-welded along the entire length to the inside edges of two tips of one of the C-shapes. Then the two Cs can be put around the concrete column and tacked together at the exposed backing. Then, field groove welding along the full length of the column, on two sides would take place. Finally, the gap between the HSS and the concrete column should be filled with a non-shrink concrete grout to achieve integral action between the HSS and the original concrete column. To install this grout, pumping from the bottom via a pre-installed access hole in the steel casing would be optimal (as is done for concrete-filled HSS columns – see photo below). If you are looking for eventual behavior as a composite column, bear in mind that there is little confinement pressure exerted on concrete infill within a square (or rectangular) HSS composite column. For a circular HSS composite column, on the other hand, it is possible to achieve considerable concrete confinement action – and thus there has been rehabilitation of circular reinforced concrete members using this steel encasement technique described above.
Disclaimer
These answers to these questions were prepared by Dr. Jeffrey Packer. While it is believed to be accurate, it has not been prepared for conventional use as an engineering or construction document and should not be used or relied upon for any specific application without competent professional examination and verification of its accuracy, suitability, and applicability by a licensed engineer, architect or other professional. Dr. Packer disclaims any liability arising from information provided by others, or from the unauthorized use of the information contained in this document.
