Question and Answers about HSS
Welding
Thank you for your interest in steel hollow structural sections (HSS).
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Question #1: Welding of HSS and “Workable Flat”
I need to weld an HSS branch member to a thick HSS chord member, but the Specification limits the workable flat width that is available, so how can I make it work? Can I get HSS with a bigger “workable flat” dimension?
Answer #1: HSS are manufactured in North America by cold-forming and the corner radii are intentionally fairly large to limit the loss of ductility and toughness in the corner regions, promote weldability and to address potential corner cracking. ASTM A500-10 permits an outside corner radius of up to 3tnom (where tnom is the nominal HSS wall thickness) but, on average, the outside corner radius is typically around 2 times the actual wall thickness. In fact, the geometric section properties tabulated for square/rectangular HSS are calculated based on an outside corner radius of 2tdes (where tdes is the design wall thickness, equal to 0.93tnom) [1]. Thus, the corner radii can be relatively large for thick-walled HSS members, but it is inadvisable to try to source cold-formed HSS with very tight corners because of the problems with mechanical properties that this might entail.
For determining the cross-sectional compactness of a square/rectangular HSS, one needs an estimate of the largest flat dimension that may possibly occur (to determine b/t and h/t), and for this the outside corner radii are taken as 1.5tdes [1]. For detailing, one needs an estimate of the smallest flat dimension that may possibly occur, and for this the tabulated dimensions (in Tables 1-11 and 1-12 of the AISC Manual) are calculated using outside corner radii of 2.25tnom [1]. This means that for making a connection to an HSS member one can rely on a flat width = (B – 4.5tnom), where B is the outside width of the HSS. This does limit the space for fillet welding all around a branch member, if the fillet weld leg along the branch longitudinal side is to land on the flat of the HSS “thru member”. However, welded joints can still be made – even up to matched-width HSS connections – by using partial joint penetration (PJP) flare-bevel welds along the branch longitudinal sides (as shown in the illustration below).
Reference:
[1] AISC, 2011. “Steel Construction Manual”, 14th. edition, American Institute of Steel Construction, Chicago, Illinois, pages 1-5 to 1-6.
Square HSS-to-HSS T-connection with equal-width members: fillet welded along the transverse branch face and PJP flare-bevel welded along the longitudinal branch face
Question #2: HSS-to-Wide Flange Truss Connections
Our company is designing a steel truss with wide flange chords and square/rectangular web members. I see here in your book, “Hollow Structural Section Connections and Trusses” that rectangular webs are only permitted in overlapped K-connections. I am curious as to why they are not allowed in gap connections. Is this a hard rule, or is there simply a reduction in capacity if the rectangular webs are used in gap connections? Where would you recommend I get more information on the subject?
Answer #2: As you noted, the book “Hollow Structural Section Connections and Trusses – A Design Guide”, 2nd. edition, by J.A. Packer and J.E. Henderson, Canadian Institute of Steel Construction, 1997, does include connection design provisions for HSS web members welded to wide flange (I-shaped) chord members. In Table 3.4(a), it does indicate that the HSS web members are restricted to aspect ratios of 1.0 for K and N gapped connections, but 2.0 for K and N overlapped connections, when using I-section (wide flange) chord members.
Research on HSS web member-to-I section chord member connections is actually very limited. In the latest international guidelines on this topic [1] – which are now a draft international standard (ISO 14346) – the range of validity for square/rectangular HSS web members to I-section chord members has now be restricted to HSS aspect ratios of 1.0 for both gapped and overlapped K and N connections, to reflect the scope of the supporting research.
References:
[1] IIW, 2012. “Static design procedure for welded hollow section joints – Recommendations”, IIW Doc. XV-1402-12, International Institute of Welding, Paris, France.
Question #3: HSS manufactured by SAW
I am responsible for a design of what my company calls a “lift beam”, which is a piece of equipment involved in lifting and transporting a maximum load of 34 metric tonnes. The main body of the equipment is based on an ASTM A500 square steel tube. Our procurement team is asking me to allow a supplier to substitute such a manufactured HSS with another that is fabricated from two different flat plates. The plates are each bent into a “U-shape” and then welded together. I think that accepting this substitution is possibly unwise and potentially unsafe, so your comments would be appreciated.
Answer #3: Square and rectangular HSS are made in North America by three different manufacturing processes: continuous forming with electric resistance welding (ERW) of the longitudinal seam and converting from round to square/rectangular shapes, the most common method, direct forming (or form-square) with ERW of the longitudinal seam, and by submerged arc welding (SAW) of two longitudinal seams joining two C-shaped plates together. These three production methods are described in a STINA brochure on “Hollow Structural Sections – Dimensions and Section Properties”, available here: http://www.hss-steeltubing.org/brochures.html
This STINA brochure indicates the HSS sizes produced by SAW using two C-shaped plates in the manner that you have described. They are mainly produced by Valmont (a member company of STINA), and their website also describes all their available sizes and product grades: http://www.valmont.com/page.aspx?id=611
These are regularly-accepted HSS and are commonly specified for very big sizes. They are made to the standard ASTM A1065-09 (which is similar to ASTM A500-10), with welding performed to AWS D1.1 and with outside corner radii of at least 3t.
Question #4: We have received some rectangular tubing 3x2x14ga ASTM A500 Grade C which has short lengths of weld (1"-2") where the outer surface of the weld is not flush. There appears to be mis-alignment of the wall. HAZ visibly wanders in these areas. We cut a section and tried unsuccessfully to break the weld. Is there a standard for allowable tolerance of weld defects?
Answer #4: The manufacturing standard, ASTM A500/A500M-10, to which your rectangular HSS was produced, requires that … “The longitudinal butt joint of welded tubing shall be welded across its thickness in such a manner that the structural design strength of the tubing section is assured”. This criterion was arguably achieved, it seems, as you were not successful in breaking the HSS at the weld seam. With regard to permissible variations in dimensions, the A500 standard states that rectangular HSS may not vary from the specified outside dimensions (including convexity and concavity) by more than +/- 0.020 in. (for wall dimensions up to 2.5 in.) or +/- 0.025 in. (for wall dimensions between 2.5 and 3.5 in.) If your mis-alignment causes the measured dimensions to exceed these tolerances then that would be cause for rejection of the material, I believe, and the issue should be taken up with the HSS supplier.
The wall thickness tolerance in ASTM A500, of +/- 10%, only applies to the centers of the flats, for square and rectangular HSS, and this is perhaps not the location of the seam weld in your product.
Question #5: Design of Fillet Welds to HSS
Table J2.5 of the AISC 360-10 Specification, for fillet welds loaded in shear, indicates that two limit states are pertinent – failure of the weld metal and failure of the base metal. Does that mean that weld design in HSS joints is based on checking weld failure through the effective throat, plus base metal shearing along the fusion face?
Answer #5: No. Fillet weld strength is determined on the basis of failure of the weld metal, in shear, along the weld effective throat. The note in Table J2.5 about shear failure on the base metal being governed by Section J4 is meant as a reminder that shear through the thickness of the connected wall may need to be checked. Hence, for a stepped HSS-to-HSS T-connection as shown below, fillet-welded all around, the weld is only proportioned on the basis of failure along the effective throat – indicated by the AISC 360-10 line in the detail below. This requirement is the same in the current Canadian standard for steel structures too (CSA S16-09). Note that the prior Canadian standard (S16-01) did require a check for shear failure along the fusion face. All of the foregoing is based on weld filler metal with a strength level equal to, or less than, “matching filler metal”. If over-matching filler metal is used then it would still be prudent to perform an additional check for shear failure along the fusion face.
Question #6: Stainless Steel HSS
Are HSS tubes available typically in stainless steel? If so, we will have highly stressed corner joints between regular HSS and these (few) stainless steel HSS members. Do you have any suggestions for the welding specifications?
Answer #6: Stainless steel square and rectangular HSS are not typically available in North America. Stainless steel tubes, for example to ASTM A1016, are more commonly available. It is certainly not wise to have mixed welded fabrication between stainless steel and regular carbon steel members.
The requirements for welding stainless steel structural assemblies are given in ANSI/AWS D1.6/D1.6M “Structural Welding Code – Stainless Steel”.
Question #7: Welding of Mitered L- or Knee-Connections
Are different AWS D1.1 prequalified welds required around the perimeter of a 45o miter butt joint used to connect two pieces of HSS at 90 degrees? Or, can the same weld be used around the entire perimeter? In either case, what is the prequalified weld typically used? A partial-joint-penetration groove weld?
Answer #7: In direct answer to your question, edge preparation (beveling) of the HSS would be required along 3 of the 4 edges of the miter joint, in order to successfully accomplish PJP welds along those 3 edges.
The capacity of such welds is limited, and the welds themselves (to matched box sections) need to be made very carefully. For such miter joints, especially if there is reasonable applied loading on the HSS members, it is recommended that the two HSS each be separately welded (usually by fillet welds) to a 45o stiffening plate. This type of “knee connection” is shown and discussed on pages 67 to 69 of CIDECT Design Guide No. 3 – “Design Guide for Rectangular Hollow Section (RHS) Joints under Predominantly Static Loading”, 2nd. edition, 2009.
Question #8: Sequencing of Welds in HSS Truss Connections
I'm a Structural Engineer working for the Dept. of Transportation of the State of California. We
have a number of structures under our review using rectangular and round HSS sections for the construction of trusses. We know that rectangular sections require a definite sequence of weldments for the development of complete penetration bevel welds in a TYK connection. I would suppose that round sections have the same or more detailed sequences. Are there sources of literature that discuss recommended shop standards for the welding of such structures.
Answer #8: The standard for the welding of hollow structural section connections in the US is: “Structural Welding Code – Steel”, AWS D1.1/D1.1M:2010, 22nd. edition, American Welding Society, Miami, Florida (available from www.aws.org ).
Disclaimer
This answer to this question was 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.
