Question and Answers about HSS
Connections
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Question #1: HSS Truss Connections with Small Branch Angles
I have a mitered tube steel truss connection (round sections) with a small angle between the chord and the branch, 16 degrees. The chord is a 10 inch tube and the branch is a 6 inch tube. AISC Design Guide 24 limits you to a 30 degree minimum branch angle. Is that one of the limitations of the underlying research, or is there a different reason for the limitation? Since the capacity has the sine of the branch angle in the denominator, I would guess that using a 30 degree angle in lieu of the 16 degree angle should provide a conservative result, is that correct? I meet all of the other limitations of AISC 360-10 Table K2.1A. The angle I’m using is a limitation of the structure’s geometry. It’s a submerged structure, so I’d like to use a mitered joint rather than a knife plate that would leave the interior of the tube open to the water.
Secondly, AWS D1.1 Figure 3.5 (AISC 13th edition manual Table 8-2) limits you to a 30 degree minimum heel angle (upsilon). I understand that I can’t have a pre-qualified joint in the small portion of the connection that is less than 30 degrees, but I’m planning on neglecting that portion of the weld for capacity and only using the weld there to seal the connection. Is there anything else to watch out for in the weld at small acute angles such as this?
Answer #1: The limitation requiring acute branch angles greater than 30 degrees, in AISC Design Guide No. 24 and AISC 360-10 (which is taken from International Institute of Welding recommendations), is primarily due to the problems of producing good welds in the heel positions at very low angles. Smaller angles may be acceptable, although not pre-qualified, if you can confirm with the fabricator that proper structural quality welds can be achieved. The difficulty is greatest for square/rectangular HSS connections, but tends to be less of a problem with round-to-round HSS connections – which you have – because that small acute angle is only at one point (the chord crown position). At other positions around the round branch member this local intersection angle “opens up”. I would agree that a fully-sealed welded joint is preferable to a knife plate connection, in your situation. Thus, in conclusion, you need to liaise with the fabricator to obtain confirmation that a specific effective throat can be obtained in the heel, while welding all around.
Question #2: Round-to-Rectangular HSS Connections
I have a Warren truss with round HSS branches welded to square HSS chord members. This appears to be outside the scope of the AISC 360 Specification, so is there any way that I can check the adequacy of these connections?
Answer #2: Yes. You are correct that AISC 360-10 Chapter K does not cover such connections, which is also confirmed in the Commentary to this chapter, but it also states that for … “round branch members joined to a square or rectangular chord member … other verified design guidance or tests can be used”. Fortunately, some experimentation on such K/N gapped and overlapped connections has been performed (see the test specimen photographs below) and research (Packer et al., 2007) has shown that the round branches can be successfully replaced (for calculation purposes) by square branch members.
The failure modes (limit states) of statically loaded hollow section connections are predominantly related to the branch footprint on the chord (hence branch perimeter) or branch cross-sectional area. Both the perimeter and area of a round and square HSS, having the same width or diameter (i.e. B=D) and being relatively thin, are approximately in the ratio of π:4 = 0.785. It has hence been shown that a “conversion method” can be implemented whereby round branches, of diameter Db, are replaced by equivalent square branches, of width Bb = (π/4)Db and the same thickness, and then design rules for rectangular HSS truss connections (i.e. AISC 360-10 Table K2.2) can be used.
Reference:
Packer, J.A., Mashiri, F.R., Zhao, X.L. and Willibald, S., “Static and Fatigue Design of CHS-to-RHS Welded Connections using a Branch Conversion Method”, Journal of Constructional Steel Research, Vol. 63, No.1, 2007, pp. 82-95.
Question #3: Through-Bolts and HSS
I am looking for some references regarding the use of through bolts in HSS connections. Specifically, guidance on whether or not the through bolts need to be designed for bending.
Answer #3: The main concern with “through-bolts” is that when they are tightened the walls of the HSS are deformed and pulled towards each other (inwards). To get over this problem a tube or pipe is often inserted inside the HSS, and the bolt is passed through this pipe. When the bolt is then pre-tensioned the HSS walls remain in alignment and the pipe is placed in compression. With this arrangement there should not be a design concern regarding bolt bending.
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.
