Question: 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: Yes. You are correct that AISC 360 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 D_b, are replaced by equivalent square branches, of width B_b = (π/4)D_b 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.