Static and fatigue design of CHS-to-RHS welded connections using a branch conversion method

This paper evaluates the hypothesis that welded hollow section connections, which have circular hollow section (CHS) branch (or bracing or web) members welded to a rectangular hollow section (RHS) chord member, may be converted into "equivalent" connections whereby the circular branch member is replaced by a square branch member. Although this notion has existed for the static design of such connections, its implementation into design recommendations or specifications has never been checked against existing test data. This is performed herein, using a database of gapped and overlapped CHS-to-RHS N-connection tests undertaken by others, and the suitability of this procedure is examined for both existing and proposed new static design recommendations for RHS welded connections. For fatigue design, no design guidance exists for CHS-to-RHS welded connections. Thus, a database of welded CHS-to-RHS T-connection fatigue tests, undertaken by both the authors and others, under branch axial loading or branch in-plane bending, is used to evaluate the replacement of the circular branch with an equivalent square branch, in the context of contemporary fatigue design procedures. It is shown that, for the connection types examined, this substitution of a circular branch member by an equivalent square branch member is a valid operation, with an adequate level of safety, for both the static and fatigue design of such connections. This indicates that existing design rules for planar RHS-to-RHS welded connections can likely suffice for the unusual case of CHS-to-RHS welded connections.