Validation of a CHS connection used in X-bracing diagonals

Abstract

The present paper investigates the behaviour of a connection widely used at the co-planar intersection of circular hollow sections (CHS), a case very common in X-bracings. The performance of this type of connection is validated by means of finite element modelling. Our connection is made up of two separate gusset plates, welded externally along the generatrices of the cylindrical surface of a CHS (in an ear-like manner), so as to accommodate the terminal gusset plates of the interrupted CHS: in this way the two bars intersect and remain on the same plane. Shell elements are used to model the CHS bar connection along several diameters on either side of the gusset plates (ears). The stiffness of the cylindrical shell that is called upon to undertake the axial force of the interrupted bar, drew our attention as it affects the structural behaviour of the X-brace well before local strength phenomena assume any importance. The actual result of this paper is that the stiffness of the interrupted length of the CHS bar is significantly less than that of the continuous bar: the interrupted bar of the X-bracing cannot develop any stresses whenever this type of connection is used, as any strain applied to its end will be cumulated in the connection zone. This effectively causes our X-brace to function with only one of its diagonals, i.e. the continuous one. Ending-up with a single-diagonal bracing can be dangerous even in the absence of material non-linearities or large displacement effects, not to mention an earthquake situation. These results are also compared with a formula derived by plane stress considerations and are found in good agreement. A total of 117 variants of this connection are examined for the purpose of this analysis and the stiffness reduction results are presented in a schematic form.