Experimental and numerical investigations on double-skin CHS tubular X-joints under axial compression

• 18 double-skin, 2 empty and 2 grouted CHS tubular X-joints were tested.
• Buckling of brace occurred for specimens with small hollow ratio of chord.
• Behaviour of specimens with small hollow ratio is close to grouted counterparts.
• Behaviour of specimens with large hollow ratio is close to empty counterparts.
• Design equations were proposed for double-skin CHS tubular X-joints.

This paper presents the experimental and numerical investigations on double-skin circular hollow section (CHS) tubular X-joints under axial compression, in which PVC pipes were used as the internal members. A total of 22 X-joints with different brace to chord diameter ratio (β), hollow ratio of chord (φ) and shapes of internal members was tested, in which two traditional CHS tubular X-joints and two grouted CHS tubular X-joints were tested for comparison. The joint strengths, failure modes, load-deformation curves, load-strain distribution curves and ultimate capacity evaluation of all specimens are reported. The effects of brace to chord diameter ratio (β), hollow ratio of chord (φ) and shapes of internal members on the structural behaviour of double-skin CHS tubular X-joints under axial compression were evaluated. It is shown from the comparison that the ultimate strength, initial stiffness and ductility of double-skin CHS tubular X-joints benefit from the increase of brace to chord diameter ratio (β). Furthermore, the double-skin CHS tubular X-joints with large hollow ratio of chord (φ) show good ductility. On the other hand, the ultimate strengths of double-skin tubular X-joints significantly increased by grouting the chord member. The ultimate strength and initial stiffness of double-skin tubular X-joints with small hollow ratio of chord (φ) are close to their grouted counterparts. However, the ultimate strength and initial stiffness of double-skin CHS tubular X-joints with large hollow ratio of chord (φ) are much smaller than their grouted counterparts. The corresponding finite element analysis was performed and calibrated against the test results. The design equations are proposed based on the test and numerical results for double-skin CHS tubular X-joints, which were verified to be more accurate.