Flexural behaviour of concrete-filled CHS X-joints with curved chord under out-of-plane bending

This paper presents experimental and numerical investigations on empty and concrete-filled circular hollow section (CHS) X-joints with curved chord under out-of-plane bending. A total of 16 specimens were fabricated by hot-rolled bending the chord members into curvature with three different radii to apply out-of-plane bending to the brace members, in which 6 specimens were tested with empty curved chord, 6 specimens were tested with concrete filled in the curved chord only, and 4 traditional empty and concrete-filled CHS X-joints with straight chord were also tested for comparison. The typical failure modes, axial load-vertical displacement curves, bending moment-chord deformation curves, bending moment-rotation curves and strain distribution curves of all specimens are reported. The effects of curvature radius of chord member and concrete filled in the chord member on the joint strength and behaviour under out-of-plane bending were evaluated. It is shown from the comparison that the ultimate strengths of the CHS X-joints with curvature radius of chord less than 12d0 under out-of-plane bending are closer to those of the traditional CHS X-joints with straight chord with the increase of the curvature radius of chord member. Whereas, the ultimate strengths of the CHS X-joints with curvature radius of chord less than 12d0 under out-of-plane bending are generally similar to those of the traditional CHS X-joints with straight chord. Furthermore, the out-of-plane flexural behaviour of CHS X-joints are improved with the increase of the β ratio. Whereas, filling the concrete in the chord member cannot enhance the ultimate strengths of CHS X-joints under out-of-plane bending. The joint strengths obtained from the experimental investigation are compared with the design strengths calculated using the current design rules for traditional CHS X-joints with straight chord under out-of-plane bending, which were demonstrated to be quite conservative. In addition, the corresponding finite element analysis was also performed and calibrated against the test results. The design equations are proposed based on the test and numerical results for empty and concrete-filled CHS X-joints with curved chord under out-of-plane bending, which were verified to be more accurate.