Experimental analysis of hysteretic behavior and strain field of external diaphragm joints between steel beams and CFDST columns

External diaphragm joints with high structural reliability are used widely in composite steel and concrete structures. In this study, the external diaphragm joint was improved to be a steel beam-to-concrete filled double steel tubular column connection. The digital speckle correlation method was used to measure and investigate the strain field in the panel zone and relative beam-to-column rotation in a low-cycle reciprocating loading test. The obtained hysteresis curves of moment-rotation ( M-θ) and shear force-deformation ( V-[Formula: see text]) showed that the external diaphragm joints had higher strength, higher ductility, and better energy dissipation capacity. Decreasing the axial compression ratio resulted in the deterioration of initial rotational stiffness. Wider external diaphragm produced better ductility and larger initial shear stiffness. The ribbed anchorage web was effective to increase the bending resistance by 10%. Beam-to-column bending stiffness ratio can not only influence the bending resistance and energy dissipation capacity significantly but also affect the shear deformation capacity in the joint core. The magnitude of the shear strain in the panel zone was large, especially for the specimens under column failure mode, and shear deformation in the panel zone should not be neglected for it accounted for 30%–40% of the beam-to-column rotation. Beam-to-column rotation and shear deformation obtained by the digital speckle correlation method offered better predictions to analyze the mechanical behavior of external diaphragm joints in the concrete filled double steel tubular structures.