Experimental Study on the Shear Performance of Headed Stud Connectors Under Axial Pressure in Steel–Concrete Composite Structures

Headed stud connectors are critical components for ensuring composite action in steel–concrete composite structures. In practical infrastructure applications, welded headed studs may be subjected not only to interface shear forces but also to additional compressive actions induced by structural restraint, self-weight, traffic effects, and other service conditions. However, the influence of axial pressure applied along the stud height direction on the post-fatigue performance of stud-connected interfaces remains insufficiently understood. This study experimentally investigates the post-fatigue shear performance of welded headed stud connections subjected to different levels of axial pressure along the stud height direction. Nine push-out specimens were divided into three groups with axial pressures of 0, 20, and 40 kN applied to each specimen. A fatigue–static loading procedure was adopted, including initial static loading, intermittent static tests after every 400,000 fatigue cycles, and final static failure tests after two million fatigue cycles under a fatigue load range of 70–130 kN. The failure mode, load–slip response, total shear-transfer capacity, shear stiffness, and slip capacity were analyzed. The results show that all specimens maintained load-carrying capacity after two million fatigue cycles under the investigated loading conditions. The final failures were mainly characterized by shear failure at the root of the welded headed studs accompanied by local concrete crushing. Axial pressure improved the total shear-transfer capacity and shear stiffness of the stud-connected interfaces, while the ultimate slip capacity remained relatively stable. The enhancement is considered to be associated with improved interface interaction and local restraint effects, although these mechanisms cannot be independently quantified using the current test setup. These findings provide experimental evidence for evaluating welded headed stud connections subjected to combined axial pressure and fatigue loading within the investigated parameter range.