Static behavior of stud shear connectors in high-strength-steel-UHPC composite beams

High-strength structural steel and ultra-high-performance concrete (UHPC) are being researched in many countries to expand their potential for wider application in civil engineering. This study investigated the shear performance of stud connectors in high-strength steel (f(y) = 690 MPa)-UHPC composite beams. A total of 6 pushout specimens, classified into three groups, were tested under static load. The test parameters included the diameter (13 mm/19 mm) and layout of the studs (single-stud/grouped-stud). The grouped-stud specimens were studied for assembly construction in composite beams. Based on the experimental results, empirical formulae for the load-slip curve of single-stud and grouped-stud specimens were proposed. Experimental results indicated that the failure mode of all the high-strength steel-UHPC pushout specimens was stud shank failure. The diameter of the studs significantly affected their shear performance. The shear capacity and shear stiffness of studs with a 19 mm diameter were 82.4% and 46.0% greater than those of studs with a 13 mm diameter, respectively. The layout of the studs had little effect on their shear capacity, with a 2.7% difference in shear capacity between single-stud and grouped-stud specimens on average. However, the stud arrangement had a great effect on the shear stiffness per stud. When the studs were arranged in a group, the shear stiffness per stud was 19.6% lower than the single-stud specimen on average because of the group effect. Although the grouped-stud specimens take advantage of assembly construction, the shear stiffness per stud decreased in the grouped-stud specimens. It was found that high-strength steel has little effect on the shear performance of studs compared with normal steel in pushout specimens. However, UHPC significantly improves the shear capacity and shear stiffness compared with normal concrete, but the ductility become relatively poor because of the higher elastic modulus of UHPC. In this paper, a formula for calculating the shear stiffness of grouped-stud specimens is also proposed based on the energy equivalent model. The main reason for the group effect is thought to be the ineffectiveness of concrete between studs in shear resisting owing to the short distance of studs. Thus, the shear stiffness per stud decreased in the grouped-stud specimens.