Study on shear behavior of inclined cross lag screws for glulam-concrete composite beams

Abstract

The mechanical performance of shear connectors has an important effect on the load bearing capacity and the bending stiffness of timer-concrete composite (TCC) beams. Most studies reported so far have focused on the shear behavior of the single inclined screw working only under shear–tension stress. However, there are few researches on the shear behavior of inclined cross screws. In this paper, a total of thirty-six push-out tests were conducted to investigate the shear behavior of the inclined cross lag screws for glulam-concrete composite beams. Based on the push-out test results, the influences of concrete strength, screw diameter, penetration length of screw into timber, screw inclination angle and screw arrangement with respect to the shear plane on the shear bearing capacity and slip modulus were investigated in detail. Two types of failure mechanisms of inclined cross lag screws were observed from the push-out tests: cone expulsion of concrete ahead of the shear-compression loaded screw and the shear-tensile loaded screw withdrawn from timber. When the inclined screws worked only under shear-compression load, the screw connectors were in the worst stress state. Since the direction of interface shear force in timber-concrete composite beams changed constantly under seismic load, the optimal screw arrangement was the crossed screws working simultaneously under shear-compression and shear–tension load. In addition, the analytical models were proposed to predict the shear bearing capacity and slip modulus of the inclined cross screws, and the calculated results were in good agreement with the test results.