Horizontal shear strength of indirectly loaded composite concrete beams

Four composite concrete beams with cylinder compressive strength f'_c of approximately 40 MPa were tested to failure under indirect two-point loading. The first beam, CB1, did not have any ties, and it served as a control specimen. The second and third beams, CB2 and CB3, had 0.11 percent tie content. The main difference is in the shape of ties; ties in CB2 consisted of individual legs, while CB3 had dosed ties. Tie content in CB4 was doubled to 0.22 percent. The first three beams failed in horizontal shear mode, while the fourth beam failed flexure. Test results clearly show that, under indirect loading, the ultimate horizontal shear strength of composite beams can be significantly lower than the AC1 Code predictions; the specified minimum clamping stress of 0.35 MPa appeared to be unsuitable for indirectly loaded beams. The study also demonstrates that the shape of ties does not have significant influence on the horizontal shear strength, provided there is adequate anchorage of ties on both sides of the interface.