Experimental study on seismic performance of concrete composite beams reinforced with overlapping hoops

In recent years, overlapping hoop, consisting of a U-shaped stirrup and a crosstie, has gained popularity because it could enhance construction efficiency of concrete composite beams. The U-shaped stirrups (U-shaped stirrup with 135/180 hooks, namely A and S) and crossties (crosstie with 135 (90)/90 hooks, namely J and C) are typical in the application. This study focused on four corresponding combinations of overlapping hoops, i.e., A+J, S+C, A+C and S+J. Ten full-scale concrete composite/monolithic beams were tested under cyclic loading, with parameters of the above four overlapping hoops and traditional enclosed hoop. The results revealed that all overlapping hoops had no fracture or early detachment prior to the peak load. The hysteresis curves of the four overlapping hoop composite beams were similar in full shape, demonstrating a good energy dissipation capacity. The four overlapping hoop composite beams had differences in flexural strength within 8% and 1% with the enclosed hoop composite beam and monolithic beam, respectively. The displacement ductility of the four overlapping hoop composite beams ranged from 6.1 to 6.8, while that of the enclosed hoop composite beam and monolithic beam was 7.3 and 6.3, respectively. Ten specimens exhibited similar stiffness degradation laws, and their three-linear restoring force models were proposed. Overall, the concrete composite beams reinforced with the four different overlapping hoops demonstrated similar seismic performance to that of the two enclosed hoop concrete beams. Considering the integrity and workability, the overlapping hoop A+C, consisting of a U-shaped stirrup (two 135 hooks) and a crosstie (two 90 hooks), would be the preferred combination.