Study On Structural Performances Of Composite RC Beams With Overlapping Hoops

In all types of precast concrete building structures, RC composite beams are the primary horizontal structural elements. Their structural performance plays a significant role in maintaining the integrity of slabs. According to current Chinese standards, closed stirrups should be protruded out when precast parts of the beams are produced. This can bring about inconvenience in laying longitudinal reinforcement in the after pouring parts of the beams. With reference to the related codes of practice in the United States, this paper proposes a new form of stirrups-overlapping hoops, which are made up of two pieces of reinforcement:one open stirrup with135-degree hooks at both ends, and one crosstie closing the ends.. Applying such stirrups in composite beams can greatly facilitate the installation of longitudinal reinforcement in the after pouring parts, and as a result the new stirrups can have a great value in engineering practice. In order to promote the use of the overlapping hoops in the composite beams, systematic studies on the structural performances of the beams with the new stirrups are necessary.To this end,22RC composite beam specimens are designed and tested, with the aim to study the structural performances of these beams with overlapping hoops under flexure, shear and torsion, respectively. Based on comparison and analysis, the following conclusions can be drawn:(1) Basically the composite beams with overlapping hoops have the same flexural performance as that of the beams with conventional closed stirrups. Hence the overlapping hoops can be applied in the composite beams governed by bending moment.(2) Basically the composite beams with overlapping hoops have the same shear capacity as that of the beams with conventional closed stirrups. The failure modes of the beams having shear span-effective depth ratio of2.62are slightly different, in terms of interface slip, from those beams with conventional closed stirrups, while as for the beams having shear span-effective depth ratio of1.50, the failure modes are nearly the same. The overlapping hoops can be applied to the composite beams subject to combined bending and shear, provided that the failure of the composite interface occurs later than the shear failure.(3) Basically the composite beams with overlapping hoops have the same torsional capacity as that of the beams with conventional closed stirrups. Before reaching the ultimate capacity, the torsional performance of both types of the composite beams are similar. Beyond the ultimate capacity, the ductility of the beams with overlapping hoops anchored by the90-degree hooks turns worse; the corners of the beams tend to spalling off; the90-degree hooks become distorted. The torsional performance of the beams with overlapping hoops anchored by135-degree hooks are the same as that of the conventional closed stirrups. Therefore the overlapping hoops anchored by135-degree hooks are recommended for the composite beams carrying unidirectional torque, while the overlapping hoops anchored by90-degree are not recommended. The structural performance of the later needs to be further investigated.(4) The test results and analysis indicate that the flexural capacity, shear capacity and torsional capacity of the composite beams with overlapping hoops under monotonic loads can be calculated according to the Code for design of concrete structures GB50010-2010.