In practice, web openings are usually drilled in reinforced concrete (RC) beams to accommodate pipes and cable installations. But introducing web openings always causes shear degradation of RC beams. This study aims to establish a rational position design of multiple small web openings, which can improve the shear behavior of RC beams, by controlling the path of critical diagonal crack. Three-point bending tests on four RC beams without and with web openings (shear span depth ratio: 2.59 and 3.56) were firstly carried out to evaluate the strengthening effect attributed to the proposed placement of web openings. Besides, the Three Dimensional Rigid-Body-Spring Method (3D RBSM), a discrete numerical methodology, was applied to simulate the test process, and the strengthening effect due to the placement of web openings was parametrically investigated. Finally, the decoupling of shear components (beam and arch actions) was conducted to comprehensively understand the shear strengthening mechanism due to drilling web openings, by using RBSM-based methodology. The test and numerical results confirmed that the proposed position design of web openings could evidently improve the shear strength and deformation ability of RC beams, because the multiple web openings could increase the angle of critical diagonal crack, by predetermining it propagation path. Moreover, it was clarified that the strengthening mechanism of shear behavior owing to web opening setting was dependent on the more dramatic development of arch action with the increasing angle of critical diagonal crack. The present paper provided a rational design to solve the problem of shear degradation caused by small web openings in the field of concrete member design, and proved a possibility to strengthen the shear behavior of existing concrete beams by drilling web openings.

This paper evaluated the improvement effect of multiple small circular transverse openings (diameter is 1/10 of member height) on the shear performance of RC beams without shear reinforcement and explained the shear resistance mechanism, using three dimensional Rigid-Body-Spring-Method (3D RBSM). First, 3D RBSM was employed to reproduce the shear loading test on the RC beams with multiple small circular transverse openings, and the contributions of beam and arch actions to the shear resistance at each loading stage was evaluated. The analytical load-deflection relationship and cracking behavior were in good consistent with the experiment, and it was noted that the connecting line of the opening centers (line of openings) could control the path of critical diagonal crack, and transfer the failure mode of one of the beams from the diagonal tension pattern to the shear compression pattern. Moreover, the shear strength and ultimate deflection of this beam with openings were significantly improved due to the increase in the shear resistance contribution of arch action. Furthermore, the analysis of additional cases with three transverse openings at each shear span were conducted aiming to explore the possibility that a proper setting of openings can reliably improve the shear performance of RC beams. The line of openings was on a straight line and passed through the loading point at the same side, and the main parameter was set as the angle of the line of openings to beam axis (25°-50°). As a result, the role of the transverse openings in improving the shear performance was confirmed and the corresponding mechanism was explained based on the investigation on the beam and arch actions and crack behaviors.