Study On Shear Behavior Of Reinforced Concrete Beams Considering Crack Control With Multiple Openings

With the development of social economy,underground engineering,high-rise and super high-rise buildings have become the main trend,and with the increasing degree of building intelligence,various pipeline lines will be arranged at the bottom of the beam.In order to improve floor height and save construction cost,the design needs to consider the provision of relevant holes for various pipes and cables to pass through the web of reinforced concrete beams.At the same time,it is inevitable to increase openings for existing buildings to meet the needs of new pipeline equipment,and it needs to be reinforced.All these will reduce the stiffness and stability of the beam,so the study of the shear performance of the open-pass beam has been widely concerned by scholars.However,at present,the influence of web opening on the shear properties of beams is still incomplete,the failure mechanism of open-hole beams is still unclear,and the design method of open-hole beams is still lacking of guidance.Based on a large number of experimental studies,numerical calculations and theoretical analysis,this paper comprehensively and systematically studies the influence of multiple openings in the web on the shear performance of reinforced concrete beams,proposes an open hole design method that can improve the shear performance of the open hole beams,analyzes the mechanism for improving the shear performance,and compares the shear capacity of the open hole beams with the domestic standards.The main research contents and conclusions are as follows:(1)In this paper,a systematic test of 8 open-pass beams is designed,with shear to span ratios of 2.59,3.0 and 3.56,respectively.Each group has control beams and open-pass beams.Through the implementation of three-point bending load test,the load-deflection relationship curve,axial tensile steel strain change,concrete compression zone strain,cracking path and failure mode and DIC image results were analyzed.The results show that the open hole in the beam belly does not necessarily weaken the beam.When the size and location of the open hole in the beam are appropriate(such as 3.0-O and 3.56-O in this test),the crack path of the critical diagonale crack of the concrete beam can be controlled,the failure mode can be changed,and the shear performance of the open hole beam can be improved.Therefore,according to the test results and analysis,the design method of opening holes which can improve the shear performance of reinforced concrete beams is proposed.(2)In this paper,the 3D rigid body spring element method(3D RBSM)is used to simulate8 specimens of concrete beams.The load-deflection curve,crack mode and shear failure mode in failure stage are successfully reproduced by the model,and the applicability of the method to simulate the shear behavior of open-pass beams is verified.The section stress calculated by3 D RBSM decoupled the shear capacity of the beam,and the decoupling was the shear contribution of the beam model and the arch model.The results show that the increase in shear capacity and ultimate deflection caused by the proposed web opening method is mainly due to the significant lifting effect of the arch model,and the lifting of the arch model is due to the development of the critical oblique crack as the target failure curve guided by the opening,and the formation of a high-bearing arch bar in the shear span of the beam.(3)Through parametric analysis,the structural parameters of concrete such as strength,shear span ratio and opening Angle are simulated.The results show that the opening design has a great effect on the improvement of shear performance when the shear span ratio reaches 3.0or above,and the improvement effect is significant when the opening center line Angle is selected between 35° and 45°.In addition,a similar study was carried out on a circular beam with open holes,and it was found that the method proposed in this paper is also suitable for circular holes,and the effect of improving the shear performance of the beam with open holes is still significant.(4)In this paper,the effects of different pass shapes on the shear properties of open-pass beams are studied and the mechanism of improving the shear properties of open-pass beams with circular openings at appropriate opening angles is analyzed.The results show that it is feasible to use the proposed opening setting method to control the critical diagonale crack.The concave critical diagonale crack can be introduced to improve the shear capacity of the beam at larger opening angles(35°,40°,45°,50°),which is related to the enlargement of the arch model.