| Because of the addition of diagonal reinforcement and rhombic reinforcement in the conventionally reinforced coupling beams,the better seismic performance can be obtained even under high shear to compress ratio.At present,the coupling beams with small span-to-depth ratio has been widely used in the shear wall structure.However,there are two main problems in the design proposal of the current Code for Concrete Structure Design based on the test results of L/h≤2.5:(1)The shear bearing capacity of the compositely reinforced coupling beams is mainly composed of concrete and two types of oblique reinforcement,and the shear capacity of the stirrup is negligible,which is not in accordance with the effect of 1.5<L/h≤2.5;(2)Because the two ends of the coupling beam are bound by the shear wall,the axial force of the beam is exited and changed with different stress stages except for the bending moment and shear force in the horizontal earthquake,which results in a different result with bending component test.Based on the test results analysis of four synthetically reinforced coupling beams with small span-to-depth ratio 1.5<L/h≤2.5 and the finite element analysis of 10-storey frame-shear wall structure in 8 degrees 0.2g area,the following main content studies are listed:(1)Based on the transformation of strut-tie model of coupling beams with small span-to-depth ratio 1.5<L/h≤2.5 without axial constraint,this paper sets up the strut-tie model which can effectively reflect stress mechanism under the axial constraint of the compositely reinforced coupling beams with small span-to-depth ratio 1.5<L/h≤2.5 under the horizontal earthquake.Through changing the diagonal strut-tie model of the concrete and the deformation coordinate equations,the physical conditions and the balance equations in four types models of transfer mechanism which are diagonally reinforced truss model、diamond reinforced truss model、beam span stirrup reinforced truss model and beam end stirrup reinforced truss model,the LL-P program is established by VB language in order to analyze the coupling beams,and adjusting the axial deformation of the beam in the program leads to axial force near zero.At this time,the results of the P-△(macroscopic aspect)and the main reinforcement strain(micro aspect)are compared with the experimental results of the coupling beam specimens and the rationality of LL-P is verified.(2)Some information are obtained through the analysis program in the first two times yield displacement and the failure state,which are the ratio of bending deformation and shear deformation in the two cases of the axial unconstrained(WYS)and the axial complete constraint(YS)、the proportion of four shear models.This paper compares the change rule of the ratios of various shear models in L/h≤1.5 and 1.5<L/h≤2.5.The influence rule of axial restraint on the ductility of coupling beams and some revised suggestions on the shear bearing capacity of L/h≤2.5 in the current Code for Concrete Structure Design are provided(3)A 10-storey frame-shear wall structure with coupling beams with span-to-depth ratio of 2.0 under three rare seismic waves are analyzed with ABAQUS.Mechanical performances including the displacement distribution of each floor,the force distribution of the coupling beams,the axial deformation of the coupling beams,the displacement composition and ductility demand of the coupling beams are studied.The following main achievements have been obtained.(1)In this paper,the program LL-P can analyze the mechanical performance of the coupling beam under different loading stages and different axial constraints(2)Comparison of the ratio of four models’ shear bearing capacity of L/h≤1.5 with axial constraint,the proportion of shear components of diamond reinforced truss model is increased by 10% or so,and the proportion of shear component of the diagonally reinforced truss model is reduced by 35%~40%,and the proportion of shear component of the stirrup truss model is increased by 25%~30% in the 1.5<L/h≤2.5 with axial constraint It is showed that diamond reinforced truss model and stirrup truss model play an increasingly important role in the compositely reinforced coupling beams with small span-to-depth ratio 1.5<L/h≤2.5.and the shear contribution of the diagonally reinforced truss model is reduced(3)Compared with axial unconstrained and axial completely constraints of coupling beam with small span-to-depth ratio 1.5<L/h≤2.5,the displacement ductility coefficient reduces from about 4.5 to 2.0 or so,but the shear bearing capacity of the coupling beam increases by about 5% ~ 10%,.It is shown that the existence of axial restraint reduces the seismic performance of the coupling beam,but it improves the load-bearing capacity of the beam to a certain extent.(4)The difference of shear bearing capacity calculation results of stirrup truss model of four coupling beam specimens without axial constraint and the calculation value of Code for Design of Concrete Structure is big.In this paper,based on the shear bearing capacity calculation results of the stirrup of four coupling beam specimens,the coefficient of the stirrup shear bearing capacity of the coupling beam in the current Code for Design of Concrete Structure is analyzed.It is suggested that this coefficient should be increased from 0.6 to 0.9 to more accurately consider the shear contribution of stirrup in the span-to-depth ratio 1.5<L/h≤2.5.At the same time,the existence of axial constraint can make the ductility of coupling beam become worse.Therefore,this paper proposes to consider the influence of the axial constraint by using the common frame beam to set the stirrups encryption area.(5)The elastic-plastic time analysis of the frame-shear wall structure is obtained by ABAQUS.The results show that the internal force and ductility demand of coupling beam in the middle of floors are greater than that of the upper and lower floors.More attention should be paid to the earthquake-resistant design of coupling beam in the middle of floors. |
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