Study On Seismic Behavior And Design Method Of Shear Wall With Small Shear Span-to-depth Ratio Under High Axial Loading

The application of reinforced concrete shear wall structures becomes wider and wider in modern high-rise buildings,attributing to the high in-plane stiffness,small lateral deformation and good seismic performance,etc.While shear wall with small shear span ratio are often formed due to various reasons,such as window walls,split layer,architectural needs.In this dissertation,focusing on shear wall with small shear span-to-depth ratio under high axial loading,its seismic performance and design method were systematically investigated.Researches include the experimental investigations,theoretical calculations and finite element analyses.The shear behavior and seismic behavior of squat shear wall under high axial load and reversed cyclic lateral load were experimentally investigated.Five squat shear walls were tested under lateral cyclic loading and one specimen with same condition was tested monotonically.By considering the axial load ratio and stirrup ratio as variables,the failure mode,shear load capacity,ductility,energy dissipation and stiffness degradation of tested shear walls were investigated.For better measuring the strains in horizontal and vertical reinforcements,a novel measuring method with embedded strain gages was applied.With this method,the damage of the strain gage due to concrete cracking can be effectively prevented.The results showed that with the increase of axial load,the load capacity increased,however,the degradation of strength and stiffness become more severely after reaching the peak load.As the stirrup ratio of lateral resisting member increased,the load capacity of squat shear walls under high axial load changed little,while ultimate displacement increased.The contribution of transverse and longitudinal reinforcement to the shear capacity of shear walls were equal when the axial load was low.The reinforcement did not fully used when the axial was high.A database includes 514 squat shear walls was first established for discussing the effect of the main factors,such as shear span-to-depth ratio,concrete strength,web reinforcement ratio,longitudinal reinforcement ratio in restraint flange,axial load ratio,etc.,on the shear behavior of squat shear walls.Then,a novel model based on truss-arch model,so called equivalent inclined web tie truss-arch model,was proposed with the consideration of the characteristics of bidirectional reinforcements in the web of shear wall and strain compatibility condition for calculating the effective shear stiffness of RC shear walls after cracking.For considering the both contributions of horizontal and vertical reinforcements to the shear load capacity of the squat shear walls,the bi-directional reinforcements in web were equivalent to an inclined tie in the proposedIV model.The orientation of the equivalent inclined tie was coincided with the direction of the resultant force in the orthotropic reinforcements.And the inclination angle of inclined crack was theoretically derived according to minimal energy principle.Since the theoretical calculation formula is too complicated to apply,a simplified semiempirical formula was proposed based on the database for practical engineering application.The calculation formulas for effective shear stiffnesses of both equivalent truss model and arch model were also derived based on the minimal energy principle.The effective shear stiffness of the shear wall at yield point can be obtained by summing up the shear stiffnesses of equivalent truss and arch actions.Finally,the stiffness of tested shear walls was calculated with the proposed model and compared with measured results.The comparison results indicate that the proposed equivalent inclined web tie truss-arch model can calculate the stiffness of squat walls at yield point with reasonable accuracy.A new discriminative approach for failure modes of squat walls was proposed.Based on the proposed equivalent inclined web tie truss-arch model,considering the deformation compatibility conditions and different failure modes,a new method for calculating the shear load capacity of shear walls with small shear span-to-depth ratio.The accuracy of the proposed method was verified with the failure modes and load capacities of 514 shear walls in the aforementioned database.Comparison results show that the proposed method can predict both failure mode and shear load capacity with reasonable accuracy.The calculation results by the proposed model were also compared with the results by calculation methods proposed by various codes or scholars.Comparison results validate that the calculation accuracy of the proposed model is relatively high and its calculation results give smaller variations,since the proposed model can calculate both contributions of bi-directional reinforcements and arch action for the shear load capacity,and rationally consider the deformation compatibility between the truss and arch actions.The finite element modeling method for simulating small shear span-to-depth ratio shear wall under high axial load ratio was studied.In this chapter,an approach for determining the parameters in hysteretic restoring force model for squat shear walls was proposed based on the proposed equivalent inclined web tie truss-arch model.Six experimental shear walls were simulated with a micro model(CSMM)and a macro model(General wall model in PERFORM-3D),respectively.The modeling results indicate that both models can capture the macro hysteretic response of small shear span-to-depth ratio shear wall under high axial load ratio.A parametric study using CSMM was applied to investigate the influence of shear span-to-depth ratio and axial load ratio on the shear capacity.The calculation results by equivalent inclined web tie truss-arch model were compared with the FEM results,and the comparison shows reasonable agreement between the results.The finite element modeling method using PERFORM-3D provide a basis for time-history analysis and performance-based seismic evaluation of whole structures.The cause of small shear span-to-depth ratio of shear walls were studied and analyzed.It was validated through linear elastic finite element analysis that shear concentration can be found in the bottom of exterior frame supported shear walls close to the transfer structures in high-rise buildings.The effect of shear concentration results in the reduction of generalized shear span-to-depth ratio of exterior shear walls close to the transfer structures,which will make the walls fail like squat shear walls,i.e.brittle shear failure.For measuring the distribution of shear stress in the shear walls above transfer structure and quantificationally determine the amplification of shear force in exterior walls,a shear concentration factor(SCF)and a shear concentration scale factor(SCSF)were defined.Five finite element models were designed based on practical structures,and the main factors for affecting the shear concentration were studied through finite element analysis.A time-history analyses for a high-rise building with transfer structure were carried out,focusing on the seismic performance of the shear walls 1-2 stories above the transfer level with the effect of shear concentration under strong earthquake.Investigations for seismic responses and damage levels of models with different SCSF were performed through time-history analyses.Analytical results indicate that the damage level of the shear walls highly affected by shear concentration will rise with the increasing value of SCSF.And therefore,the severe local damage caused by shear concentration can be effectively controlled by adjusting the value of SCSF.Since SCSF can be easily obtained with linear elastic finite element analysis,it would be convenient for its application in structural design.A strengthening approach that replace regular RC shear wall with steel plate-concrete composite shear wall to diminish the local damage caused by shear concentration effects was investigated.The effect of vertical slots and circular openings of the steel plate on the shrinkage stresses in composite shear walls was first investigated by using FEM software ANSYS.Rational forms of slotting were determined based on the analysis.The seismic behavior of a slotted steel plate-concrete composite shear wall under high axial load and reversed cyclic lateral load were experimentally investigated and compared with regular RC shear wall.And at last,time-history analyses were also performed by replacing the RC shear walls within 2 stories above transfer level to steel plate reinforced concrete shear walls.Analytical results prove that the damage level of the replaced shear walls was reduced obviously,and thus,setting steel plate reinforced concrete shear walls in the place where is highly affected with shear concentration can be applied as a strengthening approach for improve the structural seismic performance locally.