Study On The Compressive And Shear Behavior Of Cfsts Squat Wall Under High Axial Compression Ratio

Concrete-filled steel tubes(CFSTs)squat wall is a kind of composite shear walls with several steel tubes evenly distributed in its cross section and its shear span ratio less than 1.5.The steel tubes embedded in the section can not only improve the vertical and lateral bearing capacity,but also greatly improve the deformation capacity of the squat wall to avoid brittle failure.The CFSTs squat walls are especially suitable for the shear walls located at the bottom story of a high-rise or super high-rise building and the shear walls on the transfer floor.Up to now,studies on this type squat wall is very limited.In this dissertation,experiment and finite element method have been used to study the lateral load resisting mechanism of the CFSTs squat wall and some theoretical models have been proposed for its practical applications.The main work and conclusions are as follows:(1)The axial compression test of three RC columns,three CFSTs,three steel tube composite columns and three CFSTs squat walls(a total of 12 specimens)have been carried out.The stress state of the steel tubes,the axial force distribution between the CFSTs and its outside reinforced concrete and the failure mode of the CFSTs squat wall specimens during loading history have been studied.The experimental results showed that the CFSTs squat walls have a favorable vertical deformability as well as a vertical bearing capacity.The axial force in each part of a CFSTs squat wall was distributes according to its respective axial stiffness ratio before the peak load.(2)The parametric analysis of the axial performance of the composite columns and the CFSTs squat walls have been carried out.The influences of the steel tube numbers,steel ratio,confined area ratio and confinement coefficient on the axial compression performance of a CFSTs squat wall have been analyzed using ABAQUS.Based on the experimental and numerical analysis results,the formula of the axial compression ratio and the vertical bearing capacity of a CFSTs squat wall have been proposed.(3)Pseudo-static test of 5 CFSTs squat wall specimens have been carried out to study the influences of the steel tube layout,the axial compression ratio,the shear connector type and the steel braces between steel tubes on the seismic performance of the CFSTs squat wall.The failure modes,bearing capacity and deformability,stiffness degradation and energy dissipation of each specimen have been studied.The failure modes of the shear connectors and the steel braces were checked and verified as well.The experimental results showed that the CFSTs squat walls under high compression ratio behaved ductilely and have excellent deformability and stability.Numerical simulation of the 5 specimens and the parametric analysis were fulfilled using OpenSees to study the influences of the aspect ratio,concrete strength,horizontal reinforcement ratio,axial compression ratio and steel tube numbers on the seismic performance of the CFSTs squat walls.(4)Based on the simplified composite strut and tied model,the formula of the lateral bearing capacity of a CFSTs squat wall have been proposed.A four-line skeleton model and a set of restoring force rule for a CFSTs squat wall have been put forward according to the experimental data and proposed method.A method to determine the corresponding value at each key point and the loading stiffness at each stage were suggested as well.The relationship between the unloading stiffness and the lateral displacement were obtained by data fitting of the test specimens.