A Study On The Seismic Performance Of Steel-Polypropylene Hybrid Fiber Reinforced Concrete Frame Joints

The seismic performance of reinforced concrete frame joints subjected to earthquake loading is of critical importance to the accurate prediction and design of reinforced concrete(RC)structures.Under seismic load,the frame joints are subjected to complex stress state because of the coefficient of shear force,axial force and bending moment from the ends of beams and columns,which makes it very vunerable.Allocating a large number of stirrup in the core area of the frame joints is adopted in practical engineering to enhance the strength of the frame joints.Moreover,the cast quality of frame joints is difficult to guarantee due to the large amount of stirrup incorporated in the joints which increases the difficults to construction.As demanstrated by previous studies,an effective way to improve the seismic performance of RC frame joints and reduce the amount of stirrup is the addition of fibers into the core joints.In the present study,an experimental investigation and finite element analysis on the seismic performance of steel-polypropylene hybrid fiber reinforced concrete(hereinafter referred to as “HRFC”)frame joints were conducted using cyclic loading,with concrete strength,axial compression ratio and the influences of hybrid fibers analyzed in particular.Based on the test results,the shear failure mechanism was revealed as well as a shear capacity calculation and model of restoring force behavior were proposed.The present study can provide test data and theory basis for compiling industrial standard “Technical specification for fiber reinforced concrete structures”.The main research and conclusions extracted from this thesis are the following:(1)The seismic performances of HFRC frame joints were investigated subject to low-cyclic loading test.The concrete strength,axial compression ratio of column top,volume fraction and aspect ratio of steel fibers,volume fraction and aspect ratio of polypropylene fibers were selected as the main variables.The results show that addition of fibers into RC frame joints can effectivily delay the time of the initial cracks,reduce the the width of the cracks in core regions and increase the amount of micro-cracks,and then equal the stress in the core joint regions.The effect of hybrid fibers is shown to be the most effective,whilst the effect of polypropylene fibers is insignificant.Moremover,the yield load,ultimate capacity,ductility deforamtion capacity and energy dissipated capacity of the RC frame joints can be greatly improved by the incorporation of hybrid fibers.In the meantime,the stiffness degradation and stength deterioration can be alleviated.The seismic performance with respect to ductility and energy dissipated capacity of frame joints are enhanced with the increasing concrete strength,however,its stiffness degradation and srength deterioration are worse.In addition,the seismic performance,ductility and energy dissipated capacity of frame joints are enhanced and the stiffness degradation and srength deterioration are alleviated with the increasing volume fraction of steel fiber.Finally,it also can be seen that the effects of polypropylene fibers on the seismic performance of frame joints is little,with just a small enhancement.(2)According to the test results,use the finite element method to deeply analysis the influence factors and rules of seismic performance to HFRC frame joints.The influence factors consist of axial compression ratio,the strength grade of concrete matrix,the stirrup ratio of core area and the range of the beam end that the fiber mixed into.The results show that the load bearing capacity of frame joints is obviously improved when the strength grade of concrete matrix and the stirrup ratio of core area is improved in a certain range,and the increase of the stirrup ratio of core area can obviously improve its post-peak ductility;the load bearing capacity of frame joints decrease as the axial compression ratio is improved;when the range of fiber concrete incorporation is in the beam end plastic hinge,the load bearing capacity of the frame joints increases with the increase of fiber concrete incorporation length.(3)The effects of axial compression ratio of column top,concrete strength,volume fraction and aspect ratio of steel fibers and polypropylene fibers on the initial-cracking and ultimate shear capacity are studied based on the present test results.The results indicate that analogy to plain concrete beam-column joint,the shear mechanism of HFRC frame joints can be interpreated by the struct-truss mechanism.The variation of ultimate shear capacity is insignificant for different considering variables.The enhancement degree is approxiamte 8.1%~15.8%.The ultimate shear capacity increases with the increasing of concrete strength,steel fiber volume fraction and polypropylene fiber aspect ratio,and increases first and reduces afterwards with an increase in axial compression ratio of column top,steel fiber aspect ratio and polypropylene fiber volume fraction.In addition,an increase in axial compression ratio of column top,concrete strength,steel fiber aspect ratio and volume fraction leads to an increase in initial-cracking shear capacity by approximate 2.7%~25.7%,however,it has a small fluctuation by 1.5%~4.0% for an increase in polypropylene fiber volume fraction and aspect ratio.Using the above analysis as a foundation,according to 《Code for Seismic Design of Buildings》,calculation formulations of shear capacity of HFRC frame joints are developed by the incorporation of experimental investiagtion and theoretcial analysis,in which the contribution of steel fibers to the shear capacity is added.The formulations are then verified by test data reported in literatures,it is observed that the prediction yields a close estimation between the calculation and test results.(4)A trilinear storing force model of HFRC frame joints is estabished based on the conbination of original load-displacement curve in the loading end of the beam and theoretical analysis,with hibrid fiber,stiffness degradation and strength degradation taken into consideration.The calculation methods of characteristic points(i.e.yield displacement,yield load,peak displacement,peak load,failure displacement and failure load),stiffness degradation and strength deterioration as well as the laws of the hysterectic curves are suggested.