Seismic Performance Of Precast Restressed Self-centering Reinforced Concrete Frame Structures

Almost every survey of structural seismic damage after severe earthquakes revealed that the reinforced concrete(RC)frame structures,designed strictly following the current seismic design code,can achieve the seismic design goal of“no collapse under large earthquake.” However,it is often severely damaged and difficult to repair,causing many “standing ruins” in the earthquake area and causing substantial economic losses.Therefore,the precast prestressed self-centering(PPSC)reinforced concrete frame structure was proposed in the 1990 s and gained much attention because of its light seismic damage and quick recovery of service function after an earthquake.However,after nearly thirty years of research,it still fails to move towards large-scale engineering applications due to two main problems.First,this structural system mainly relies on prestressing tendons to connect precast members in series.The failure of prestressing tendons will lead to devastating consequences.Therefore,what is the structural system's collapse safety margin under the earthquake beyond the design expectation? Reliable assessments are lacking.Second,most of the previous works focused on the components.There is still a lack of studies to verify the whole structure system's actual seismic performance and the collapse resistance performance under rare earthquakes in detail employing large-scale shaking table tests.Based on the above background,the seismic performance and collapse resistance of PPSC reinforced concrete frame structures installed with external dampers were studied in this paper.The main research contents are as follows:(1)Two types of external dampers(BRB dampers and steel plate dampers)suitable for self-centering RC frame joints were aimed at.The mechanical properties of the two dampers were studied by means of quasi-static tests and finite element simulation.The numerical simulation methods for the dampers' mechanical performance were then put forward based on Open Sees,which laid a foundation for the subsequent shaking table tests and the overall structure's seismic performance evaluation.(2)A two-story PPSC reinforced concrete frame model structure(1/2 scale)used for shaking table tests was taken as the research object.Aiming at the RC frame joints with two types of external dampers installed(column-base joints,X-direction beam-column joints,Y-direction beam-column joints),the design mechanism of each type of joint was described.The corresponding design methods were presented,including self-centering capability design,shear design,and anti-slip design.(3)For a two-story PPSC reinforced concrete frame model structure(1/2 scale),a series of shaking table tests in two-direction was carried out.It was tested whether three kinds of self-centering joints installed with two types of external dampers can produce deformation according to the designed action mechanism.The actual seismic performance,damage degree,and self-centering performance of the structure under six different levels of ground motions(from small earthquakes to rare earthquakes)were studied.The experimental results showed that both the BRB dampers and the steel plate dampers could play a useful role in the structure's energy dissipation.The three self-centering joints installed with the BRB dampers and the steel plate dampers can deform according to the design mechanism.The X-direction frame with steel plate dampers(X-direction of the structure)and the Y-direction one with BRB dampers had a relatively good seismic performance.Under rare earthquakes,the concrete beams and columns only suffered slight damage,and the structure still had an excellent self-centering ability.(4)The numerical simulation method of PPSC reinforced concrete frame structure based on Open Sees was studied.The method of simulating the mechanical property of prestressing tendons and the open-close behavior of self-centering joints were given.For the prestressing tendons,they were presented by the truss element in Open Sees,with the material constitutive defined by the Steel02 material with initial stress and in series with the elastic no compression(ENC)material.The open-close behavior of the self-centering joint was simulated by a series of parallel zero-length axial spring elements that can be compressed only.These numerical simulation methods were then verified by comparing the simulating data with the shaking table test results.(5)Based on the seismic fortification requirements in the current seismic design code,a 5-story PPSC reinforced concrete frame benchmark structure was redesigned.Then the Open Sees finite element model was established.The model considered the failure of different self-centering elements in the structure(yielding and fracture of prestressing tendons,fracture of dampers)and the influence of randomness of ground motions(site conditions,epicenter distance,etc.)on seismic responses of the structure.Based on the numerical model,the structure's seismic performance under different intensity of ground motions was systematically studied,especially the collapse resistance of the structure under large earthquakes and different levels of rare earthquakes.The results provided a reliable basis for the practical application of the PPSC reinforced concrete frame.