Study On Progressive Collapse Resistant Mechanism Of Prestressed Precast Concrete Frame Structures

In recent decades,terrorist attacks or accidental explosions occurred frequently in the world,leading to serious casualties and huge economic losses.Moreover,the blast wave might cause local damage,or even progressive collapse of the surrounding building structures.Therefore,conducting progressive collapse resistant design of building structures to enhance their ability of explosion protection is critical for ensuring the safety of urban.With the development of industrialization of building construction,the precast concrete(PC)structure develops promptly.However,there is lack of research on blast resistant performance of PC structure,while the investigation on its seismic resistant performance is much more common.And there is even no basis on the progressive collapse resistant design of PC structure.This paper investigates the progressive collapse resistant mechanism of prestressed PC frame structure through the static and dynamic tests,numerical simulations and parametric studies,analytical and pseudo analysis,which supplies theoretical basis for the progressive collapse resistant design of PC structure.The main research works and innovative results are listed as follows:(1)Static experiments on collapse resistance of prestressed PC structure are carried out,and the collapse resistant mechanism of prestresed PC structures is investigated.The specimens,which are plane sub-frame with two stories and two spans and 1/2 scaled,include two prestressed PC structures and one cast-in-place reinforced concrete(RC)structure.Under the middle column removal scenario in the first floor,the push-down test is conducted through applying a static load on the top of the middle column vertically with middle joint displacement(MJD)control.The results show that compressive arch action(CAA)and tensile catenary action(TCA)occur in succession in the beams of the two types of the structure during the process of loading.Compared with the beams in RC structure,the damage is mainly concentrated at the beam end sections in prestressed PC structure,and the CAA is weakened and TCA is enhanced in prestressed PC structure.the side column of the two type structures get through three stages,including intial outward-push,outward-push and inward-pull.Due to the lateral deformation of side column,the restrained stiffness at beam ends in the second floor is reduced,and the effect of CAA is also reduced correspondingly.TCA is fully displayed in the beams of the two stories due to the reliable lateral restraints at beam ends.(2)A static numerical analysis on progressive collapse resistance of prestressed PC structure is conducted,and a sub-structure method of collapse resistant analysis of frame structure and a rapid calculation method of collapse resistant capacity of prestressed PC structure are proposed.Based on the experiment described above,a high-fidelity finite element model(FEM)of prestressed PC substructure is established using the software of LS-DYNA.Internal force analysis of structure and parametric studies of beam vertical resistance is conducted.The results show that the damage mode of beam ends can be simulated more accurately through setting a series of discrete elements between the weakened segment of energy-dissipated bars of beam and stirrups in FEM.Decreasing the span-depth ratio of beam,increasing the reinforcement ratio of energy-dissipated bars or increasing the anchor length of shear bars of beam ends can enhance the vertical resistance of beam at CAA stage effectively.With regard to the frame structure with multi stories and multi spans,the contribution of beam in the first floor to the structural collapse resistance is the most prompt,and the beam collapse resistance is reduced with elevation of the floor.The proposed method about the structural collapse resistant analysis based on substructure and the rapid calculation on structural collapse resistant capacity can realize a high efficiency.(3)A static analytical model on beam vertical resistance is established,and static analytical method about collapse resistance of prestressed PC structure is proposed.Taking a failed span with single story(beam-column sub-assemblage with one story and two spans adjacent with the failed column)as the research objective,the boundary condition is simulated by setting translation and rotation spring at side joint,and the mechanical character of weakened segment,non-weakened segment of energy-dissipated bars of beam and the bars embedded into the side column and connected with the energy-dissipated bars are considered respectively.The static analytical model of beam vertical resistance is established according to force equilibrium condition and deformation compatibility condition.The mechanical behavior of beam end section is investigated further,and parametric studies about the beam vertical resistance are conducted.The results show that,the beam vertical resistance can be obtained accurately according to the established model.Increasing the length of weakened segment of energy-dissipated bars can enhance the structural ductility at TCA stage.Increasing the initial tension of steel strands,or increasing the restrained stiffness of beam ends can enhance the beam vertical resistance at CAA stage,and the effect of translation stiffness is more prompt than the rotation stiffness.(4)Dynamic experiments about collapse resistance of prestressed PC structure are conducted,and collapse dynamic response mechanism of prestressed PC structure is investigated.The test specimens,which are 1/2 scaled beam-column sub-assemblages with one story and two spans,include two prestressed PC structures and two RC structures.The material strength,geometrical size and reinforcement ratio of the specimens are identical with those in static experiments.An impact load is applied on the top of middle joint to consider the effect of axial tension action of failed column on the failed span when a frame is under explosion.The results show that,the damage mode of beam after impact is similar with that under static loading in the two structural types.At the moment of impact,an upward deformation of reverse arch occurs in beam instantaneously,and reverse arch action(RAA)is formed.With the increase of impact velocity,RAA of beam is enhanced.The RAA is reduced and CAA is emerged when the inertial effect is decreased.The residual MJD is smaller in prestressed PC structure,which shows a perfect ability of self-reset.The tension of steel strands accelerates the compressive damage of concrete at beam end sections,and thus reduces CAA of beam.(5)A dynamic numerical analysis on progressive collapse resistance is conducted,and analytical method on collapse dynamic response of prestressed PC structure based on traveling plastic hinge is proposed.Based on the dynamic experiment described above,a high-fidelity FEM of prestressed PC sub-assemblage is established using LS-DYNA software,and internal force analysis of beam and parameter studies of dynamic response of structural collapse are conducted.According to theory of traveling plastic hinge,a mechanical model of sub-assemblage under vertical concentrated dynamic loading is established,and an analytical method of calculating the plastic hinge position in beam in the failed span is proposed.The results show that,with increase of impact velocity,the damage mode at beam end near middle joint transfer from flexure to shear.When the velocity is invariable,and a flexural damage is occurred at beam end,the peak value of MJD can be reduced through increasing the collapse resistant capacity of beam.The plastic hinge moves toward to middle joint gradually when the peak value of impact load is increased.Enhancing the reinforcement ratio of top bars of beam can keep the plastic hinge away from the middle joint,and thus the distribution of bending moment along beam is changed,and the performance of structural collapse resistance is enhanced.(6)Dynamic analysis of progressive collapse of prestressed PC structure under blast load is conducted,and a pseudo dynamic analysis method of structural collapse resistance is established.Based on the above numerical models under static and dynamic loading condition,a dynamic response analysis of frame structure with multi stories and multi spans under blast load is conducted.According to theory of work-energy,a pseudo dynamic analysis method of structural collapse is established considering blast load.The results show that,under blast load,the strain energy at the peak MJD is equal to the summary of work done by vertical load in the failed span and axial force of failed column.The proposed method of pseudo dynamic analysis can predict the peak MJD of the failed span rapidly,and the predictive results are generally coincident with the results of numerical simulation.