Progressive Collapse Resistance Of Prefabricated RC Frame Structure Considering Bond Slip Effect

The bond slip between steel bar and concrete is an important factor that can not be ignored under large deformation of progressive collapse.Due to the expensiveness and time-consuming of experiment,it is impractical to study the influence of various parameters in the form of test.The existing research can consider the bond slip by preset the macro element node of spring,but it needs complex pretreatment steps to deal with the pairing of the master node of solid element and the slave node of truss element,and the increase of degree of freedom and element significantly increases the calculation cost and convergence.In this paper,the slip between reinforcement and concrete is calculated by assuming the distribution of bond stress.Based on the UMAT（user defined material subroutine）of ABAQUS and the calculation of the slip amount of steel bar,a modified constitutive model of steel bar is developed by using FORTRAN statements.That is to say,the value of slip is equivalent to the constitutive relation of steel bar in plastic hinge area.The modified steel bar constitutive model is used in plastic hinge area,so that five assembled monolithic substructures and one cast-in-place substructure are simulated.By comparing the test values,the influence of bond slip on the performance of resistance of progressive collapse is discussed in the load displacement curve and the beam rotation angle two aspects.The simulation results show that the peak load capacity of the pressure arch of substructure will be improved without considering the bond slip between the reinforcement and the concrete,and the fracture of the reinforcement at the bottom of the beam will be advanced,and the interval displacement in the catenary stage will be reduced.This paper analyzes the influence of the roughness of new and old PC concrete composite surface,joint forms,spatial effect,concrete strength grade of beam,reinforcement ratio（bottom and top of beam）,beam span length（beam span height ratio）,lateral stiffness of side column,and the number of floors on the progressive collapse resistance of substructure of PC substructure.The simulation results show that:（1）When the friction coefficient of the composite surface reaches 1.0 or more,the rotational capacity of the beam is almost equal to that of the RC structure.The selection of integral joints is particularly critical,and the slotted horizontal lap joint can meet the requirements of progressive collapse resistance.Before the peak value of arch compression,the spatial model can be approximately superimposed by the plane model and the cantilever model.With the further increase of the vertical displacement,the preferential fracture of the reinforcement at the bottom of the beam near the end of the central column and the bending and axial elongation of the out of plane transverse beam lead to the torsion effect of the central column head in and out of plane.Torsion effect is disadvantageous to the progressive collapse resistance of substructure.（2）The influence of beam concrete strength on progressive collapse resistance is limited.The reinforcement ratio of the longitudinal reinforcement at the bottom of the beam has a great influence on the arch compression,but has little influence on the catenary.Higher reinforcement ratio of beam top plays a positive role in the development of arch compression effect and catenary effect in substructure.The substructure with smaller span to height ratio is easier to provide resistance through arch compression rather than catenary action,and with the increase of span to height ratio,catenary effect increases gradually,when it reaches a certain value,catenary effect is greater than arch compression effect.Considering the restraint effect of the surrounding structure in the actual structure,the bearing capacity of arch and catenary can be significantly improved,which is beneficial to the progressive collapse.With the increase of the number of vertical stories,the arch compression and catenary effect in the substructure gradually decrease.（3）Zhou YL / Wang X’s CAA formula is more appropriate to the test value,while the standard formula is conservative and safe.Through nonlinear fitting,the reduction factor considering the effect of torsion is obtained.The proportion of non conservative points corresponding to this formula is 8.8%.