Analysis Of The Effect Of Steel-FRP Composite Bar On The Progressive Collapse Resistance Of Frame Structures

The progressive collapse behavior of conventional reinforced concrete（RC）frame structures caused by the loss of columns has been widely investigated.However,few studies have focused on the enhancement of structural collapse resistance by improving mechanical properties of reinforcements.Steel-FRP composite bar（SFCB）is composed of an outer fiber layer and an inner steel bar.The typical feature of the SFCB is stable and controllable post-yield stiffness.Therefore,ordinary steel bars are replaced by SFCBs as longitudinal bars of RC frame beams,and the progressive collapse characteristics of SFCB concrete frames are investigated according to alternate load path method in this paper.The main contents of this paper are as follows:Main design methods and current research progresses of progressive collapse for conventional RC frame structures are discussed,and considering the disadvantages of RC frames against progressive collapse,main application researches of SFCBs are described.The collapse criterions of SFCB frame structures are proposed at the material level and at the component level.Progressive collapse behavior of a RC beam-column sub-assemblage,an SFCB beam-column sub-assemblage and a RC plane multi-story frame structure are analyzed.The characteristics of capacity development and deformation of the above specimens under different mechanisms are summarized,which is the basis of investigating the progressive collapse characteristics of space frame structures.According to Chinese building codes,a typical three-story RC frame structure is designed.According to the composite principle,five types of SFCBs with different secondary stiffness ratios are designed.Based on the principle of equal yield load,steel bars in RC frame beams are replaced,and five kinds of SFCB frame structures with different secondary stiffness ratios are obtained.With the help of ABAQUS software,the center column of long side,corner column,center column of short side and inner column of the above frames are removed respectively,and then the nonlinear static analyses of the residual frame structures are carried out.Change principles of load factors,axial forces and moments of frame beams with the increase of vertical displacement are revealed,and the robustness of SFCB frame structures is qualitatively evaluated.Based on the nonlinear dynamic analysis,the collapse failure modes of SFCB frame structures and the hinge sequence of beam ends under different column removal scenarios are studied.Compared with the nonlinear static analyses,the change principle of dynamic increase factors of frames with the increase of secondary stiffness ratios of SFCBs is revealed.On the basis of the displacement ductility factor,a coefficient is proposed to quantitatively evaluate the robustness of SFCB frames and RC frames.To reveal the effects of different structural parameters on the capacity of SFCB specimens under compressive arch action and catenary action,parametric analyses by numerical simulations are conducted on the collapse resistance of SFCB beam-column assemblages after the failure of a middle column.The structural parameters include secondary stiffness ratios of SFCBs,beam reinforcement ratios,column spacings,beam span-to-depth ratios and the failure time of columns.Theoretical calculation formulas of the capacity of SFCB beam-column sub-assemblages under flexural action,compressive arch action,transition mechanism and catenary action are proposed,and collapse resistance mechanisms under different stages are revealed.Finally,based on the above analysis results,main conclusions are summarized,and design suggestions for resisting progressive collapse of SFCB frame structures are given.