Study On Flexural Mechanical Properties Of FRP-concrete-Steel Double-Skin Tubular Beams With PBL Shear Connectors

The fiber-reinforced polymer（FRP）-concrete-steel double-skin tubular beams（DSTBs）have a very ductile load-deflection response but may experience a relatively large slip between the concrete and the inner steel tube.In this study,for the first time,perfobond（PBL）shear connectors are employed in DSTBs to tackle the excessive slip problem.This study mainly includes two parts: the first part is the shear performance and pred model of PBL shear connector in DSTBs;The second part is the flexural performance of hybrid DSTBs with PBL shear connector.In this study,13 groups of 26 specimens were tested by push-out test.The variables included specimen form（with or without FRP constraint）,hole diameter,steel plate thickness,hole number,steel tube shape,GFRP tube thickness,steel plate row and steel plate number in each row.The test results showed that the external failure modes of the specimens are all circumferential cracking in the middle of GFRP tubes,while the internal failure modes are mostly open-hole steel plate disconnecting at the connection between steel pipe and perforated steel plate（only 8mm open-hole steel plate disconnecting at the weld）,the concrete is broken,and large bending and shear deformation occurs through the steel bar.By analyzing the loadslip curve of each specimen,two typical load-slip behavior can be obtained,namely threephase load-slip curve and four-phase load-slip curve.The existing calculation methods can not accurately predict the shear capacity of PBL shear key in this thesis,so a new model for the calculation of PBL shear capacity is established,and the results matched the best capacities of PBL shear keys well.In this study,Four-point bending tests are performed on eight DSTBs with two crosssectional shapes of inner steel tubes,two types of PBL plates,and four different PBL arrangements.Results identify that a typical load-deflection curve consists of three segments:an initial linear segment up to the yielding of steel tube bottom surface,a hardening segment with crack development,and an almost flat load plateau segment.The slips between concrete and steel tubes increase in a faster rate when yielding occurs at steel tube bottom surface.Overall,specimens with PBLs placed along the full span have the smallest slips（around 0.2mm）,whereas maximum slips occur for specimens with foams inserted at the spacings between PBLs（i.e.,2.33 and 0.59 mm for specimens with a circular and a square steel inner tube respectively）.In the end,the predictions of a conventional sectional analysis are compared with test results,and demonstrated to be satisfactory and conservative.