| Ultra-high performance concrete(UHPC)was regarded as an ideal solution to develop the bridges with high service performance,high construction performance and high durability due to its superior strength and ductility,as well as the excellent ductility.In order to overcome the shortcomings of the traditional composite bridges with heavy dead loads and the brittle cracking behavior of normal concrete(NC)bridge decks,the NC deck was necessary to be replaced by the thin UHPC bridge deck to develop the composite bridges with lightweight,long life and high durability.One hand,steel girders should be compatibility with UHPC deck so that their high strength can be fully used.On the other hand,the effective load transferring on the interface ensured the high performance of lightweight steel-concrete composite bridges,however,which in turn triggered for the new requirements of short studs.In terms of the composite bridges with new materials,new structural system and new connection detail,how to realize the goal of ―materials with high performance-effective interfacial connection-structure with high performance‖ is the major scientific and technologic problems to achieve the full use of high strength of the UHPC.Therefore,this paper focuses on the development of structural systems of interfacial connections and lightweight steel-concrete composite bridges.The performances of both interfacial connections and structural systems are studied to reveal the interfacial load transfer and failure mechanisms based on parametric experimental,finite element and theoretical analyses.The interfacial theoretical load-slip relationship was established.The flexural performances of steel-UHPC composite bridges were further studied,in which the ultimate bearing capacity,flexural stiffness,ductility and the interfacial slip were also investigated to establish the calculation methods.The detailed research contents and results are as follows:(1)Study on the short stud-UHPC connection behaviorA total of 33 push-out tests were carried out to consider the influences of stud diameter(22 and 30 mm diameter)、UHPC slab thickness(35-150 mm)、stud aspect ratios(1.0-5.5)and cover thicknesses(5-30mm)on the interfacial shear connection behavior.Based on the test results,the stud-UHPC interactions and failure modes with vary stud aspect ratios were revealed.The influences of different parameters on the ultimate bearing capacity,connective stiffness and slip ductility were further explained.Test results indicated that the stud aspect ratio of 1.5 was the critical value of full stud fracture and partial stud fracture.The UHPC failure modes of shear-compression,shear-uplift and tension-uplift potentially overlapped,thereby decreasing the shear strength by 25%,comparing partial stud fracture with full stud fracture.UHPC pryout failure may occur for short studs of aspect ratio 1.0,owing to insufficient anchorage,while the complete stud fracture failure occurred for 22 mm studs.Thus,observed shear resistance increases were smaller proportional to the increases in stud cross-sectional area,but were influenced by local crushing,cracking and UHPC pryout.(2)Theoretical analyses on the load-slip relationships for short stud-UHPC connectionsThe widely parametric analyses were carried out based on the test-validated finite element model(FEM)to consider the influences of stud diameter,stud aspect ratios and cover thicknesses on the interfacial shear strength.Based on the elastic foundation beam,a theoretical model was established to illustrate the load-slip behavior by three typical stages,i.e.elastic,elasto-plastic and plastic phases.In this model,the yielding slip amount of the concrete and the concrete plastic region at stud yielding were obtained from the FEM results.Good agreement was observed between the proposed model and experimental results from different studies,covering the concrete strength of 40 MPa-200 MPa,indicating that the proposed model could predict the interfacial load-slip relationship well.(3)Shear strength and mechanisms analyses on the grouped short stud-UHPC connectionsThe three dimensions nonlinear finite element model was established and validated by the experimental results,including the grouped long studs and single short stud-UHPC connections.The influences of stud number,longitudinal stud spacing and stud aspect ratios were studied on the interfacial shear strength to reveal the shear mechanisms of grouped short stud connections.Then the shear strength of equivalent large stud diameter-UHPC interaction was established,which agreed well with the results of finite element and published experimental results.(4)Experimental studies and mechanisms analyses on shear strength of innovative steel wedge block-crossed inclined stud-UHPC connectionsThe innovative steel wedge block-crossed inclined stud-UHPC connection was proposed.The study entailed 36 push-out tests to consider the influences of the inclined stud angels,stud aspect ratios and cover thickness on the shear strength,stiffness and ductility,which was also compared with the conventional stud connections.Based on the test-validated three dimensional nonlinear finite element analysis and the test results,the inclined studs-UHPC interactions were studied.The applicability of such a connection to the composite beams was evaluated based on the stiffness reduction methods.(5)Flexural performances of lightweight steel-UHPC composite beams with ultra-short stud connectionsTen steel-UHPC composite beams under four-point loading were tested to study the flexural performances of steel-UHPC composite beams with ultra-short stud connections.The influences of longitudinal stud spacing,stud aspect ratios,UHPC slab thickness and shear span ratio were studied.The theoretical load-deflection was proposed,characterized by the yielding of steel girder,cracking of UHPC slab and UHPC crushing.Through the 3D nonlinear finite element model,the influences of stud diameter,stud spacing and UHPC slab thickness were investigated on the ultimate bearing capacity.Only 9%-16% flexural resistance was observed when the slab thickness was reduced from 55 mm to 35 mm and the mid-span deflection achieved 1/47-1/36 and 1/53-1/32 of the span length,respectively.It is shown that the thin UHPC slab-steel composite beam exhibited both good bearing and deformation capacities.(6)Flexural performances of composite beams for innovative steel wedge block-crossed inclined stud-UHPC connections(SWBCIS connections)In order to study the flexural performances of composite beams for innovative steel wedge block-crossed inclined stud-UHPC connections,a total of six steel-UHPC composite beams were tested to consider the influences of vary slab thickness、inclined stud angels and stud spacing.The results indicate that the flexural strength of the composite beams with SWBCIS connections and elastic stiffness were increased by 6% and 8%,respectively,while the mid-span deflection decreased by 13%,compared with the conventional stud connections.This indicated that SWBCIS connections had better interfacial load transfer and composite actions.Combined with the load-slip relationships from the push-out tests,the prediction of load mid-span deflection was proposed to consider the interfacial elasto-plastic slip distribution,which agreed well with the test results.The influences of vary slab thickness、UHPC slab thickness,concrete strengths and stud spacing on the ultimate bearing capacity of composite beams were further carried out by widely parametric analyses. |
