Study On The Shear Behavior Of High Strength Bolts In Steel-SFRC Composite Beams

Steel-concrete composite beams have been constructed extensively in civil engineering for many years,owing to its outstanding properties,such as favorable mechanical properties,easy construction and better economical performance.The shear connector,as an important component of composite beams,directly affect the overall work performance of composite beams.As one of the most important shear connectors,high-strength bolts are preferentially used in the prefabricated composite beams,owing to the characteristics of high strength and stiffness,tight connection and convenience for dismantling.However,due to lower load at the initial slip and cracking in concrete slabs,the properties of high-strength bolts may not be fully developed.The application of steel-fiber-reinforced concrete(SFRC)with high tensile strength and crack resistance to composite beams with high-strength bolts will improve the shear behavior of high-strength bolts and enhance the potential of composite beams.Therefore,the shear performance and its calculated method for the high strength bolts in steel-SFRC composite beams have been studied.The main contents of the thesis are stated as follows:(1)Experimental study on the optimum volume content of steel fiber in concrete.Since the mechanical properties of SFRC are affected by many factors,it is not clear about the suitable steel fiber volume content in SFRC.Based on the study on mechanical properties of SFRC,the appropriate amount of steel fiber was determined as 1.0% in SFRC,comprehensively considered the mechanical properties and workability of SFRC.(2)Experimental study on the shear behavior of high-strength bolts.Twenty-four push-out specimens with different high-strength bolt pretensions,its diameters and tensile strengths and concrete properties,were tested.Based on the push-out test results,the failure modes and load-slip curves of the tested specimens were discussed accordingly.The load at the initial slip,the initial shear stiffness,the post slipping stiffness,the ultimate shear strength and the ultimate slipping for the high-strength bolts,were defined reasonably to study the shear behavior of high strength bolts quantitatively.The differences of shear behavior between studs and high-strength bolts were discussed.Based on the results of push-out tests,the performances of high-strength bolts in steel-normal concrete(NC)specimens and in steel-SFRC specimens were discussed.The tested results showed that the ultimate shear strength and the post slipping stiffness of high-strength bolts in steel-SFRC specimens was higher than that of in steel-NC specimens.Compared to NC specimens,the extension of cracks on SFRC slabs were limited and the width of cracks efficiently decreased owing to the addition of steel fibers in concrete and the properties of high-strength bolts were fully developed in steel-SFRC specimens.(3)Study on the load transfer mechanism and the failure mechanism of high-strength bolts,as well as the extensive parametric influenceing on the shear behavior of high-strength bolts.Finite element models,which confirmed by the tested results,were also employed to perform the parametrical analysis involved in concrete compressive strength,as well as bolt pretension,and diameter and tensile strength.And the load transfer mechanism and the failure mechanism of high-strength bolts were also analyzed by the finite element models.Based on the orthogonal experimental method,finite element models were further designed to investigate the significant influencing factors on the shear behavior of high-strength bolts.(4)Study on the shear behavior of high-strength bolts under shear and tensile force.Combining the results from push-out test and the finite element models under shear and tensile force,the load transfer mechanism,the failure mechanism of high-strength bolted connectors and the influences on the shear behavior of high-strength bolts under different tensile force were analyzed.The results showed that the load at the initial slip,the initial shear stiffness,the post slipping stiffness and the ultimate shear strength for the high-strength bolts decreased with the increase of the tensile force.(5)Study on the calculation method of mechanical behavior of high-strength bolts in composite beams.The shear properties of high-strength bolts in composite beams were analyzed by finite element models.The results showed that the shear behavior of high-strength bolts obtained from the push-out specimens are able to feasibly predict the shear behavior of high-strength bolts in composite beams.Upon the experimental and the FE results,practical calculation formulas were further proposed to properly estimate the load at the initial slip,the initial shear stiffness,the post slipping stiffness,the ultimate shear strength,as well as the relationship between tensile and shear forces and the load-slip relationship of high-strength bolts in steel-SFRC composite beams,which provide a convenience for the application of high-strength bolts in steel-SFRC composite beams.