Mechanical Behavior And Bearing Capacity Calculation Of Steel Fiber Reinforced High Strength Concrete Pile Cap

Pile cap forms an important connecting link between the preceding and the following in all forms of pile foundation. With the development of modern hydraulic structures, offshore drilling platforms and high-rise buildings, further requirements on the mechanical behavior of pile caps are presented. The current design methods of pile cap were mainly derived from the research results of ordinary strength concrete pile cap, but little has been reported on the mechanical behavior of high strength concrete pile cap. Based on the model experiments, finite element numerical calculations and theoretical analyses, the mechanical behavior and bearing capacity calculation of steel fiber reinforced high strength concrete (SFHSC) pile cap were investigated in this dissertation. The main contents are as follows.(1) The experiments of 10 specimens of steel fiber reinforced high strength concrete (SFHSC) three-pile caps were conducted to clear the mechanical behavior of pile caps. The crack distribution, load-deflection curve, concrete strain and longitudinal reinforcement strain of pile caps were measured. The factors, such as the steel fiber volume ratio, effective thickness, reinforcement ratio and reinforcement layout, and the concrete strength, on the bearing capacity of the pile caps were analyzed. The results indicate the failure mode of the SFHSC three-pile caps is the punching failure, and its transfer mechanism can be similar to strut-and-tie model. The specimens failed as a result of diagonal splitting or crushing of compression struts. With the increase of the steel fiber volume ratio and the effective thickness, the bearing capacity of pile caps improves evidently. The reinforcement ratio and reinforcement layout also have some effects on the bearing capacity of pile caps.(2) Based on the finite element model of SFHSC three-pile caps, the load-deflection response, the stress distributions of concrete and reinforcement in the whole loading process from initial loading, cracking, yielding until failure were investigated, and the bearing capacities of SFHSC three-pile caps predicted by finite element analysis were compared with the experiment results. It is further demonstrated that the failure mode of the SFHSC three-pile cap is the punching failure.(3) The experiments of 17 specimens, which have various steel fiber volume ratio, effective thickness, reinforcement ratio and concrete strength, were conducted to observe and analyze the crack distribution, load-deflection curve, concrete strain and longitudinal reinforcement strain, and failure mechanism. The results indicate the failure mode of the SFHSC four-pile caps with the moderate reinforcement is the flexural failure. The flexural tensile stress is resisted by reinforcement and steel fiber reinforced concrete. The ultimate moment capacity of the pile cap is improved evidently with the increase of the effective thickness and the steel fiber volume ratio. Also, the increase of the longitudinal reinforcement ratio enhances the strength and deformation characteristics of pile cap.(4) The failure mechanism and bearing capacity calculation of the SFHSC three-pile caps were studied. The results indicate the strut-and-tie model more accurately explains the mechanical behavior of the SFHSC three-pile caps. Based on the results of the study, the strut-and-tie models were respectively presented to describe the punching behavior of pile caps which fail in the diagonal splitting or diagonal compression, and the relative formulas for calculating the punching shear capacity of SFHSC three-pile caps by strut-and-tie model approach were proposed.(5) The failure mechanism and bearing capacity calculation of the SFHSC four-pile caps were studied. The results indicate the failure mode of the SFHSC four-pile caps is the flexural failure. The plastic calculation formulas of flexural capacity for the SFHSC four-pile caps, which correspond with the suggested failure mechanism and consider the effect of the steel fiber characteristic value, were put forward by using yield line theory. Furthermore, the practical formula of bearing capacity for the SFHSC three-pile caps according to the failure mode of the cantilever beam was proposed.The study of this paper serves as a reference for the ’Technical Specification for Fiber Reinforced Concrete Structure’ in China.