| Pile foundation is widely used in high-speed railway construction as the main foundation form of railway bridge.The high pile foundation has a free length,and there is no lateral constraint around it.It is easy to become a vulnerable part of the bridge foundation.The concrete structure adopts high strength steel bars to reduce the amount of steel bars,which has the important significance of environmental protection and sustainable development.500Mpa fine grain high strength steel bar(HRBF500)has high strength,high elongation,good plasticity and toughness,and has remarkable economic and social benefits.Based on this,according to the design characteristics of high-pile cap pile pier,the seismic performance and cumulative damage performance of the top of 500Mpa fine-grained reinforced high-pile cap pile are studied by combining experimental research,theoretical analysis and numerical simulation.The main efforts and results are as follows:(1)The damage evolution mechanism of the top of fine grain reinforced concrete piles under cyclic loading is studied.By changing the strength of the longitudinal bars,the ratio of the longitudinal bars,the strength of the concrete,the strength of the bars,the reinforcement method of the BFRP fiber cloth,the number of layers,and the loading system,low cycle repeated loading tests were carried out on the top specimens of 15 fine-grain reinforced concrete piles The variation laws of limit load deformation and energy dissipation ability of the specimens after different times of cyclic loading are obtained.The effects of different design parameters and loading system on the load-displacement curve,skeleton curve,stiffness degradation,ductility and hysteresis energy consumption were analyzed.The results show that the damage accumulates gradually with increasing the number of cycles and displacement.Compared with variable amplitude cyclic loading,the damage evolution of the sample under constant amplitude cyclic loading is relatively slow,and the total amount of hysteresis energy consumption is relatively large.The yield load and peak load of the top part of concrete pile equipped with 500 Mpa fine crystal high strength reinforced steel are higher than those of ordinary reinforced concrete,which shows good ductility.When using BFRP fiber cloth to strengthen,we should use the effective package method to provide enclosure pressure,and the marginal effect of multi-layer fiber cloth enhancement decreases rapidly.(2)Considering the equivalent constitutive model of damaged reinforcing bar,the user material subroutine UMAT of finite element software ABAQUS is used to develop the numerical stress calculation of the top specimens of fine grain reinforced concrete piles under reciprocating cyclic loads,The numerical simulation results are in good agreement with the experimental data,which vaifies the feasibility of the numerical calculation method.On this basis,the influence of axial compression ratio,longitudinal reinforcement strength,reinforcement ratio and concrete strength on the seismic behavior of the top of fine grain reinforced concrete piles is systematically studied.From the point of view of damage degradation,the damage development,stiffness degradation and energy dissipation capacity of concrete in plastic hinge zone are analyzed.(3)According to the moment-curvature analysis method of pile top and the test data of fine grain reinforced concrete pile top under six standard loads,the characteristic points of the three-fold skeleton curve and the calculating formula of the skeleton curve stiffness are deduced.According to the characteristics of the hysteresis curve obtained from the test,the hysteresis loop is simplified.The cyclic degradation index considering energy dissipation as damage index is introduced to describe the degradation of mechanical properties of fine grain reinforced concrete pile top under repeated loading.A restoring force model of fine grain reinforced concrete pile top considering damage effect is established.(4)The classical Park-Ang two-parameter damage model can not consider the effect of loading process on the cumulative damage of structure and the defect of boundary conditions.The modified factor based on low-cycle fatigue damage is introduced.The functional relationship between normalized hysteretic dissipation energy and normalized lateral displacement ratio is analyzed by parametric regression,and a modified damage model for top members of fine-grained reinforced piles is established.By controlling the damage threshold,the basis of seismic damage evaluation for the top of fine grain reinforced pile under various state levels is defined,and the existing damage model damage evaluation criteria are improved.(5)A theoretical model of low cycle fatigue life based on plastic strain energy density theory and inelastic buckling theory of compressive members is proposed.According to the theoretical model and the results of finite element analysis under constant amplitude loading,the corresponding program is compiled to calculate the low cycle fatigue life of the pile top model.The results show that the low-cycle fatigue life of the model is generally larger when the longitudinal bar fatigue fracture occurs;the fatigue life of the model is generally smaller when the longitudinal bar buckling failure occurs;it shows that the former failure makes better use of the materi al properties and has better fatigue performance. |
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