Study On Low-cycle Fatigue Cumulative Damage And Lateral Capacity Of Reinforced Concrete Pier Columns Subjected To Flexural-shear Failure

Structures are subjected to displacement reversals under strong earthquakes,so the earthquake-induced structural damage is essentially caused by the cumulative damage of low-cycle fatigue.Therefore,it is critically important to study the low-cycle fatigue damage of the structure under earthquake loads.Under such loading scenarios,piers and columns of bridges or other structural systems may sustain damage in the form of flexure failure,flexural-shear failure,or shear failure.Research to update has been focused on the low-cycle fatigue behavior of piers and columns in the case of flexure failure rather than flexural-shear failure.Additionally,the lack of studies on the correlation between the flexure behavior and flexural-shear behavior of piers and columns under the critical factors that influencing their failure mechanisms,the load capacity of such components as determined by empirical equations differed substantially from the experimental results.In this dissertation study,the low-cycle fatigue tests of reinforced concrete pier columns were conducted to investigate the effect of low-cycle fatigue behavior on the pier columns governed by flexural-shear failure mechanism.Based on the result analysis of the reinforced concrete pier columns subjected to flexural-shear failure mechanism,this study proposes an enhanced method for determining the lateral capacity of such structural components.The major scope and findings of this dissertation study are summarized as follows:(1)Pseudo-static loading tests of 23 reinforced concrete pier columns were conducted to investigate the low-cycle fatigue behavior of pier columns in response to the variation of loading cycle numbers(i.e.,3,10,and 20 cycles)during both constant-amplitude and variable-amplitude load reversals.The test results demonstrate that increasing of the displacement amplitude reduces the numbers of loading cycles at the occurrence of specimen failure and the cumulative hysteretic energy as well as significant strength and stiffness reduction at the initiation of failure.In addition,the residual deformation increased as a result of the increase of the loading cycle number.(2)Based on the test results of this study and those in the literature,the relationship between the low-cycle fatigue life of the reinforced concrete pier columns subjected to flexure and flexural-shear failure and the drift ratio is established,and the criteria for the nonlinear low-cycle fatigue cumulative damage of piers and columns are established,and consequently the evaluation procedure of low-cycle fatigue cumulative damage of reinforced concrete piers and columns under earthquakes is proposed.Since the earthquake loading histories are considered in the proposed method,the analysis results indicate that the method as proposed in this study enhances the seismic damage predictions compared to those displacement-based damage prediction results.(3)The qualitative analysis of the transformation process of flexure failure to shear failure of reinforced concrete pier columns is carried out,and the design parameters that affect the conversion of flexure failure to shear failure are analyzed,including the aspect ratio,the characteristic value of the longitudinal reinforcement,the characteristic value of the hoop and axial compressive ratio.A formula for calculating the displacement ductility coefficient of the descending point and the failure point of the lateral capacity curve of the pier columns with these parameters as variables is established.The results show that the fiber model method of flexure analysis is used to calculate the lateral capacity of the flexure control section of pier columns,and the lateral capacity of the shear control section is calculated using the curve of falling point and the failure point determined in this paper.The lateral capacity curve of the flexural-shear failure pier columns based on the proposed method is in good agreement with the hysteretic curve envelope obtained from the test.(4)The lateral deformation of reinforced concrete pier columns members consists of deformations induced by flexure,slip and shear effects.Based on the studies on constitutive relation of materials and deformation mechanism of each deformation components,the rotation of plastic hinge region of pier columns is used to monitor flexure and shear failure initiation,a macro-element load-deformation analysis model of reinforced concrete pier columns is proposed considering the influence of the stiffness and strength degradation subjected to loading cycle number.The analysis results show that the load-deformation curve calculated by the model in this paper agrees well with the test hysteresis curve,and can be used to calculate and analyze the nonlinear hysteretic behavior of the reinforced concrete pier columns subjected to flexural-shear failure considering the cyclic degradation effect.