Study On Ductility And Seismic Behavior Of Concrete Hollow Pier Based On Quasi-static Test

In view of the shortcomings of the seismic design theory of concrete hollow bridge piers in China’s current code,this paper carried out the tests on 19 hollow pier model with typical hollow piers of highways and railway bridges as prototype based on a summary of the results of pseudo-static test of hollow piers at home and abroad,quantified the seismic performance level of bridge pier,and conducted a deep study on ductility index,plastic hinge model and the amount of confined stirrups in the plastic hinge,hoping to provide a powerful reference for the seismic ductility design of hollow piers.The range of ductility indicators of bridge piers under various seismic performance levels was quantified.Based on the evolution process of pier cracks during the failure process of hollow piers with round end and thin wall,the damage levels of five damage grades of bridge pier,including initial micro-crack,slight damage,medium damage,severe damage,and near collapse,were described,and the ductility index range of the bridge pier under each performance level was given.Combined with the quasi-static test at home and abroad and the statistical analysis of the bending failure pier column in the PEER database,this paper obtained that the displacement ductility ratios of solid pier and hollow pier with 90%guarantee rate were 4.5 and 4.2 respectively in the limit state,and the displacement ductility coefficients were 3.3 and 3.0 respectively.The curvature’s distribution characteristics along the pier body proposed by Park and Paulay was modified,and the formula for calculating the equivalent length of plastic hinge of the hollow pier was proposed.Based on the analysis of the applicability of existing models,the results of the plastic hinge length test and the main influencing factors of 35 rectangular hollow piers,the regression formula of equivalent length of plastic hinge of rectangular hollow pier was given.According to the experiment,the curvature of variable cross-section hollow pier with round end as nonlinearly distributed along the pier body,and the plastic hinge was moved up as a whole with extended range.The curvature distribution along the pier and deformation characteristics assumed by Park and Paulay were not applicable to variable cross-section hollow pier with thin wall.Therefore,the paper proposed to modify the theory of the curvature of plastic hinge along the pier body.The study shows that the equivalent length of plastic hinge is mainly affected by geometrical dimensions,axial compression ratio,compressive strength of concrete,the diameter of longitudinal bar and tensile strength,and is most affected by the calculated height of the pier and the material characteristic parameters(f _y d_b f_c’).Compared with the rectangular hollow pier with equal section,the variable-section hollow pier with round end had longer areas of plasticity hinge,which is 1.1 times of that of the rectangular hollow.The relationship between the design ratio of stirrup and the displacement ductility index of the plastic hinge area of concrete bridges with different section type was established.After combing the existing formula of stirrup content in plastic hinge area and analyzing the factors affecting the displacement ductility of the bridge pier,the paper obtained formula of the stirrup content based on the regression of the experimental data,and then evaluated the its rationality and effectiveness.The results show that the effects of the shear span ratio,ratio of stirrup and reinforcement ratio on displacement ductility coefficient of the pier are different;the effect of the mechanical stirrup ratio on displacement ductility coefficient is the most significant and it increases with the increase of the mechanical reinforcement ratio;the stirrup ratio calculated by the existing formula is generally lower than the actual value;compared with the formulas ACI318,AASHTO,and Eurocode 8 whose results are close to the actual values,the results obtained by formula in this paper agrees better with the actual value.The proposed formula can be used to estimate the amount of restrained stirrups in the plastic hinge area of solid and hollow piers under different axial compression ratios,shear span ratios and displacement ductility coefficients,so as to provide reference for the seismic design of bridge piers.Finally,the practicality of the research results through the calculation of displacement capacity of two engineering examples was discussed.The paper first analyzed the seismic performance of the railway hollow pier with round end and the highway rectangular hollow pier based on the Midas and CSiBridge finite element software,and then calculated the displacement capacity of the engineering example according to the results of the research on plastic deformation ability of the hollow piers,and finally combined with the seismic performance level and the quantitative ductility index criteria evaluate the ductile seismic performance of engineering examples.