Research On Aseismic Ductility Of Railway Round-end Hollow Piers Based On Quasi-static Test And Shaking Table Test

Compared with solid-section piers,hollow-section piers have the advantage of saving materials,reducing the self-weight of structure and mitigating the seismic response.The central and western parts of China are mountainous areas,where the main features are high terrain,steep descent of the earth surface height,difficulty in construction of highways and railways,high ratio of bridges to railway line and a number of major traffic routes passing through active earthquake regions.High-pier and long-span bridges are widely constructed in these major traffic routes.To ensure the seismic safety of these bridges,it is necessary to study the seismic design theory and method suitable for hollow piers.In the present literature,researchers concern mainly on rectangle section and circular section hollow piers,but the study on round-ended section hollow piers which are widely used in railway bridges barely yields recognized results.Besides,experimental study is an effective way to discuss the aseismic properties of reinforced concrete piers.The existing research on the hollow piers is mainly based on the quasi-static test,while the shaking table test is relatively rare.It is the key of performance-based seismic design for bridges to understand the nonlinear seismic response laws and failure mechanisms of piers.A comprehensive research was carried out from the perspectives of the failure mechanism,deformation component and ductility performance of the round-end hollow piers based on large scale quasi-static test and shaking table test.The main contents and results are as follows:(1)Experimental phenomena of hollow piers with solid part at the bottom shows that,the macroscopic damage process is similar to that of the existing hollow pier test,but the damage details has its particularity.The specimen of flexural failure experienced concrete cracking,steel reinforcement yielding,concrete flaking,concrete flaking area expansion,core concrete crushing and buckling of steel reinforcement.In the failure details,due to the end part of piers is solid section,under cyclic loads,the most vulnerable area no longer locates at the bottom of the pier,but the border chamfering area between hollow section part and solid section part.Observed cracks of shaking table test can reach similar conclusions.There is a difference of inclined cracks distribution between static specimens and shaking table specimens.Static specimens of high axial pressure have obvious inclined cracks after equivalent yield point,but shaking table specimens are observed less inclined cracks.This is due to the large shear span ratio and varying axial force of the shaking table specimens.(2)On the basis of qualitative analysis of specimen damage,the damage status of static specimens was quantitatively classified,and through the study,quantitative assessment damage state of dynamic specimen.Based on the summary of damage process in quasi-static test,combining bridge performance levels proposed by Hose,performance levels of round-end hollow piers are classified by displacement ductility,the quantization results were consistent with the existing results,and the index range of I and II was refined.According to performance levels of specimens researched by quasi-static test,damage levels of specimens in shaking table test under a series of PGA were evaluated.The results show that even under high-level earthquake and super-intensity earthquake,only minor damage can be found in the specimens which means the aseismic performance is excellent.The results of the evaluation are consistent with the actual damage condition of the dynamic specimen.Therefore,for this kind of high piers,it is advisable to use displacement ratio to evaluate the damage state of elastic and weak nonlinear(3)Considering the damage characteristics of specimens,the theoretical formula was adopted to calculate the deformation components of the round-end hollow piers in bending failure.In the calculation of bending deformation,the plastic deformation of the hollow pier was calculated from the failure section,and the solid part of piers was calculated as elastic deformation.The results show that: The bending deformation in the yielding stage is almost 90% of the total deformation of the pier,and in the ultimate stage is 69%~73%.The slip deformation accounted for about 7.5% of the total deformation in the yielding stage,and accounted for 22%~26% in the ultimate stage.High axial pressure can reduce slip deformation and for the specimens with well ductility,the transverse rebar rate has little effect on the slip deformation.The shear deformation was barely produced during the yielding stage and SB-1 shows least shear deformation which proves that low axial pressure can reduce shear deformation.Shear deformation ratios of all specimen are less than 10%.Therefore,although quasi-static specimens with high axial pressure have obvious inclined cracks,the shear deformation is negligible.(4)Based on deformation components results,the static and dynamic specimens were simulated numerically,and the applicability of existing calculation method for displacement ductility was discussed.fiber beam models in OpenSEES which take flexural and slip deformation into consideration were applied to simulate quasi-static test and calculate displacement ductility capacity.The results of numerical simulation were compared with theoretical formula calculation.The research shows that: Pushover and formula method are applicable to calculation of displacement ductility of middle and low pier.Pushover results has higher precision and theoretical formula method is relatively simple.When adopting theoretical formula method,results based on current highway bridge seismic design code is conservative and based on Leman separate formula fits the experiment better.The natural vibration characteristics of shaking table specimens and the analysis results of dynamic response show that the contribution of its high-order mode is not negligible,and this type of bridge pier needs to be analyzed by IDA method.In order to verify the necessity of IDA method for the calculation of the displacement ductility of the hollow round-end high pier,the dissertation compares the calculation results of IDA and static Pushover and static formula method.The calculation result of static method is larger than that of IDA,and the static method overestimates the displacement ductility of the hollow high pier.Therefore,it is necessary to use IDA method to calculate the displacement ductility capacity of hollow high pier.