Research On Repairable Seismic Design Theory And Method For Hollow Piers In Near-fault Region

The performance-based seismic design conception requires structures to meet the corresponding performance objectives at multi-level seismic fortification,which is embodied in three-level fortification principles,non-damage during small earthquake,repairable during medium earthquake and non-collapse during large earthquake,where the fortification target of reparability requires the bridge to possess repairable value under design earthquake.However,there is a lack of quantitative calculation method for bridge reparability in the current seismic design code of China,which may result in the loss of repairable value for some bridges suffering non-extreme earthquake and causes unnecessary wastes of human and material resources.At present,scholars home and abroad have made some achievements in the research area of bridge reparability.However,most of the existing research results focus on the conventional far-field ground motion,which are limited in the applicability for nearfault pulse-velocity ground motion.In order to meet the construction requirements of a large number of near-fault bridges in the traffic network of western China,it is of great theoretical significance and practical value to establish a repairable seismic design method for near-fault bridges.To establish a near-fault reparability design method requires solving two core problems.First is objectively and accurately classifying the level of reparability and determining the aseismic capability of the structure.Second is establishing a near-fault residual displacement coefficient spectrum to clarifying the earthquake demand of the structure.The main contents of this paper are organized as follows:(1)The pseudo-static tests of 14 reinforced concrete rectangular hollow piers are carried out to discuss the influence of shear span ratio,reinforcement ratio,stirrup ratio and section shape on the seismic behavior of bridge piers.A constitutive formula of confined concrete is deduced to explain the difference between hollow section pier and solid section pier in seismic property.The repairable level of reinforced concrete rectangular hollow bridge piers is quantitatively defined by a variety of performance indexes,mainly residual displacement and displacement ductility ratio,supplemented by stiffness degradation coefficient and Park-Ang damage index.(2)A modified BWBN hysteretic model is established,which is suitable for the research of bridge reparability.The hysteresis model is developed by introducing peak displacement influence factor r into BWBN hysteresis model based on the analysis of unloading stiffness variation of hysteretic curve.A numerical solution on the basis of the fourth order RungeKutta method is compiled.Local sensitivity analysis method and global sensitivity analysis method are carried out to classify the parameters of hysteretic model into core parameters,main parameters and secondary parameters according to their importance,which lays a foundation for the further study of the hysteretic model.(3)A parameter identification method based on PSOGSA hybrid optimization algorithm is established to identify the hysteretic model parameters corresponding to each experimental model.The parameter interval of hysteresis model for rectangular hollow pier is summarized.The correlation between pier design parameters and hysteretic parameters is developed by regression analysis,and the empirical prediction method of core parameters and main parameters in hysteretic model is deduced.It is suitable for predicting hysteretic characteristics and seismic performance of hollow bridge piers without measured hysteretic curve,which provides an effective way for engineering application of the new hysteretic model.(4)Based on the new hysteretic model,a single-degree-of-freedom elastoplastic system was constructed,and the velocity-pulse effect of near-fault earthquake is considered by the two-period normalized method(TTN).226 pulsed ground motions in NGA-West2 are taken as the input seismic set,and the TTN residual displacement coefficient mean spectrum is generated by using MATLAB to compile the dynamic calculation and statistical analysis program.The influence of several factors on residual displacement is discussed.A TTN residual displacement coefficient design spectrum is generated by nonlinear numerical fittings.(5)Based on the quantization results of repairable performance and TTN residual displacement coefficient design spectrum,a repairable seismic design method for near-fault bridges is proposed.Taking a rectangular hollow pier simply supported bridge as example,the basic process of the repairable seismic design method is illustrated by using displacement ductility ratio and residual displacement as target performance indexes.The accuracy of TTN residual displacement coefficient spectrum is verified by nonlinear time history analysis.