Study On Displacement-based Seismic Design Method For Pile-supported Wharves

The displacement-based seismic design method has been adopted in several foreign codes and standards for pile-supported wharves.In comparison with traditional force-based method,this method is intended to allow the engineers to design for structural safety while considering the economic aspects of seismic performance and post-earthquake recovery.Although this method has been applied in overseas wharf engineering for many years,some problems still need to be studied deeply.In the displacement-based method,the deternination of demand displacements with accuracy and reasonability is the key.In current practice,the substitute structure method is one of the major methods for demand analysis,in which the evaluation of equivalent damping ratio is an important step.Unfortunately,the calculating equations for damping ratio in current practice do not account for the structural characteristics of wharves and the contribution of soil to the dissipated energy of wharf system.Moreover,the equations determining dynamic magnification(DMF)which is utilized in demand analysis to address simultaneous seismic excitation in orthogonal horizontal directions and torsional effects,have limited applicability.Moreover,the determination of free-field earthquake-induced permanent embankment deformation is a significant step during the evaluation of kinematic loading for codes and standards with displacement-based method.Though many calculating equations for embankment deformation have been proposed,all of them cannot be adapted to the site classification and earthquake characteristics in China.Therefore,based on the displacement-based method in overseas codes and standards,the wharf damping and demand analysis methods are studied in this paper,the main contents and conclusions are as follows:(1)The influencing factors for nonlinear static demand analysis(Pushover analysis)of wharf are studied.By using substitute structure method and N2 method,a case study was conducted to determine the displacement demands.In the study,the effects of Pushover curve linearization methods,calculating equations for damping ratio and design response spectra were taken into account.It is found that the differences between the demand displacements determined by substitute structure method and N2 method depend on the damping equation utilized by substitute structure method.It is also shown that the Pushover curve linearization methods have a little influence on the demand displacements determined by the substitute structure method,while the damping equations and response spectra have a significant one.(2)The equivalent damping ratio equation for pile-supported wharves is derived based on the dissipated energy of pile plastic hinges and soil springs.By considering the energy dissipated by wharf under earthquake action as the sum of energy dissipated by pile hinges and soil springs,the damping equation was determined based on the Jacobsen’s approach.Then the equations determining energy dissipated by pile hinges were established on hinge moment-rotation curves and relevant hysteretic models,and equations for soil springs were determined based on p-y curves and the Masing rule.Finally,the accuracy and applicability of proposed damping equation were validated by conducting nonlinear time-history analyses for a wharf with 60 ground motions.(3)Based on typical hysteretic models,the equivalent damping ratio equations for wharves are proposed.Through cyclic pushover analyses of typical wharves,hysteretic characteristics of wharves were investigated.It is found that the Pivot hysteretic model and Masing rule can mimic the hysteretic characteristics of concrete and steel wharves,respectively.Then,the damping ratio equations were determined based on the Jacobsen’s approach with above two hysteretic rules.To verify the accuracy and applicability of proposed damping ratio equations,three case studies were conducted by comparing the demand displacements determined by time history analyses and substitute structure method with various damping equations.(4)The equation determining dynamic magnification factor(DMF)to address the simultaneous seismic excitation in orthogonal horizontal directions and torsional effects is proposed.By developing an equivalent mechanical model for actual wharf and considering the bi-directional horizontal seismic inertial forces applied on the wharf simultaneously,a DMF equation is derived based on conditions for static equilibrium,which can be expressed as a function of eccentricity ratio and aspect ratio of wharf segment.Finally,the accuracy and applicability of proposed equation were verified by conducting nonlinear time-history analyses of actual wharves with 40 pairs of bi-directional horizontal ground motions.(5)Based on the artificial records compatible with the design spectra of code for water transportation engineering in China,a simplified calculating equation for earthquake-induced permanent slope deformation is proposed.By conducting the Newmark sliding block analyses with 3600 spectra-compatible records,the equation for permanent deformation was determined,as well as its coefficient of variance and probability distribution.It is found that permanent deformation can be expressed as the function of yield acceleration for wharf slope,and it seems to follow logarithmic normal distribution.Moreover,by referring to seismic design codes and standards in abroad,the checking criteria for permanent deformation are recommended.