A Study On Damping Modification Factor For Acceleration Response Spectrum From Subduction Slab Earthquakes

Earthquake is one of the major natural disasters.At present,it is not possible to accurately predict the time and location of earthquake occurrence,and an effective earthquake disaster mitigation is to improve the seismic performance of buildings,thereby to reduce the major economic losses and human casualty caused by large earthquakes.China is one of the most earthquake-prone countries with a large population and a large number of large earthquakes,and it is especially important to improve the aseismic design of buildings and other structures.The seismic code in China provides a design spectrum with a damping ratio of 5% and a simple damping modification factor(DMF)model.A damping ratio of 5% is not suitable for all types of structures.For example,a steel structure may have a damping ratio of 2% whereas the structures with seismic isolation devices or a structure with added damping devices may have a damping ratio of up to 25%.The simple DMF model in the code for China may not be suitable for the ground motions for subduction slab earthquakes.Most of the researches on the damping ratio modification factor(DMF)in the past has been mainly for developing DMF models for the displacement spectrum.When the acceleration value is large,the building contents would be damaged,and some important buildings such as hospitals would not be able to resume the normal functions the building is designed for.A total acceleration spectrum is therefore required to estimate the acceleration of the structure.Although pseudo-acceleration can be estimated from a displacement spectrum,at a long period and a large damping ratio,the pseudo-acceleration spectrum differs significantly.Therefore,a DMF model for an acceleration spectrum is very important for the design of these structures with energy dissipation device.In this study,4,695 strong-motion records obtained by the K-net and Ki K-net networks from subduction slab earthquakes were used to develop two DMF models.The first model is referred to as the simple DMF model.This model does not include the moment magnitude,focal depth,source distance but include spectral periods and site effect.This model can be used to scale a 5% damped design spectrum that is not associated with a specified earthquake with a known magnitude,fault top depth and source distance.The second DMF model includes akl groundmotion parameters,i.e.,magnitude,fault-top depth,and source distance.This model can be used to adjust the design spectrum from a specified scenario earthquake in the subduction zone.The main contents of this thesis include:1.Establish a simple DMF model that is a simple and continuous function of both damping ratios and spectral periods.To avoid error propagation from one damping ratio to another,model coefficients were determined in two steps by a fixed effects method,coefficients for damping ratios first,and spectral periods second.The continuous functions of damping ratios and spectral periods were used to model the effects of these two parameters,leading to reduce the model parameters significantly.Next,a random effects model was used to separate the residual and standard deviation into between-and within-event parts and the within-site residuals and standard deviation were further separated into within site and between site parts.The goodnessof-fit parameters include the maximum log-likelihood,the distributions of various residuals with relevant model parameters,and various standard deviations;2.Using residual analyses to investigate the effects of the ground motion parameters on the simple DMF model.The distributions of various residuals and the trend lines were used to define the functional forms for various ground motion parameters.The functions accounting for moment magnitude and fault-top depth were designed and the coefficients were derived by a fixed effects regression method for each period and each damping ratio.Through the analyses of within-site residuals,coefficients for the anelastic and geometric attenuation terms were determined for each period and each damping ratio.The initial values for the coefficient derived from residual analyses were fit to functions that satisfy boundary conditions,i.e.,at a damping ratio of 5% all functions have a value of 1.0,and all functions approach 1.0 as period approaches to 0.0.All coefficients were derived by a fixed effects method in an iterative manner.Because the parameters were obtained in a step by step manner,a constant term was added to correct the errors accumulated in the step by step method.3.The total,between-event and within-event residuals and standard deviations were obtained by using a random effects model;the within-event residuals and standard deviation were further separated into the between-site and the within-site residuals and standard deviation.4.Combine the simple DMF model and the functions of each ground motion parameter to form a full DMF model.The random effect model was used to separate the total standard deviation into various standard deviations of the full DMF model.Analyses of residuals were performed again to ensure that the distributions of all residuals were not significantly biased.A linear function of the logarithmic damping ratios can be used to model these standard deviations.5.The expressions of the two models are multiplied by the spectral values of the Zhao(2016c)model,respectively,to obtain smooth spectra with different damping ratios,suggesting that the DMF models are suitable for engineering designs.