Ground-motion Non-stationarity Controlling And Its Application In Engineering Practice

The non-stationarities of ground motion have great impact on seismic site responses and structural responses.However,because of lacking an efficient controlling method for ground-motion non-stationarities,it is difficult to quantitively study the influences of nonstationary parameters on structural responses.Thus,the application of ground-motion non-stationarities is still a niche in engineering practice.To facilitate engineers choosing proper non-stationary parameters based on site conditions and characters of structures,we first propose a new stochastic ground-motion model for engineering practice which can describe the non-stationarities of ground motion statistically;then,based on the new model,we propose a new ground-motion simulation method for matching multiple targets to achieve quantitative control of non-stationarities.Using the proposed nonstationary modal parameters and ground-motion simulation method,we further study two applications on the ground-motion non-stationarities.First,we study the impact of bedrock-motion non-stationarities on seismic site responses by controlling the non-stationary characters of bedrock motion.Second,improving design of experiments(DOE)methods,we establish surrogate models of the quantitative relationships between non-stationary parameters and intensity measures(IMs)of structural responses(with various structural characters).Based on the surrogate models,we obtain the non-stationary parameters corresponding to the maxima of the intensity measures to consider the adverse effect of non-stationarities.The framework of this study can facilitate engineers to choose proper ground-motion nonstationary parameters for both artificial ground-motion simulation and record selection according to structure characters,which can provide suitable seismic input for seismic design.The results of this study are as follows.(1)A new stochastic ground-motion model is established based a novel energy-compatible equivalent intensity modulating function.This model uses physical-based,simple,and uncoupled intensity and frequency models to describe the temporal and spectral non-stationarities of ground motion,suitable for engineering practice.The statistical study of the modal parameters produces the ranges of the parameters and the correlation among them.The results show there is strong correlation among the three temporal nonstationary parameters but week between the two spectral parameters.(2)Based on the proposed stochastic ground-motion model that captures non-stationarities of ground motion,we established a method for simulating ground-motion compatible with multiple targets.This method adjusts spectral acceleration period by period by adding acceleration impulse response functions to match targets such as the peak ground acceleration,response spectral acceleration,peak ground displacement,and temporal and spectral non-stationarities.The new method can quantitively control the nonstationary ground-motion characteristics approximately.(3)The impact of bedrock-motion non-stationarities on seismic site repones are analyzed by controlling the nonstationary characters of bedrock motion.The results show as follows:i.For bedrock motion with significantly time-varying frequencies,ignoring of spectral non-stationarities in ground-motion simulation may underestimate the hazard on ground surface.The harder the soil and the higher PGA_rock is,the more the results will be underestimated.ii.The influence of the time-varying frequency on the high-frequency parts of SA_surf correlates with the concentrated time of high-frequency contents(2?5Hz)and characters of the site:if the high-frequency contents of bedrock motion concentrates at the strong-motion phase,SA_surf obtained from the temporal-nonstationary-only artificial motions on harder site is more conservative.iii.The influence of the time-varying frequency on surface/bedrock response spectral ratio(RSR)correlates with the site classification and natural vibration frequency(1))of structures.If PGA_rock is high,the influence is more distinct at?5Hz on harder site;while,on softer sites with classification of III or IV,the influence is more distinct at?1Hz.iv.When simulating bedrock motions,especially those with high peak values,one should use the fully-nonstationary method and combine the factors like natural vibration frequencies of structures,site condition,and requirement of a specific project,to consider the spectral nonstationary characters.(4)Surrogate models for quantitative relationship between ground-motion nonstationary parameters and IMs of structural responses are established by introducing the DOE method into the study on ground-motion characters.The results show as the following.i.Conducting statistical analysis of non-stationary parameters and grouping by correlated parameters,we design four groups of experiments using the uniform design method.Grouped uniform design not only guarantees uncorrelation between factors and space filling of factors in each group,but also avoid unreal parameter combinations in the simulation-based experiments.ii.Two intensity measures,i.e.peak accelerationand root mean square acceleration₈)of structural responses,are chosen based on structural characters as suitable target response indexes for their low dispersion and high correlation with nonstationary parameters.For every intensity measure,there are acceleration,velocity,and displacement indexes,respectively,which should be chosen according to their sensitivity to non-stationary parameters in engineering practice.iii.Introducing in the surrogate-based method,surrogate models between temporal and spectral non-stationarities of ground motion and intensity measures of elastic and elastoplastic structural responses are established.Using these models,impact of ground motion non-stationarities on structural responses can be analyzed.One can also easily choose proper nonstationary parameters for seismic input in engineering practice.iv.Main effects of nonstationary parameters play a predominant role in the proposed surrogate models;while,whether the interactions make significant contribution is related to the natural vibration frequencies and intensity measures of structures.v.When conducting simulation-based experiments using the uniform design method based on simulated ground motions,the surrogate models established by simulating only one ground motion of each parameter combination are almost the same with those by 7 uncorrelated motions,which can further reduce the experimental number.This can massively save costs for analysis on complex structures.vi.Based on the surrogate models proposed in this paper,the nonstationary parameters of ground motion corresponding to the maxima of intensity measures can be obtained to consider the adverse effect of ground motion on structures.The influencing trend of non-stationarities on structural response is different for not only various structural characters but also different types of ground motion.Therefore,one should consider factors including site conditions, structural characters,and ground-motion types(synthesized accelerograms/records)to choose proper ground-motion nonstationary parameters in engineering practice.