Study On The Numbers Of Earthquake Ground Motions For Nonlinear Time-history Analysis Of Structures

Along with the rapid development of economy and performance-based seismic design concept constantly deepening in the field of earthquake engineering,dynamic time-history analysis has been one of the main methods for nonlinear dynamic calculation of structures.As a basic work of time-history analysis,ground motion input has been concerned by engineering and academia.However,the calculation results of time history analysis are greatly influenced by input seismic records,The results of different ground motion input sometimes can produce several times or even a dozen times difference.Therefore,how to select records and how many records should be selected as the input of time-history analysis is the most concerned problem for engineers at present.Futher more,proper quantity of earthquake ground motions is one of the key problems to ensure the reliability of structural nonlinear time-history analysis.The limiting values for the number of records in Cord for Seismic Design of Buildings are not clear enough and based on engineering experience.So,this study examines the required number of records statistically,such that the selected records provide accurate,efficient,and consistent estimates of "true" structural responses.With the conventional "double-spectrum" record-selecting method,in this study,The structures considered are 4 reinforced concrete frame with vibration stories equal to 3,6,9,12.Peak inter-story drift angles,top displacement and base shear were chosen as the damage measures.The results were compared with the benchmark values of large samples,and the reliability of the results under different quantities of earthquake ground motions were analyzed.Results show that,when the maximum value of the calculated results from various earthquake ground motions is taken as the final result,"2+1" is the most appropriate combination,it has the minimal computational effort and best reliability.When the average value is taken,the arithmetic mean values and geometric mean values show no significant difference,and "8+4" is the most appropriate combination,it is the best balance point between the reliability and the computational effort.Considering the structure under nonlinear state,the ground motion records are selected employing structure yield cycle instead of basic cycle.Comparing the results of small samples and large samples.With multiple ground motion records input,the study indicates that "2+1" is optimum when the maximum is set as the calculated result,which has simple amount and better reliability;"9+4" is optimum when the mean is chosen,which is the best balance point between the reliability of the results and the workload of calculation.