Study On Seismic Response Of RC Frame Structure Considering Two Consecutive Earthquakes

A large number of earthquakes show that the case only a single event occurs during the earthquake (i.e. isolated earthquake) is less, indicating that the probability of isolated earthquakes is pretty low and generally within 11%. The occurrence of strong mainshock is usually accompanied by many aftershocks.In most case,short time interval between the aftershock and mainshock results in further damages to structure without any reinforcement,which means more serious damage to structures or even collapse. However, in most current codes all over the world, the seismic design methods consider the effects of single earthquake, usually the mainshock, on structures only.Therefore, it is of important theoretical significance and engineering value to study the proper method to consider the influence of multiple earthquake effects on the structure.In this thesis, seismic analysis method for structures under the excitation of two earthquakes is studied with a typical RC frame as an example, utilizing collected ground motions at home and abroad which are widely used in the seismic responses analysis.The method for determining the adjustment coefficient of design PGA is discussed to consider the excitations of two consecutive earthquakes. According to the differences of damage states of the structures under single and two consecutive earthquakes considering various fortification intensities, site conditions,and ground motion characteristics, the adjustment coefficient of design PGA is determined. The main contents include the following parts:1. The damage index and strength reduction factor model are chosen. The relation and difference between the single and two consecutive earthquakes are compared in terms of the acceleration time history, the response spectrum, the Fourier spectrum and the structural damage. By analyzing the application range of different damage indexes,the overall damage index is finally used. Through the comparison of three strength reduction coefficient models, the Vidic's model is selected.2. A seismic analysis method considering the excitations of two consecutive earthquakes is proposed. The seismic analysis methods commonly used to analyze single seismic action are selected, and the advantages and disadvantages of these methods are analyzed by taking typical structures as examples. Combining dynamic time history analysis, static cyclic loading-unloading analysis and Pushover analysis, a new method for seismic analysis of structures considering the excitations of two consecutive earthquakes is proposed. The feasibility of this method is validated with a three layers of typical RC frame structure.3. Based on the principle of equivalent damage, a method for determining the adjustment coefficient of design acceleration peak considering the excitations of two consecutive earthquakes is proposed. Through selecting and constructing sequence type ground motions, the dynamic elastic-plastic time history analysis are carried out for a typical three layers RC frame structure with 600 single earthquakes and 3600 consecutive two-earthquakes. The statistical characteristics of the structural overall damage index (DI) are analyzed. By defining the amplification coefficient of overall DI and the equivalent adjustment coefficient of PGA, the suggested value of the adjustment coefficient of the acceleration peak value for structure design considering the action of two consecutive earthquakes is finally given.4. Empirical equations for the amplification coefficient of DI and the equivalent adjustment coefficient of PGA under the excitations of two consecutive earthquakes are established. The dynamic elastic-plastic time history analysis of different typical RC frame structures are carried out and the characteristics of the structural overall damage index are studied. The conclusion that the amplification coefficient and the equivalent adjustment coefficient are proportional to the structural vibration period is obtained.5. The influence of the fortification intensity, site conditions and input ground motion records on amplification coefficient and equivalent adjustment coefficient are studied. Through the dynamic time history analysis of the four layers RC frame structures considering the influence of fortification intensity, site conditions and input ground motion records, and according to the statistical analysis of the overall damage index, it is found that amplification coefficient and equivalent adjustment coefficient are proportional to the fortification intensity, and are inversely proportional to the average of the input records VS30. The suggested value for the effects of input ground motion on the amplification coefficient and the equivalent adjustment coefficient is finally given.