چكيده به لاتين
A large main shock may consist of numerous aftershocks with a short period. The aftershocks induced by a large main shock can cause the collapse of a structure that has been already damaged by the preceding main shock. These aftershocks are important factors in structural damages. Furthermore, despite what is often assumed in seismic design codes, earthquakes do not usually occur as a single event, but as a series of strong aftershocks and even fore shocks. For this reason, this study investigates the effect and potential of consecutive earthquakes on the response and behavior of concrete structures. At first, six moment resisting concrete and steel frames with 3, 5, 7, 10, 12 and 15 stories are designed and analyzed under three different records with seismic sequences from real and artificial cases. The damage states of the model frames were then measured by the Park and Ang’s damage index. Also, This study compares the vulnerability of initial shock-damage reinforced concrete (RC) with steel frames in successive scenarios, as an essential part of developing a framework to consider seismic sequence hazard into structural design. From the results of these investigations, it is observed that the sequences of ground motions can almost double the accumulated damage and increased response of structures. Therefore, it is certainly insufficient to ignore this effect in the design procedure of structures. Also, the use of artificial seismic sequences as design earthquake can lead to non-conservative prediction of behavior and damage of structures under real seismic sequences. Furthermore, the results of damage evaluation also reveal that RC frames have better performance than steel frames in seismic sequence phenomena. Furthermore, steel frames damage is about 53% more than the damage caused by RC frames under critical successive earthquakes. In the following, the current study introduces a new approach for development of damage index to obtain the maximum damage in the reinforced concrete frames caused by as-recorded single and consecutive earthquakes. To do so, two sets of strong ground motions are selected from “PEER” and “USGS” centers. Consecutive earthquakes in the first and second groups, not only occurred in similar directions and same stations, but also their real time gaps between successive shocks are less than 10 minutes and 10 days, respectively. The idealized multilayer artificial neural networks, with the least value of Mean Square Error (MSE) and maximum value of regression (R) between outputs and targets were then employed to generate the empirical charts and several correction equations for design utilization. To investigate the effectiveness of the proposed damage index, calibration of the new approach to existing real data (the result of Park–Ang damage index 1985), were conducted. The obtained results show good precision of the developed ANNs-based model in predicting the maximum damage of regular reinforced concrete frames. In the last step, the present study proposed a methodology based on wavelet packet transform, principal component analysis and neural networks to generate artificial critical aftershock accelerograms, which are compatible with response spectra. In fact, the proposed methodology contains two steps and expands the knowledge of the inverse mapping from first-shock response spectrum to second-shock’s and second-shock response spectrum to WPT coefficients of the following shocks. This procedure results in a stochastic ensemble of response spectrums of second-shock (in the first step), WPT coefficients of them (in the second step) and, they are used to generate the following shocks applying the inverse wavelet packet transform. Finally, in order to demonstrate the proposed method effectiveness, two examples are presented which uses recorded critical successive ground motions to train and test the neural networks.
