روش سه مرحله اي انتخاب شتابنگاشت هاي زلزله براي تحليل ديناميكي سازه ها

A 3-stage Method for Selection of Ground Motion for Dynamic Time History Analysis

تحليل ديناميكي خطي و غير خطي سازه ها با استفاده از شتابنگاشت هاي زلزله، روشي مرسوم در تحقيقات مهندسي زلزله و نيز در طراحي سازه هاي مهم مي باشد. امروزه آيين نامه هاي زلزله لازم مي دارند كه شتابنگاشت ها اولا سازگار بوده و ثانيا به مقياس در آيند. براي سازگاري شتابنگاشت ها معيارهايي همچون مشابهت ساز و كار كانوني و بزرگا، فاصله هاي مشابه از مركز زلزله و بالاخره يكسان بودن نوع خاك محل ثبت زلزله عنوان شده است. در اين مقاله يك روند غربالگري سه مرحله اي ارائه شده كه در طي آن تعداد ركورد هاي منتخب به سرعت كاهش داده مي شود. ركورد هاي نهايي از نظر معيارهاي ارائه شده مناسب ترين شتابنگاشت ها براي سازه مورد نظر مي باشند. استفاده از اين روش نسبت به روش هاي موجود به كاهش بيشتر پراكندگي پاسخ هاي تحليل غيرخطي سازه منجر خواهد شد و با كاهش پراكندگي، ميزان اعتماد به ايمني سازه طراحي شده بيشتر خواهد بود.

Results of any dynamic analysis are clearly influenced by the type of the selected acceleration records as the use of inappropriate records can lead to a misunderstanding of the real behavior of structures against earthquakes. For compatibility of accelerograms various criteria including similarity of focal mechanism and magnitude, the same distance from the epicenter, and finally the same soil type of recording stations has been mentioned. Even with respect to these criteria, referencing to databases of earthquakes results in finding several hundreds and sometimes several thousands of apparently suitable accelerograms. Additional measures should be considered to minimize the number of appropriate records. In this paper a three-step screening process is presented during which the number of selected records is quickly reduced from a few thousands to a few number. The finally selected records are the most appropriate for the studied structure regarding the chosen requirements. In the first stage, called the coarse sieve, ground motion parameters and geological parameters are used to select the records. In the second stage, known as the intermediate sieve, a suit of earthquake records that are more similar in terms of spectral parameters are extracted and in the third and last stage, known as the fine sieve, the final records are selected with studying the spectral shapes of the records. It is shown that the proposed selection method results in a much less scatter in the nonlinear structural responses among the selected earthquakes resulting in an enhanced confidence in the safety of the designed structure.