ارزيابي احتمالاتي اثر كاهش مقاومت درون چرخه در رفتار لرزه‌يي سازه‌هاي با قاب خمشي فولادي با ارتفاع متوسط

Probabilistic assessment of the effect of in-cycle strength degradation on seismic behavior on mid-rise steel moment frames

پس از زلزله‌ي نرتريج و مشاهده‌ي رفتار شكننده در اتصالات سازه‌هاي فولادي، مطالعات زيادي جهت بررسي اثرات كاهندگي در رفتار اين اتصالات انجام شد. در نتيجه‌ي اين تحقيقات، نوعي از رفتار كاهشي در چرخه‌هاي هيسترزيس مورد توجه بسيار پژوهشگران قرار گرفت، كه كاهش مقاومت درون چرخه‌يي نام گرفت. در اين تحقيق به‌منظور مطالعه‌ي اثر اين نوع كاهش، سازه‌هاي 3، 5، و 7 طبقه‌ي فولادي با انواع اتصالات قبل و بعد از زلزله‌ي نرتريج و اتصالات ايده‌آل درنظرگرفته شدند و پس از انجام تحليل ديناميكي غيرخطي افزايشي با استفاده از روش ارزيابي احتمالاتي، درصد قابليت اطمينان سازه در سطوح عملكرد موردنظر محاسبه شد. نتايج حاكي از آن است كه كاهش مقاومت درون چرخه، باعث كاهش قابل ملاحظه‌ي درصد قابليت اطمينان سازه و نيز ظرفيت مقاومت سيستم در برابر ناپايداري جانبي كلي در سازه‌هاي مذكور مي‌شود

For many years, providing sufficient strength for buildings while trying to avoid collapse prevention was the most important purpose in seismic codes. After Northridge earthquake (1994) and many damages due to brittle behaviors in steel structure connections, many studies about degrading effects have been done. One the most important degrading behaviors were, In-cycle Strength Degradation. In recent years many researchers have been focused on in-cycle strength degradation in hysteresis cycles. In this phenomenon degradation occurs in the same cycle of loading. Recent investigations done by FEMA-P440A, have shown that the effects of in-cycle strength degradation can be critical in determining the possibility of lateral dynamic instability in Single Degree Of Freedom (SDOF) systems. Multi degree of freedom (MDOF) systems is more complicated behavior than SDOF systems. In this research, in order to study the effects of such kind of degradation, 3, 5 and 7 story steel moment frame buildings with three types of connections (Pre-Northridge, Post-Northridge and Elastic-Perfectly-Plastic), have been modeled. In order to have more precise study, probabilistic seismic design approach has been used and then the reliability of structures in pre-defined confidence levels have been calculated, By performing Incremental Dynamic Analysis (IDA), the required parameters in probabilistic approach such as demand and capacity have been calculated. Results of this study have been shown that in-cycle strength degradation can highly increase the likelihood of global dynamic instability and decrease the reliability of mid-rise steel moment frame structures.