توسعه‌ي ضوابط آيين‌نامه‌ي زلزله‌ي ايران جهت اعمال اثر حوزه‌ي نزديك -كاربرد موردي: سايت‌هاي حرم مطهر امام رضا (ع) و حوزه‌ي نفتي شهر اهواز

Development of Iranian Seismic Design Provisions for Estimating Near-Fault Earthquake Effects- Case-study: Imam Reza Holy Shrine and Ahwaz Petrol-field

تفاوت چشم‌گير در گستره و نوع خرابي سازه ها ناشي از زلزله هاي حوزه‌ي نزديك و دور، موجب بذل توجه پژوهشگران به زمين‌لرزه هاي نواحي نزديك گسل شده است. هدف اين نوشتار، توسعه‌ي روش برآورد نياز لرزه‌يي سازه ها براي شبيه ‌سازي اثر زلزله هاي حوزه‌ي نزديك است. در اين راستا، تعميم روابط مربوط به منظور پيشنهاد استفاده در طراحي ساختمان ها در دستور كار قرار گرفته است، كه با اقتباس از نتايج مطالعات تحليل خطر لرزه‌يي براي تعدادي از مناطق حوزه‌ي نزديك كشور، ضرايب اصلاحي پيشنهادي ارايه مي‌شوند. نتايج بررسي هاي انجام‌شده حاكي از افزايش قابل ملاحظه‌ي نياز لرزه‌يي سازه در محدوده‌ي دوره‌هاي تناوب كوتاه و متوسط در مقايسه با مقادير پيشنهادي در آيين نامه‌ي طرح لرزه‌يي است. به عنوان نمونه در مورد سايت حوزه‌ي نفتي شهر اهواز از ديدگاه روش طراحي براساس ظرفيت، نياز لرزه‌يي سازه در محدوده‌ي دوره‌هاي تناوب كوتاه و متوسط در حدود 7/2 برابر مقادير پيشنهادي در آيين‌نامه‌ي طرح لرزه‌يي است. شايان ذكر است كه در اين پژوهش رويكردهاي تعيّني و احتمالاتي مورد استفاده قرار گرفته و با كاربرد روابط كاهندگي مختلف و تركيب نتايج حاصل در درخت منطقي، پارامترهاي طراحي موردنظر ارايه شده‌اند.

Significant differences between specifications of structural damage occurrence for buildings subjected to near-field and far-field earthquakes have been observed in recent years. According to recent experiences, there has been extensive damage occurrence in structures subjected to near-field earthquakes compared to far-field ones. The adopted approach in existing design codes is the consideration of coefficients, to take into account the effects of near-field earthquakes in evaluating the design base shear of structures. The aim of the present paper is the development of structural design methods for more adequate simulation of near-field earthquakes effects in the structural design process in order to take into account an increase in the seismic demand of structural systems. In this method, the effects of near-field earthquakes are not limited to only consideration of additional design base shear demands, but extra capacity, in addition to the ultimate strength of the structure is demanded also. In the present study, the proposed development of current Iranian seismic design provisions for estimating near-fault earthquake effects in the structural design process of buildings is investigated. In this manner, some near-field coefficients are proposed for some of the near-fault areas of the country, based on the results of seismic hazard analysis. It should be noted that in this investigation, both deterministic and probabilistic approaches are followed in the seismic hazard assessment procedure and various attenuation relationships have been considered with judgmental weighting factors and the obtained results have been combined in a logic tree. In addition, the proposed modifications and corresponding results are compared with the current code-conforming seismic demands for two special cases; the Imam Reza holy shrine site and Ahwaz oil field. The obtained results lead to a significant increase in design base shear, especially for structures with low and medium natural periods in near-fault areas, when compared with corresponding structures located on far-field sites.