اثر روش هاي مقياس كردن ركوردهاي جنبش زمين بر توزيع تغيير مكان نسبي قاب هاي خمشي فولادي طراحي شده به روش پلاستيك مبتني بر عملكرد

Effect of Ground Motion Scaling Methods on Inter-story drift Distribution of Steel-Moment Frames Designed Based on Performance-Based Plastic Design

در اين پژوهش اثر شش روش مقياس كردن ركوردهاي جنبش زمين بر پاسخ حداكثر تغيير مكان غير الاستيك قاب هاي خمشي فولادي طراحي شده به روش پلاستيك مبتني بر عملكرد، با دو طيف هدف حد اكثر زلزله محتمل و طيف طرح، مورد مطالعه قرار گرفته است. در اين روش از يك دريفت هدف و مكانيزم تسليم از پيش تعيين شده به عنوان حالت حدي عملكردي استفاده مي گردد. اين روش طراحي پيشنهادي بر اساس تئوري طراحي پلاستيك مي باشد كه در آن مقادير نيروهاي طراحي با استفاده از اصل بقاي انرژي استخراج مي گردد. براي رسيدن به اين هدف سه قاب 4، 8 و 16 طبقه كه براساس روش فوق طراحي شده اند تحت 42 ركورد حوزه دور كه با 6 روش مقياس شده اند، تحليل ديناميكي غيرخطي شده اند و توزيع حداكثر تغيير مكان غير الاستيك در ارتفاع محاسبه شد. هدف از اين مطالعه، ارزيابي دقت روش هاي مختلف مقياس كردن جنبش زمين در در محاسبه حداكثر تغيير مكان غيرالاستيك قاب هاي خمشي و تعيين مناسب ترين روش را با توجه به كارايي و دقت آنها به صورت پارامتريك است. نتايج حاكي از آنست كه روش مقياس كردن آيين نامه اي براي تخمين پاسخ غيرخطي سازه هاي طراحي شده به روش پلاستيك با توجه به حساسيت زياد به ركوردهاي زلزله مناسب نيست. همچنين در بين روش هاي بررسي شده، بسته به تعداد طبقات سازه، سه روشي كه اثر مودهاي بالاتر در آن لحاظ شده است به همراه روش مقياس به بيشينه شتاب زمين، پاسخ هاي مناسب تري براي تخمين بيشينه نياز لرزه اي سازه هاي مورد مطالعه داشته اند.

Time history analysis, which is the most important analysis tool in performance-based seismic design, has become more and more popular worldwide. In the seismic design, seismic demand is mainly governed by three factors including the peak value of ground motion, the characteristic of earthquake spectrum and duration. An earthquake intensity index of ground motions is normally used as a scaling parameter that is critical for seismic analysis and design. A number of researchers have, from their own perspective, proposed various intensity indices. However, due to the complexity and randomness of earthquake motion, it has been a difficult task to accurately evaluate the applicability of various existing intensity indices. In addition, an objective and quantitative method is lacking in the evaluation of the applicability of such indices. This has been a challenging issue in seismic engineering research and has become a fundamental problem in performance-based seismic design. Nonlinear structural response is often highly sensitive to the scaling of input ground motions. Thus, many different ground motion scaling methods have been proposed. The “severity” of an earthquake ground motion is often quantified by an intensity measure, IM, such as peak ground acceleration, PGA, or spectral acceleration at a given period. The PGA of a record was a commonly used IM in the past. More recently, spectral response values such as spectral acceleration at the fundamental period of vibration have been used as IM. Scaling of ground motions to a given spectral level at the fundamental period of vibration significantly decreases the variability in the maximum demand observed in the structural system. However, it is widely known that for records with the same spectral acceleration at the fundamental period of vibration value, spectral shape will affect the response of multi-degree of- freedom and nonlinear structures, because spectral values at other periods affect the response of higher modes of the structure as well as nonlinear response when the structure’s effective period has lengthened. Similar attention to the influence of nonlinear behavior of a structure on the period of vibration led to an IM that accounts for period softening to reduce variability at high levels of maximum inter-story drift ratio, drift demands larger than 5%, for composite structures. Previous studies have focused on evaluation of different ground motion scaling methods in single-degree-of freedom and buildings of multi-degree-of-freedom with shear-type behavior or common steel-moment frame structures. However, over the last decade, the performance-based seismic design philosophy has emerged as a promising and efficient seismic design approach. The novel Performance-based plastic design (PBPD) approach explicitly accounts for the inelastic behavior of a structural system in the design process itself. PBSD approaches based on plastic analysis and design concepts were recently developed for different lateral load resisting systems such as steel moment resisting frames, steel braced frames, etc. In these design methods a pre-selected yield/failure mechanism and a uniform target drift (based on inelastic behavior) were considered as performance objectives. The analytical validation of these methods showed that structures designed using these methods were very effective in achieving the pre-selected performance objectives. Considering a gradual shift towards PBSD for seismic design methods in general, this study is aimed at examining the effects of six different IMs on the estimation and distribution of the maximum inter-story drift for three short, moderate, and long-period steel-moment resisting frames designed with PBPD method buildings using the concepts of efficiency and sufficiency. An ensemble of 42 far-filed earthquake ground motion without pulse characteristics were used and scaled based on two target spectrum MCE and Design Response Spectrum to conduct nonlinear dynamics analyses by using OPENSEES. Results indicate that, the cod-compliant scaling method was not reliable for nonlinear dynamic analyses of structures designed by PBPD method, and cloud be very sensitive to the ground motion characteristics. Among them, depending on the number of stories, the three scaling methods including scaling ground motions to a given PGA and those that take into account for periods of higher modes generally decrease the variability in the maximum demand observed in the structural systems.