رمزدهي ويژه‌ي الگوريتم توارثي براي تعيين بهينه‌ي دسته‌ي شتاب‌نگاشت‌ها

IMRPOVED GENETIC CODING FOR ACCELEROGRAM SET OPTIMIZATION

تهيه‌ي شتاب‌نگاشت و طيف ويژه‌ي ساختگاه براي تحليل‌هاي غيرخطي و خطي ديناميكي خود مستلزم انجام پروژه‌هاي تحليل خطر است، كه در مواردي توجيه كافي يا امكان اجرايي ندارد. از جمله در مورد بافت قديمي ساختمان‌هاي شهري و ميراث فرهنگي كه با محدوديت‌هاي اجرايي يا اقتصادي براي گمانه‌زني و شناسايي ساختگاه همراه است. در مواردي از اين قبيل، يك راه حل كاربردي، انتخاب تعداد محدودي زلزله از كاتالوگ زمين‌لرزه‌هاي گذشته است؛ طوري كه طيف حاصل از مجموعه‌ي آنها داراي بيشينه‌ي مطابقت با طيف مبنا در استاندارد طراحي باشد، تا با حجم محاسبات كمتر تصميم‌گيري بهتري براساس پاسخ‌هاي حاصل با وجاهت آيين‌نامه‌يي طراحي تامين شود. در اين پژوهش، الگوريتم توارثي با شيوه‌ي نوين رمزدهي صحيح براي حل اين مسيله‌ي بهينه‌يابي گسسته ضمن ويژه‌سازي عملگرهاي آن ارايه شده است. كارايي روش گسترش‌يافته طي چند مثال از مجموعه‌ي شتاب‌نگاشت‌ها، براي بهبود تطبيق طيفي ارزيابي و طي بحث نظري نوآوري‌هاي روش تبيين شده است.

Time-history earthquake records are critical means for many earthquake engineering applications including step-by-step numerical solution of dynamic, nonlinear or highly damped systems, seismic design of new buildings and infrastructures, seismic control, risk assessment and vulnerability evaluation of existing structures and so on. They are provided using statistical simulated records, geological model-based artificial earthquakes or recorded accelerograms of real experienced earthquakes. The latter is preferred in many cases due to expensive computational effort and lack of reliability in simulating events of strong ground motion by the artificial methods. However, the resulted structural responses are highly sensitive to the selected earthquake record so that their variation from one record to the other is a major source of uncertainty in further design and decision making. In order to overcome such a challenge, the well-known seismic design regulations have accepted using a set of earthquake records instead of one provided that they are scaled to match a target design spectrum. It is especially beneficial in the absence of sufficient site-specific earthquake records or impossibility of extracting them via seismic hazard analyses. As no further limitation than over-riding the design spectra is implied, the resulted mean spectrum may over-estimate the target leading to economically undesired design. In addition, the corresponding scale factors will not be unique for a set of records unless taken similar or optimized by proper methods. Hence, it is reasonable to select a set of accelerograms from an available catalogue which is optimally compatible with the design spectrum; formulated as the first optimization problem in the present work. While the set of optimal records is identified their scaling factors are also optimized as the second problem formulation. The present work employs genetic algorithm as a reliable meta-heuristic and utilizes its operators to solve both the optimization problems. In addition to the conventional binary coding, an integer index coding is also utilized for the especial case of optimization problems in this research. Superiority of the integer coding over the binary type is not only discussed in reducing the search space cardinality but also shown via the results of a numerical example using a list of 136 earthquake records to match the target spectra according to the Iranian Standard of practice no.2800. The results also show the proposed method is highly beneficial in matching the target design spectrum with respect to the common practice with trial-selected uniform-coefficient set of records.