الگوي نوين هموارشده‌ي توزيع برش پايه براي طرح لرزه‌‌يي قاب‌هاي خمشي مستوي

New Optimal Smoothed Base-Shear Distribution Pattern for Seismic Design of planar Moment Frames

اعمال نيروهاي روش ايستاي معادل، توزيع خاصي از سختي و مقاومت به سازه تحميل مي‌كند كه پاسخهاي لرزه‌يي به آن وابسته‌اند. اين نوشتار آشكار مي‌سازد كه با تغيير اين الگو مي‌توان طراحي قابهاي فولادي تحت بارگذاري ايستاي معادل را براي تغيير توزيع سختي جانبي و تغييرمكان يا گريز طبقات و حتي نقطه‌ي شروع خرابي هدايت كرد. بدين منظور الگوي توزيع برش پايه به‌عنوان بارهاي معادل به صورت تركيبي از چند الگوي جديد چنان معرفي مي‌شود كه با تغيير ضرايب تركيب بتوان الگوي فعلي آيين‌نامه‌يي و يا ساير الگوهاي كاربردي قابل پيشنهاد به طراحان را بسته به اينكه كداميك از بهينه‌يابي نتيجه شود، ارايه كرد. سپس با كاربرد الگوريتم‌هاي فراابتكاري از جمله: جستجوي هماهنگي، اتفاق مورچگان و شيوه‌ي شاخه-كران در قالب يك بهينه‌يابي جامع، ضرايب مناسب تركيب مذكور به‌طور خودكار به‌دست مي‌آيد. اجراي اين مسيله‌ي پيچيده‌ي گسسته-پيوسته در مثال‌هاي قاب خمشي منجر به الگوهاي جديد هموار، ولي متفاوت از بار معادل آيين نامه‌‌يي شد كه طراحي مقاطع قاب‌ها تحت آن علاوه‌بر توزيع يكنواخت‌تر گريز طبقات، توانست محل بيشينه‌ي تنش ستون‌ها و شروع خرابي را به طبقات بالاتر و ايمن‌تر نسبت به طرح‌هاي آيين‌نامه‌ي فعلي منتقل كند.

Structural design of building frames depends on the exerted loading. The simplified lateral loading in seismic design codes is, in fact, height-wise distribution of base-shearand depends on the designed section profiles for structural members. The present study seeks for the suitable, meanwhile, smooth and simplified, distribution of base-shear for practical seismic design and compares it with the current code practice. The problem is, thus, two fold. First, a lateral load pattern is sampled using linear combination of a few simplified patterns and their corresponding scale factors, second, the optimal sizing design under such a generated loading is determined. An integrated optimization platform is presented, including Harmony Search, for the first part, dealing with both continuous and discrete variables, and Ant Colony Optimization, with further local search for the second part to do the discrete sizing design of the building frame. Consequently, a penalized objective function is developed to consider, not only code-based steel design and drift constraints, but also, remunerate pioneering plastic-hinge formation, to occur at upper levels of the frame. Using the proposed algorithm, examples of steel moment frames with different width and height are treated. New smooth patterns of base-shear distribution are obtained, including a uniform part at mid-height, with a linear increase near the roof and a linear decrease to zero at the base level. Optimal sizing designs under these patterns and the code-based pattern, were then provided to be further compared. It was found that the new designs are closer to the structural constraints, including stress and displacements limits. In another words, the code-based pattern resulted in non-economic over-designed structures. The achieved designs are verified employing several static, spectral and time-history analyses. Considering the story displacement and drift response, the corresponding spectral and static results under new-optimal patterns are found to be compatible. Standard deviations of the inter-story drift response for new-pattern designs were generally less than the code-based designed models. Such drift uniformity among the frame stories is desired in seismic design, as it leads the entire structure to uniformly participate in resisting earthquake effects. The most critical stress points as the plastic-hinge starting locations, occurred in upper stories of the designed models under optimal patterns with respect to the code-based practice. The new smoothed patterns of base-shear distribution stand superior to the current codified pattern, regarding the abovementioned points especially to prevent progressive collapse, Thus, they may be considered as safer seismic designs than the current practice.