ارزيابي مقاوم‌سازي لرزه‎اي قاب‎هاي فولادي مهاربندي شده به‌كمك ديوارهاي برشي فولادي

Evaluation of Seismic Strengthening of Steel Braced Frames using Steel Shear Walls

استفاده از ديوارهاي برشي فولادي به‌‌دليل مزاياي فراواني كه نسبت به ساير سيستم‎هاي مقاوم در برابر زلزله دارند، قابل توجه است. از مزاياي اين سيستم مي‎توان به شكل‌پذيري و جذب و استهلاك انرژي زياد با وجود سختي بالا و اقتصادي بودن آن اشاره كرد. با عنايت به خطر لرزه‎خيزي بالا و نياز به مقاوم‌سازي بسياري از ساختمان‎ها در برابر زلزله، استفاده از اين سيستم لرزه‎بر، مناسب و مقرون به‌صرفه به‎نظر مي‎رسد. در تحقيق حاضر به بررسي مقاوم‌سازي قاب‎هاي فولادي مهاربندي شده همگرا با استفاده از ديوارهاي برشي فولادي پرداخته شده است. تمركز پژوهش بر روش‎هايي است كه در آن‎ها مقاوم‌سازي تنها با اضافه كردن المان‎هاي ديوار برشي فولادي به قاب مهاربندي شده صورت مي‎پذيرد. ابتدا تعدادي قاب فولادي مهاربندي شده‌ي غيرمقاوم به‌عنوان قاب‎هاي پايه انتخاب و مدل‌سازي شده‌اند. قاب‎هاي پايه با اضافه كردن پانل‎هاي فولادي به روش‎هاي مختلف، مقاوم‌سازي مي‌شوند. سپس همه قاب‎هاي پايه و قاب‎هاي مقاوم‌سازي شده به‌صورت استاتيكي غيرخطي تحليل مي‌شوند. از مقايسه‌ي مدل‎هايي كه در آن‎ها بهسازي لرزه‎اي به‎كمك تعويض سيستم لرزه‎بر از مهاربندهاي فولادي همگرا به ديوارهاي برشي فولادي صورت پذيرفته است، با ساير روش‎هاي مقاوم‌سازي مورد مطالعه، مي‎توان به رفتار لرزه‎اي بهبوديافته‌ي قاب‎هاي بهسازي شده پي‎برد. مطالعه‌ي حاضر نشان مي‎دهد در قاب‎هاي فولادي مهاربندي شده، با رعايت پاره‎اي نكات علمي، فني و اجرايي، تعويض سيستم باربر جانبي از مهاربندهاي فولادي همگرا به ديوار برشي فولادي نازك، مي‎تواند به‌عنوان روشي مناسب براي مقاوم‌سازي و بهسازي طيف وسيعي از قاب‎هاي فولادي موجود، استفاده شود.

In recent years using steel plate shear wall system, because of its advantages in comparison with other earthquake resistant systems, has been a matter of attention. Some of its advantages relative to other systems include abundance advantage, high ductility, good hysteretic behavior and energy absorption capacity, high stiffness and economic advantages. Regarding that in Iran there is high seismicity risk and the need to strengthen old and unsafe urban textures and buildings, using this system as a lateral load resistant system seems appropriate and economical. In the present research strengthening of x-braced steel frames with steel plate shear walls is evaluated. Addition of bracing to unbraced frame spans, substituting braces with thin steel shear wall panels and adding thin steel shear wall panels to unbraced spans which do not have architectural requirements are considered as retrofitting strategies. The focus is on the methods in which retrofitting is only done by adding steel plate shear wall elements to braced frames. Some of these methods have many economic and practical advantages. Others are only proper for some special cases. In this study a number of x-braced steel frames designed by the first edition of Iranian code of practice for seismic resistant design of buildings (Iranian Standard No. 2800) are taken as basic frames which need to be retrofitted. These basic frames are retrofitted by adding steel panels with different methods. Then nonlinear static analysis (pushover analysis) with displacement control pattern has been done on both basic and retrofitted finite element frame models and the capacity curves (diagram of story displacements against base shear) of basic frames and retrofitted frames are compared. Considering the results of the pushover analysis of models in which seismic retrofitting is done by replacing x-bracing earthquake resistant system with steel plate shear walls and the results of other methods of strengthening, it is seen that seismic behavior of retrofitted frames is more desirable in terms of overstrength factor (?) and overall ductility of structure (?_s). The failure and fracture mode in most of the medium-rise frames was ductile but in the short-rise frames the fracture was brittle. Thus, replacing the braces in short-rise structure with thin steel shear walls seems irrational and unjustified economically. But it is to be mentioned that strengthening and increasing the moment of inertia of the adjacent columns of steel shear wall panels in structures with brittle fracture mode could result the change from brittle to ductile fracture. The results of this research show that in the case of steel braced frames with regard to some scientific, technical and practical points; replacing concentric steel bracing earthquake resistant system by steel plate shear walls can be used as a suitable method for retrofitting a wide range of existing steel structures in Iran.