كاربرد رويه بتن آرمه در بهسازي لرزه اي سقف هاي شيبدار طاق ضربي ساختمان هاي بنايي

Application of Reinforcement Concrete Layer Method for Retrofit of Slant Jack Arch Roofs in Masonary Buidlings

با توجه تعداد زياد ساختمان هاي بنايي در كشور و نيز ضعف عملكرد اين سازه ها در برابر نيروهاي زلزله، مقاوم سازي آن ها حائز اهميت فراوان مي باشد. مشكل اصلي اين نوع سازه ها، عملكرد نامطلوب و عدم صلبيت سقف طاق ضربي آن در اثر زلزله است. اين نقطه ضعف، در سقف هاي طاق ضربي شيب دار رايج در شمال كشور خود را بيشتر نشان مي دهد. لذا براي بررسي عملكرد سقف طاق ضربي و همچنين تاثير شيب بر آن، در اين مقاله تعداد 8 نمونه سازه بنايي يك طبقه با سقف طاق ضربي با شيب هاي صفر، 10، 15 و 20 درجه در حالت هاي بهسازي نشده و بهسازي شده با رويه بتن آرمه در نرم افزار اجزاء محدود ANSYS مدلسازي شده و تحت آناليز استاتيكي غيرخطي مورد بررسي قرار گرفته است. نتايج نشان مي دهد كه بهسازي با رويه بتن آرمه باعث افزايش صلبيت سقف ها مي گردد. افزايش شيب سقف سبب كاهش مقاومت و سختي الاستيك و نيز ضريب رفتار سازه مي گردد كه به كمك روش بهسازي با رويه بتن آرمه مي توان اين پارامترها را بهبود بخشيد. همچنين نتايج تحليل اقتصادي نيز نشان دهنده ي آن است كه روش بهسازي با رويه بتن آرمه نسبت به شيوه هاي ديگر بهسازي با توجه به عملكرد مناسب تر، ازنظر اقتصادي مقرون به صرفه تر مي باشد.

Jack arch masonry slab, developed in the 19th century in Britain has been used widely in floors and roofs of industrial and residential masonry buildings in many parts of the world. It is still in use in parts of Europe, the Middle East and Indian subcontinent. Taking into account the widespread use of the jack arch flooring and its ease of constructing as compared to the more modern concrete-based slabs, it should be pointed out that such slabs are built in traditional ways and little control is applied to their method of construction. Collapse of a large number of these composite slabs during past earthquakes pointed out the weakness of this type of flooring to seismic loads. It has also highlighted the need for developing appropriate retrofitting schemes, since a large number of buildings in Iran are roofed with masonry slabs. As an illustration, the statistics have shown that around half of the slabs used in traditional buildings of Iran are jack arch roofs. Due to their poor construction style and lack of appropriate retrofitting, these slabs cannot tolerate high seismic demands and fail to meet the seismic performance required in areas with high seismic activities (especially the slant types which have been widely used in the buildings of northern areas of Iran). Therefore, rehabilitation of these roofs must be considered. One of the effective methods is to add a thin layer of reinforced concrete over the slab. The retrofitting procedure includes three main steps: (1) Removing the top flooring finish, (2) Installing a mesh of reinforcement bars over the slab and (3) Covering the bars with a layer of concrete. To further investigate the seismic behavior of these roofs, response modification factor can be utilized as a well-known seismic parameter. This study investigates the seismic performance of masonry buildings with slant jack arch slabs retrofitted by the method of adding a layer of reinforced concrete. Two groups of one story masonry buildings with jack arch masonry slabs are designed including roofs with slopes of 0, 10, 15 and 20 degrees with and without concrete layer for roof retrofitting. Static nonlinear (pushover) analysis is carried out. Nonlinear analysis program “ANSYS” is employed for the analyses. The load–displacement curves for both types of models are obtained and variations of strength, ductility factor, stiffness and rigidity of roofs on both types of models are investigated. Response modification factors of two groups are calculated and results are compared. Results show that according to standard No. 2800 criterion, slant jack arch masonry slabs are classified as semi-rigid roofs and by retrofitting them, their rigidity can be enhanced. Also increasing the Slope of roofs inversely affects the Response modification factor (R), strength and elastic stiffness of structure. Finally For the consideration of economic factors, a cost analysis based on the tariffs of the Iranian Management and Programming Organization is carried out on three conventional methods of roof retrofitting (method of adding a concrete layer on the roof, steel grid method and tie-bracing method recommended by standard No. 2800). The obtained results indicate that method of adding a concrete layer is the most cost-effective method for jack arch retrofitting.