بررسي سطح عملكرد سازه هاي فولادي با سقف تيرچه اي با در نظر گرفتن انعطاف پذيري ديافراگم در محدوده هاي خطي و غيرخطي

Study of Performance Level of Steel Structures with Joist Floor by Considering the Diaphragm Flexibility in the Linear and Nonlinear Zones

چكيده كفها به عنوان ديافراگم افقي و اولين جز از سيستم باربر جانبي وظيفه انتقال بارهاي جانبي را به اجزاي قايم باربر جانبي بر عهده دارند. در روشهاي متداول تحليل و طراحي سازه‌ها، فرض معمول اين است كه ديافراگم كف در صفحه خود به صورت كاملاً صلب عمل مي‌نمايد و نيروهاي جانبي به نسبت سختي اجزاي قايم مقاوم بين آنها توزيع مي-گردد. در صورتي‌كه در بسياري از موارد سقفهاي ساختمان به صورت نيمه‌صلب عمل كرده و انعطاف‌پذيري آنها باعث مي‌گردد كه توزيع نيروي جانبي بين عناصر مقاوم به نسبت سختي آنها صورت نگيرد. موضوع اصلي اين تحقيق ارزيابي اثر انعطاف‌پذيري ديافراگم كف بر پاسخ لرزه‌اي ساختمانهاي فولادي مهاربندي شده با سقفهاي تيرچه‌اي در محدوده‌هاي خطي و غيرخطي در روش سطح عملكرد مي‌باشد. با مقايسه و بررسي نتايج مي‌توان نتيجه گرفت كه روش طراحي بر اساس فرض ديافراگم صلب كه در آيين نامه هاي كنوني زلزله وجود دارد، نمي تواند برآورد و ارزيابي صحيحي از جابه جايي نسبي و سطح تقاضاي شكل پذيري در اعضاي اصلي سيستم باربر جانبي يعني بادبندها، به خصوص در طبقات پايين ساختمانهايي كه داراي نسبت دهانه به عرض زياد باشند (نسبت بزرگتر از سه) انجام دهد؛ به علاوه انعطاف پذيري ديافراگم باعث تفاوت الگوي پخش بار بين بادبندها با حالت ديافراگم صلب مي‌شود. اين تفاوت باعث ايجاد نيروهاي زيادتري در بادبندهايي كه سطح بارگير بيشتري دارند، مي گردد. همچنين مدل ديافراگم انعطاف پذير جابه جايي نسبي و تغيير‌شكل بيشتري در قابهاي سازه به خصوص قابهاي مياني ايجاد مي‌كند كه اين افزايش جابه جاييها مي‌تواند منجر به افزايش نيرو درون بادبندهاي اين قابها شود، در حالي كه اين اعضا در روش ديافراگم صلب براي انتقال اين نيروها طراحي نشده‌اند.

Abstract Floors as horizontal diaphragms and first element of lateral resisting system transfer the lateral forces to the vertical resisting elements. In conventional methods of analysis and design of structures, horizontal diaphragms are typically assumed to be rigid and distribute the horizontal forces to the vertical lateral load resisting elements in proportion to their relative stiffness. In addition, this assumption is often used to reduce the degrees of freedom and simplifies seismic analysis of buildings. Although this assumption may be justified for many structures, but for some types of structural systems, the effect of diaphragm deformability cannot be disregard and the floors behave as semi-rigid diaphragms and the actual force distribution between vertical lateral load resisting elements is not proportion to relative stiffness of vertical elements. In braced steel structures with block-joist floor, the vertical components consist of braces with high story stiffness and the floors in one direction may have less rigidity in their own planes. Furthermore, inaccurate modelling of flexibility of floor diaphragms leads to misestimating of building seismic response and unsafe design of vertical lateral load resisting elements such as braces. The main objective of this study is to evaluate the impact of in-plane diaphragm flexibility on the seismic response of steel braced buildings with block-joist floors in linear and nonlinear regions using a performance-based approach. In order to investigate the diaphragm effect on the seismic response of buildings; one story, three stories and five stories buildings are designed according to the Iranian Seismic Code procedures by assuming rigid diaphragm behaviour. In order to incorporate the flexible behavior of floor diaphragms, it is necessary that the building structural system be analysed as a three-dimensional system. The structural Software SAP2000 was used for the linear and nonlinear analysis procedures. For the linear and nonlinear analysis procedures, the instruction for seismic rehabilitation of existing buildings no.360 guidelines were used to determine the response and corresponding displacements and internal forces in building. Floors are modelled with SHELL element having four nodes in each element to consider in-plane flexibility of the diaphragm. The beams, columns and bracings were modelled by FRAME element and use of the diaphragm constraint and master joint to model a rigid floor assumption. By studying and comparison the results, it can be concluded that the design procedures in current codes, based on the assumption of rigid diaphragm, can not provide a reasonable estimate of the drift and brace ductility demand especially in lower stories of the building with large aspect ratio (aspect ratio greater than 3) and the flexible models produce more frame displacement and maximum drift relative to rigid models. Also, application of rigid diaphragm assumption for structures with aspect ratio greater than 3 causes significant errors in analysis results. Inclusion of diaphragm flexibility changed the natural period, maximum total base shear and dynamic response of structures. Also the results of studies indicate that the diaphragm flexibility change the distri-bution of forces between the vertical load resistance elements and causes the middle braced frames pick up additional lateral load.