استفاده از طيف شتاب زلزله براي محاسبه نيروي وارد بر اجزاي غيرسازه‌اي

Utilizing Earthquake Response Spectrum to Calculate the Floor Acceleration of Steel Frames

چكيده- در اين تحقيق، محاسبه شتاب وارد بر اجزاي غيرسازه اي واقع بر طبقات سازه هاي قاب خمشي و مهاربندي شده هم محور و برون محور تحت اثر زلزله بررسي شود. براي بررسي اين موضوع از تحليل هاي تاريخچه زماني غيرخطي قاب هاي فولادي 3، 5 و 7 طبقه با پريودها و ضرايب كاهش مقاومت مختلف تحت اثر ركوردهاي حوزه دور و نزديك استفاده شده است. براي انجام مطالعات پارامتري از مدل برشي اصلاح شده قاب هاي فلزي استفاده شده كه تحقيقات قبلي ميزان صحت نتايج حاصل از آن را در حد قابل قبول به اثبات رسانده است. در آيين نامه هاي معتبر ساختماني معمولاً روابطي كه براي محاسبه نيروي وارد بر اجزاي غيرسازه اي ارايه مي شود، به صورت ضريبي از بيشينه شتاب زمين بيان مي شود. نتايج اين تحقيق به ارايه روشي منجر شده است كه طبق آن شتاب وارد بر عناصر غيرسازه اي براساس طيف شتاب زمين محاسبه مي شود. به اين ترتيب اثر محتواي فركانسي زلزله طرح نيز در محاسبه شتاب لحاظ خواهد شد. همچنين اثر عوامل مختلف بر شتاب وارد بر طبقات سازه از جمله اثر زلزله هاي حوزه نزديك و سيستم هاي سازه اي مختلف بررسي مي-شود.

The importance of non-structural components in seismic Performance Based Design of buildings is well known nowadays. In this research calculation of absolute acceleration applied on non-structural components located on floors of moment-resisting, eccentric braced and concentric braced frames subjected to earthquake ground motions has been studied. The results of nonlinear time-history analyses of 3, 5 and 7-story steel frames with 8 different periods and 5 reduction factors subject to 15 records of near-field earthquakes and 15 records of far-field earthquakes has been used to investigate the effects of different parameters on absolute acceleration induced in each floor of the structures. The effect of inelastic behavior of system, natural periods of primary and secondary systems, structural system type and near-field ground motions have been studied with use of modified shear-building models of steel frames. The shear building models are set to have an equivalent lateral force-deformation behavior in each story to the one’s of given steel frames. Reliability of these models to estimate the maximum roof displacement and the maximum inter-story drift of steel frames has been investigated elsewhere through probabilistic analysis of the results obtained from comprehensive incremental dynamic analyses. The relationships that are presented in building codes to calculate the force applied on non-structural components has been usually expressed as a ratio of peak ground acceleration. This method of calculating of input acceleration to non-structural elements ignores the effect of frequency content of design ground motion. The results of the present study have been used to introduce a new method for calculation the force applied on non-structural components based on ground acceleration spectrum. In this method the input acceleration to non-structural components has been expressed as a ratio of earthquake response spectrum (instead of the peak ground acceleration). For this ratio which is entitled as “spectral amplification factor” two different expressions have been proposed for use in structures with linear and nonlinear behavior. This approach explicitly accounts for the frequency content of design earthquake in calculation of peak floor acceleration. The results of this study show that Euro-code 8 and ASCE 7-2010 recommendations need to modify specially for the precise location of the non-structural element and inelastic behavior of the structure. It has been demonstrated that structural system type does not significantly affect the amount of induced acceleration on each floor of steel frames.