بهبود پاسخ تحليل هاي غيرخطي لرزه اي سازه هاي بتن مسلح برمبناي تعيين جابه‌جايي تسليم

Amelioration of the nonlinear seismic responses of RC structures based on the yield displacement (On the Comparison of CSA and YPS analysis methods

ارزيابي عملكرد لرزه اي سازه هاي مختلف نيازمند انجام تحليل هاي غيرخطي (استاتيكي و ديناميكي) است. از روش هاي تحليل ديناميكي غيرخطي مي توان به تحليل تاريخچه زماني (TH) و از روش هاي تحليل استاتيكي غير خطي مي توان به تحليل طيف ظرفيت با استفاده از مفهوم طيف غير خطي با شكل پذيري ثابت (CSA) و تحليل طيف نقطه تسليم (YPSA) اشاره كرد. در اين مقاله دقت دو روش CSA و YPSA در تعيين پاسخ به عنوان روش اصلي نسبت به روش TH به عنوان روش شاهده و دقيق، مورد مقايسه قرار گرفته است. به اين منظور 3 سازه ساختماني سه بعدي با تعداد طبقات 8، 12 و 15 با شكل پذيري متوسط كه در يك جهت شامل سيستم دوگانه (قاب-ديوار) و در جهت ديگر شامل سيستم مقاوم قاب خمشي است، انتخاب شده اند. هر يك از اين سازه ها ابتدا بر اساس استاندارد 2800 و آيين نامه بتن ايران تحليل و طراحي و سپس با سه روش CSAو YPSA و TH تحليل شدند. نتايج به دست آمده نشان مي دهد روش YPS در زلزله هايي كه سازه را تا حد زيادي وارد ناحيه غيرخطي مي كند، دقت كافي ندارد. همچنين اين روش براي تعيين پاسخ، حساسيت بالايي به جابه‌جايي تسليم تعيين شده دارد. از اين رو راهكارهايي براي رفع اين كاستي ها ارايه شده است. در ادامه نشان داده شده است كه بهترين بازه براي در نظر گرفتن جابه‌جايي تسليم در سازه هاي دوگانه بتن مسلح تا حداكثر ارتفاع 50 متر 0.6% تا 0.8% ارتفاع سازه است.

Seismic performance evaluation of different structures requires nonlinear analysis utilizing static or dynamic methods. Among the dynamic nonlinear methods is the time history (TH) method can be noted. Performing dynamic nonlinear methods is not decree because of these methods has complex process and need long time to perform so often engineers do evaluations of seismic performance of different structures by using static nonlinear methods. In static nonlinear methods, capacity spectrum analysis using the concept of nonlinear spectra with constant ductility (CSA) and yield point spectrum analysis (YPSA) can be pointed. Both of these methods determine maximum displacement of structure by comparison capacity spectrum and demand spectrum but YPS method is easier than CSA and need lesser attempts. In additional of this advantage YPSA can be used in design. In order to Performing any of these Methods seven records is needed as history of the sever ground motions. These records are proportionate with the soil of type II. To allow comparison and homogenization of results of these methods scaling of the records is necessary. Also in scaling process the effect of vertical component of ground motion is neglected. In this paper the accuracy of CSA and YPS methods in determining the structural responses, has been assessed by their comparison with the TH method as the witness and accurate method of analysis. In this comparison is used the average of the results. Results include of Displacement of roof of the structures and Inter-story drifts. For this purpose, three 3D structural models of 8, 12 and 15 stories of moderate ductility were selected. These structures consist of a dual system (wall-frame) in one direction and moment resistant system in another direction of the plan against seismic load. All these structures were analyzed and designed according to the Iranian standard 2800 (IS-2800) for seismic analysis and Iranian concrete code of practice respectively. For 12 and 15 stories structures, all records scaled base on demand spectrum given by IS-2800. In the case of 8 story structure the records scaled base on equal maximum acceleration (0.4g). All structures analyzed utilizing all the methods of TH, CSA and YPSA. In all these analysis the flexural and axial behavior of elements and the shear behavior of the walls were considered nonlinear. Also in the both nonlinear static methods, the strength reduction in the demand spectrum of applied earthquakes was not taken into consideration. Results show that the YPSA method is not accurate enough in case of the earthquakes that cause the structure to largely enter the nonlinear state. Also this method is highly sensitive to the yield displacement for determining the response. Thus, strategies to address these deficiencies are presented. It’s also been shown that the best range for considering the yield displacement in dual system of concrete structures up to 50 meter height is 0.6% to 0.8% of the height of structures