ارزيابي ضريب C1 مندرج در رابطه‌ي تغييرمكان هدف نشريه‌ي 360 براي سازه‌هاي روي خاك سخت و دور از گسل براساس شتاب‌نگاشت‌هاي ايران

EVALUATION OF C1 FACTOR IN TARGET DISPLACEMENT OF CODE 360, BASED ON NONLINEAR DYNAMIC ANALYSIS AND IRAN EARTHQUAKE RECORDS FOR FIRM SOIL AND FAR-FAULT SITES

در اين نوشتار، نتايج مربوط به مطالعه‌ي آماري بر روي نسبت بيشينه‌ي تغييرمكان غيركشساني به تغيير مكان كشساني منطبق با ضريب C1 در رابطه‌ي تغييرمكان هدف، دستورالعمل بهسازي با استفاده از ركوردهاي زلزله‌هاي ايران ارايه شده است. اين نسبت‌ها براي سيستم‌هاي يك درجه‌ي آزاد با مدل رفتاري كشسان‌خميري كامل و ضرايب كاهش مقاومت مختلف تحت تاثير 204 شتاب‌نگاشت ثبت‌شده روي خاك سخت ايران محاسبه شده است. سه نوع خاك يك، دو، و سه براساس نسبت طيفي H/V درنظر گرفته شده است. با استفاده از تحليل رگرسيون غيرخطي، رابطه‌ي ساده‌‌يي بر نتايج ميانگين برازش شده است. درنهايت، رابطه‌ي پيشنهادي با ضريب C1 مندرج در نشريه‌ي 360 و استاندارد ASCE41-06 مقايسه و مشخص شده است كه اين ضريب در نشريه‌ي 360 نيازمند تجديد‌نظر و تغيير است.

In recent years, performance based design and assessment methods are commonly used. Structural and nonstructural earthquake damage is mainly due to imposed lateral displacements. Therefore, in recent methods, design criteria are based on displacement instead of force, as used in old methods. In fact, in these methods, displacement presents the performance of structures subjected to earthquakes. On the other hand, using performance based methods, in practical cases, needs simplified procedures to estimate the inelastic displacement of structures subjected to earthquakes. In general, nonlinear time history analysis presents an appropriate estimation of the imposed displacement of a structure subjected to a specific acceleration time history, but, results of this dynamic analysis are very sensitive to chosen acceleration time history. Thus, more reliable and practical methods are needed for the seismic evaluation and rehabilitation of existing buildings. One of the most practical methods is estimating maximum inelastic displacement from maximum linear elastic displacement. Recent codes for the evaluation and rehabilitation of existing buildings are suah as FEMA273 (1997), FEMA356 (2000), FEMA440 (2005), ASCE-41-06(2006). Code360 introduced a simplified analytical method called the coefficient method for estimating the inelastic displacement of structures from equivalent SDOF systems. In the coefficient method, maximum inelastic displacement of a structure is calculated from maximum elastic displacement, using 4 coefficients. FEMA 440 proposed new coefficients in the coefficient method based on 100 California earthquake records. These new coefficients are used in the latest code of the rehabilitation of existing buildings, ASCE41-06. The goal of this paper is a presentation of the statistical study of the inelastic displacement ratio for elastic perfectly plastic SDOF systems (according to the C1 factor of coefficient method), based on Iranian earthquake records. Results of this study are restricted to firm soils. In this paper, results of the statistical study of inelastic displacement ratios related to the C1 factor in the target displacement equation of seismic rehabilitation methods, based on Iranian earthquake ground motions, are presented. These ratios are calculated for single degree of freedom systems with an elastic perfectly plastic behavior model and various strength reduction factors subjected to 204 earthquake ground motion records. Earthquake records are selected recorded on soil condition types 1, 2 and 3, based on the spectral ratio H/V method. Using nonlinear regression analysis, a simplified equation based on mean results is calculated. Finally, the proposed equation is compared with the C1 factor in Code360 and the ASCE41-06 standard, and it is realized that this factor should be revised and changed in Code360.