كاربرد شتابنگاشت‌هاي مصنوعي در براورد تقاضاي لرزه‌اي سيستم‌هاي يك درجه آزاد: از ارزيابي تا تصحيح روابط پيشنهادي آيين نامه‌هاي لرزه‌اي

Artificial earthquakes in seismic demand estimation of SDOF systems: From evaluation to the correction of the proposed equations in the seismic codes

در اين مقاله، به بررسي كاربرد شتابنگاشت‌هاي مصنوعي در براورد تقاضا در سيستم‌هاي تك درجه آزاد و ارزيابي دقت روابط تجربي ارائه ‌شده در استاندارد 2800 ايران – ويرايش چهارم و نشريه 360، به‌منظور تخمين حداكثر تقاضا در سازه، پرداخته‌شده است. در اين راستا، سيستم‌هاي يك درجه آزاد با رفتار غير خطي الاستيك – پلاستيك مطلق و با زمان تناوب 1/0 تا 0/2 ثانيه و ضريب كاهش مقاومت بين 2 تا 8، به عنوان سيستم سازه‌اي در نظر گرفته مي‌شوند. سيستم‌هاي يك درجه آزاد تحت شتاب‌نگاشت‌هاي مصنوعي كه براساس توابع پوش مختلفي نظير؛ مركب، نمايي و ساراگوني-هارت توليدشده، مورد تحليل تاريخچه زماني قرار مي‌گيرند و نتايج حاصل از انجام تحليل تاريخچه زماني با روابط تخميني ارائه‌شده در استاندارد 2800 ايران - ويرايش چهارم و نشريه 360، مقايسه مي‌شوند. بر اساس نتايج به‌دست‌آمده، مشاهده مي‌شود كه روابط هر دو آيين‌نامه در Rμ=2 به‌درستي عمل نموده ولي براي Rμ هاي بالاتر دست پايين عمل مي‌كند و داراي دقت كمتري مي‌باشد. به همين منظور در ادامه مقاله روشي براي اصلاح روابط آيين نامه جهت برآورد بهتر سطح تقاضا لرزه‌اي سازه ارائه گرديده است.

Seismic design codes provide different equations for estimating displacement demands in various buildings and structures. Such equations were usually obtained by performing regression analysis over the obtained data from numerical models under different nonlinear analyzes. On the other hand, the application of artificial earthquakes is allowed to be considered for design and demand assessment in structures when there is a lack of suitable ground motions for a specific region and site. Since the accuracy of such relations affects the reliability of demand estimating in structures, it is required to assess the efficiency of such predictive relations. Moreover, it is essential to assess the efficiency of those relations for artificially generated earthquakes. Hence, in this study, the estimated demands from the design and assessment codes relations are evaluated for artificially generated ground motions. In this regard, regulations in the fourth edition of the Iranian seismic design code (also known as Standard 2800) and the last revision of the Iranian seismic evaluation code of practice (also known as Code-360) are considered. Estimated demands from these codes are compared to the results from the nonlinear time-history analysis of a group of single degree of freedom (SDOF) systems. Although an SDOF system can not represent the complete behavior of a complex building, for the low to medium-rise buildings with a fundamental vibration first mode, such an idealization is acceptable. In this regard, a group of SDOF systems with the elastic-perfectly-plastic (EPP) nonlinear behavior was considered. SDOF systems have vibration periods between 0.1s to 2.0s (as low to medium-rise buildings) with strength reduction factors (Rμ) of 2 to 8 to cover most of the common lateral resisting systems. These systems were analyzed under the action of 24 artificially generated ground motion records. Earthquake records were generated based on three different envelop shapes including compound shape, exponential shape, and Saragoni and Hart shape. These envelop shapes are representing the general form of an earthquake accelerogram and try to impose the real characteristics of an earthquake on the generated record. After employing the time-history analysis on each SDOF system, the mean of the maximum displacement demands of SDOF systems was obtained and compared to those from the estimating equations in Standard 2800 and Code-360. It is observed that estimated demands from Standard 2800 are closer to those from time-history analysis when compared with the obtained results from Code-360. Among the considered strength reduction factors, it was observed that SDOF systems with larger Rμ lead to a higher difference between the time-history results and those from codes. This is more predominant over the period range of 0.1s to 0.8s. So, relations in both codes are required to be modified for better demand estimating. In this regard, a method is proposed for modifying the available equations in the prementioned codes to accurately predict the displacement demands in systems under the action of artificially generated ground motions. A comparison between the results from the modified equations and those from the nonlinear time-history analysis shows the efficiency of the proposed method.