اپسيلون به‌عنوان شاخصي از شكل طيف شتاب نگاشت

EPSILON AS AN INDICATOR OF GROUND MOTION SPECTRAL SHAPE

در برآورد خطرپذيري لرزه‌يي سازه ها از سنجه هاي مختلفي از شدت زلزله مي توان سود جست كه در اين بين، طيف پاسخ شتاب (Sa(T)) شناخته‌شده ترين آنهاست. در سال‌هاي اخير با تاكيد بر «ناكامل»‌بودن اين سنجه، تحقيقات دامنه داري صورت پذيرفته است، كه ثمره‌ي آن ارايه‌ي روش‌هايي جهت برآورد واقع گرايانه تر پاسخ سازه هاست. در اين نوشتار، ضمن بازتعريف اين مسيله به بررسي سير پژوهش‌هاي سال‌هاي اخير و دست آوردهاي حاصل پرداخته شده است. يافته هاي سال‌هاي اخير حاكي از آن است كه به‌كارگيري طيف يكنواخت خطر (UHS) براي تحليل سازه ها كه در آيين نامه هاي امروزي استفاده مي‌شود، منجر به برآورد محافظه كارانه‌ي پاسخ مي شود. نقد روش فعلي آيين نامه ها براي انتخاب شتاب-نگاشت هاي زلزله و نيز معرفي نسل نويني از طيف پاسخ با عنوان طيف ميانگين شرطي (CMS) بخش ديگري از اين نوشتار است.

From the various intensity measures that may be applied to evaluation of the seismic risk of structures, the acceleration response spectrum, Sa(T), is the most famous. As a key assumption in usual risk assessment procedures, such as PEER methodology, the structural response depends only upon the applied intensity measures, and not on any other properties of ground motion. This required condition is termed the “sufficiency” of the used intensity measure. The limited “sufficiency” of Sa(T) has been emphasized in recent research and, as a result, different methods have been proposed to modify structural response analysis. In this paper, the problem has been re-defined and then recent studies have been surveyed. The inelastic displacement response spectrum is another alternative for Sa(T). The intensity measure vector, as an innovative approach for bias reduction, has been also reviewed. This paper is mainly focused on the spectral shape concern. It has been discussed how the spectral shape of ground motion affects the structural nonlinear response. Epsilon, as a well known seismological parameter, is introduced as a convenient indicator of spectral shape. It has been shown that epsilon has significant effects on the nonlinear response of structures. As a convenient approach for reliable ground motion selection, the hazard related target epsilon is calculated first, and then the compatible ground motion is selected. A convenient procedure has been introduced in this paper, in order to find the target epsilon and eta values at different hazard levels. In the second half of this paper, the current code conformed approach for ground motion selection has been discussed. All seismic design codes and guidelines require scaling of a number of selected ground motions so that they match or exceed a target spectrum, i.e. uniform hazard spectrum (UHS). The recent findings show that application of the UHS for analysis of structures leads to a conservative estimation of structural response. Finally, a new generation of response spectrum, named the conditional mean spectrum (CMS), has been introduced. The CMS presents the expected response spectrum, conditioned on occurrence of a target spectral acceleration value at the period of interest. It has been shown that this is a more realistic target spectrum for ground motion selection in comparison with UHS.