تحليل آسيب‌پذيري لرزه‌يي ساختمان‌هاي بنايي با استفاده از طيف ظرفيت در روش تحليل حدي جنبشي

SEISMIC VULNERABILITY ASSESSMENT OF MASONRY BUILDINGS BY THE CAPACITY SPECTRUM KINEMATIC LIMIT ANALYSIS METHOD

در ميان روش‌هاي متعددي كه براي تحليل آسيب‌پذيري لرزه‌يي ساختمان‌هاي بنايي ارايه شده، روش «تحليل حدي جنبشي» از‌جمله روش‌هاي مناسب براي اين تحليل و نيز تهيه‌ي طرح بهسازي لرزه‌يي ساختمان‌هاي بنايي ــ به‌ويژه ساختمان‌هاي تاريخي ــ است. در اين نوشتار ضمن معرفي روش تحليل حدي با استفاده از طيف ظرفيت (غيرخطي)، آسيب‌پذيري لرزه‌يي نمونه‌يي از ساختمان‌هاي بنايي و قديميِ كشور تحليل شده است. با استفاده از اين روش، آسيب‌پذيري ساختمان در رخداد زلزله با 10درصد احتمال وقوع در 50 سال در مقابل بيست‌و‌يك سازوكار خرابي ناشي از رفتار درون‌صفحه و برون‌صفحه‌ي ديوارها و واژگوني جان‌پناه‌‌ها براساس آيين‌نامه‌ي ايتاليايي 3431P.C.M O. ارزيابي شده است. سپس نتايج حاصله با دو ضابطه‌ي طراحي موجود در آيين‌نامه‌ي زلزله‌ي كشور، شامل ضابطه‌ي كمينه سطح مقطع ديوار نسبي و ضابطه‌ي كنترل واژگوني ديوار مقايسه شده است.

Earthquakes are an important hazard for masonry buildings and historical constructions. Recent earthquakes have shown a deficient performance of masonry buildings under seismic loading. In relevance to the importance of this topic, different methods have been presented for the seismic vulnerability assessment of masonry constructions, like linear and nonlinear finite element methods, including limit analysis etc. Limit analysis is a good technique for estimating the ultimate limit state of the structure, because of the simplification of assumption, reducing the number of necessary mechanical properties and giving the ultimate condition of the structure in a critical section. On the other hand, in existing masonry buildings, partial collapses (local failure mechanisms) generally occur; very often due to the loss of equilibrium in masonry portions in earthquakes. Verifications of collapse (in-plane and out of- plane), with reference to those local failure mechanisms, can be done by means of the limit equilibrium analysis, based on the kinematics approach, which depends on selection of the failure mechanism and evaluation of the horizontal forces that generate that kinematic mechanism. Therefore, for application of kinematic limit analysis, it is necessary to identify the different collapse mechanisms. The determination of an abacus of failure mechanism is an important issue, as a good interpretation of the vulnerability of the building, which directly relates to the identification of failure mechanisms. The basis of this method is to find the safety factor of each failure mechanism, due to the kinematical stability imposed on the lateral load. For these reasons, evaluating the ultimate load bearing capacity of an entire masonry building subjected to horizontal loads is a fundamental task for the design of masonry buildings in some codes, for instance, the recent Italian Ordinance PCM 3274 & modifications OPCM 3431 of 3/05/05, which pays due attention to the kinematic limit analysis. This verification can be developed through a simplified verification, with behavior factor q (linear kinematic analysis) and the capacity spectrum (non-linear kinematic analysis). In the capacity spectrum (non-linear kinematic analysis) method, based on appendix 11.c of the Italian Ordinance, the safety verifications for local failure mechanisms in the ultimate limit state, consist of a comparison of the ultimate displacement capacity of each local failure mechanism with the displacement demand evaluated by means of a response spectrum, defined similarly to that adopted for the assessment of non-structural elements, estimated in respect to the secant period (Ts). This paper presents application of the Capacity Spectrum Kinematic Limit Analysis (nonlinear) to typical masonry buildings in Iran. The safety of the building against 21 failure mechanisms, due to in-plane and out-of-plane behavior , and also, the overturningof parapets in an earthquake with 10? probability of occurrence over 50 in years, has been studied. The the vulnerability of the structure was assessed, in compliance with the limit analysis method presented in the O.P.C.M 3431 Italian code. A comparison of these results, with those of the Iranian codes, shows some disparities.