آناليز سازه‎‌يي منارهاي تاريخي آجري ايراني

Structural Analysis of Historical Persian Brick Masonry Minarets

رفتار سازه‌يي 9 منار آجري تاريخي اصفهان مربوط به قرون پنجم و ششم هجري قمري در برابر بارهاي ثقلي، زلزله، تغييرات درجه حرارت و باد با استفاده از روش المان محدود غيرخطي، در 2 حالت منار به صورت كامل (شامل پوسته‌ي منار، ستون مركزي و راه پله) و فقط پوسته‌ي خارجي مورد مطالعه قرار گرفته‎ است. براي آناليز ديناميكي تاريخچه‌ي زماني غيرخطي از 7 شتاب‌نگاشت مناسب و مقياس‌شده براساس ضوابط استاندارد 2800 استفاده شده است. به دليل شكست منارها در هنگام وقوع زلزله، جهت تعيين وضعيتي كه شكست رخ نمي‌دهد، بررسي پارامتري تاثير ارتفاع، مقاومت مصالح، معيار شكست، و اندازه‌ي ميرايي انجام شده است. آناليز حرارتي نشان مي‌دهد كه منارها در شرايطي كه اختلاف دماي داخل و خارج آنها از مقدار مشخصي بيشتر شود، دچار شكست مي‌شوند. منارها مي‌توانند حدود 4/1 برابر باد مبناي اصفهان را بدون شكست تحمل كنند.

In this research, the structural behaviour of nine historical brick masonry minarets in Isfahan, built in the eleventh and twelfth centuries A.D., against weight, earthquake, temperature and wind, is studied. In order to investigate the effect of the central column and spiral staircase on structural behaviour, analyses have been performed for two cases: 1) the whole minaret (including the outer shell, central column and spiral staircase), and 2) only the outer shell. Frequencies and mode shapes have been obtained by modal analysis. Seven scaled, appropriate, ground motion records have been selected, according to the Iranian Seismic Code, for non-linear time history dynamic analysis. Based on the Willam-Warnke failure criterion, all minarets fail, due to selected earthquakes. Parametric studies have been undertaken to find situations in which the minarets do not undergo failure. Parameters are the height of the minarets, the tensile and compressive strengths of materials, failure criterion, and damping ratio. By decreasing minaret height, failure is delayed or does not occur. Increase in tensile and compressive strength results in delayed failure and smaller displacement. Using the Drucker-Prager failure criterion, all minarets still fail at early stages of earthquakes. The same thing occurs by changing the damping ratio from 5% to 20%, except that the values of the displacements decrease. Structural analysis of minarets against temperature changes, according to Isfahan’s thermal conditions, fire, and a dramatic decrease in temperature, shows that in cases of certain inner and outer temperature differences and very high or very low temperatures, the minarets fail. When the minaret consists of only the outer shell, it has a higher strength than the whole minaret and experiences less damage. Non-linear static analysis indicates that under Isfahan’s design wind load, (with a velocity of 130 km/h) minarets do not fail, but they will fail during winds with a velocity of 165 km/h. A minaret consisting of only the outer shell withstands larger displacements than the whole minaret.