بررسي آزمايشگاهي و تحليل عددي رفتار ارتعاشي مخازن مستطيلي بتني ذخيره‌ي آب

FREE VIBRATION OF RECTANGULAR CONCRETE LIQUID STORAGE TANKS: EXPERIMENTS AND FINITE ELEMENT ANALYSIS

براي بالابردن دقت تحليل ديناميكي مخازن ذخيره‌ي آب، بررسي مشخصه‌ها و رفتار ارتعاشي سيستم آب و سازه اهميت زيادي دارد. به‌منظور بررسي رفتار ارتعاشي مخازن ذخيره‌ي آب، يك مخزن مستطيلي بتني به ابعاد 130×130 و ارتفاع 150 سانتي‌متري ساخته و براي حالت هاي مختلف ارتفاع آب، آزمايش مودال بر روي آن انجام شد. در بخش تحليلي با استفاده از روش المان محدود، مخزن ساخته‌شده در آزمايشگاه مدل و نتايج آن با پاسخ‌هاي آزمايش مقايسه و مطابقت خوبي مشاهده شد. براساس نتايج آزمايشگاهي و تحليلي با زيادشدن ارتفاع آب درون مخزن، مقادير بسامد‌ها كاهش داشته است، درصورتي‌كه درصد ميرايي افزايش يافته است. در مورد رابطه‌ي افزايش ارتفاع آب در مخزن، كاهش بسامد‌ها، و افزايش ميرايي در اين نوشتار بحث و بررسي شده است. همچنين در تحليل، اثر امواج سطحي بر روي نتايج وابسته به ارتعاش آزاد در حوزه‌ي بسامد، يعني بسامد‌هاي طبيعي، شكل مودها و توابع پاسخ بسامدي بررسي شد. براساس نتايج تحليل‌ها درنظرگرفتن امواج سطحي، منجر به پديدارشدن تعداد زيادي بسامد‌هاي مربوط به بخش مواج و اعمال فشارهاي ديناميكي در قسمت‌هاي بالايي مخزن نسبت به حالت نبود امواج سطحي مي‌شود.

In recent years, researchers have intensively studied the dynamic response of liquid containers under ground excitation. However, most of these studies were conducted on cylindrical tanks, while those concerning rectangular tanks are not adequate. In the early investigations, approximate methods were proposed to include the effect of hydrodynamic pressure for a symmetric fluid container subjected to horizontal acceleration. The fluid response was represented by impulsive and convective components. The fluid was assumed to be incompressible and the container was assumed to have rigid walls. Recently, very strong earthquakes have caused heavy damage to many liquid storage tanks. It was concluded that the assumption of a rigid wall may not be appropriate for structural modeling. Including wall deformability in dynamic analysis requires a systematic knowledge and understanding of the fluid–structure free-vibrational characteristics. Modal testing is one form of non–destructive testing that may be used to obtain information on the dynamic behavior of actual structures. This type of experimental study is conducted to determine different modal parameters of a structure, such as natural frequencies, damping and mode shapes, or to verify the theoretical models and predictions. This technique has the advantage of determining real structural properties without using any assumptions, and taking into account the actual boundary condition. In this paper, a numerical-experimental study of the overall dynamic response of rectangular concrete tanks is presented. In order to identify the natural frequencies that mainly contribute to the response, modal tests on a rectangular concrete tank model for different liquid levels were carried out. Also, a numerical model that accounts for interaction between the fluid and structure was developed. A very good agreement between experimental and numerical results was obtained. In the numerical study, the influence of sloshing on the natural frequencies and their modal pressure distribution are investigated. The results indicate that sloshing has a significant effect on the dynamic characteristics of the system. Thus, in cases where an accurate dynamical analysis is required, sloshing should be considered.