بررسي حساسيت لرزه‌اي سدهاي بتني پشت بنددار

Seismic sensitivity evaluation of concrete buttress dams

بلند ترين پايه ي سد بتني پشتبنددار سفيدرود به صورت سه بعدي تحت اثر مولفه هاي طولي و قايم زلزله ي در در?ه ي ساختگاه سد تحليل شده است. اندركنش سد با پي و مخزن و جذب امواج هيدروديناميك در كف و انتهاي مخزن در نظر گرفته شده اند. شتاب نگاشت هاي اعمالي به سازه با توجه به شرايط ساختگاه سد سفيدرود مقياس شده ، و براي تعيين شرايط اوليه ي قبل از وقوع زلزله، يك سري تحليل هاي استاتيكي تفصيلي انجام شده اند و سپس بارگذاري ديناميكي تاريخچه ي زماني ناشي از زلزله ي طولي و قايم در در?ه ي ساختگاه سد صورت گرفته و رفتار غيرخطي سد در برابر عوامل مختلف، مانند حالت هاي مختلف بارگذاري لرزه اي، مشخصات مصالح بدنه ي سد و پي، بررسي شده است. نتيجه تحليل ها آن است كه چهار منطقه ي اصلي از بدنه ي سد شامل محلِ تغييرِ شيبِ بدنه در پاياب از نظر كششي، پاشنه ي سد از نظر كششي، پنجه و لبه ي پشتبند سد به ويژه از نظر فشار و برش، مناطق حساس و آسيب پذير سد مي باشند. البته تحليل ها نشان مي دهد كه احتمال شكست كششي در بيشتر نقاط حساس مذكور در بدنه با افزايش مقاومت فشاري بتن كاهش پيدا مي كند. ودر صورت نرمي زياد پي، تنش هاي فشاري لرزه اي در پنجه ي سد افزايش زيادي دارند كه گرچه معمولاً كمتر از مقاومت فشاري بتن باقي مي مانند، ولي ممكن است منجر به شكست سنگ پي ضعيف شود. نكته قابل توجه ديگر اين كه هرقدر پي سد بتني پشتبنددار، سخت تر باشد، در ساختگاه هاي لرزه خيز شكست موضعي در محل تغيير شيب پاياب، پاشنه و پنجه ي سد محتمل تر است. پس توصيه مي شود در چنين شرايط از بتني با مدول ارتجاعي بالاتر از حالات عادي براي بدنه ي سد استفاده شود.

Concrete buttress dams are constructed in large numbers at medium sites in many countries such as Iran because of their considerable technical and economical benefits in previous century. This type of dams is exposed to damages due to earthquakes as other structures. Some buttress dams such as Sefidroud dam in Iran, Hsinfengkiang dam in China and Honenike dam in Japan have undergone some damages due to recent earthquakes. After these incidents, some investigations have been carried out. However, these investigations have just mentioned the manner of incidents and the resulting damages. Therefore, the seismic behavior and sensitivity recognition of these dams with respect to different factors have been ignored; however the study of behavior and seismic sensitivity of this type of dams is important. In this paper, the tallest monolith of the Sefidroud concrete buttress dam is analyzed using a 3D model with massless foundation to study the seismic behavior and sensitivity of this type of dam. The interaction of the dam with the reservoir, the reservoir bottom absorption and upstream radiation of hydrodynamic waves are considered, but the cross-canyon component of earthquake is neglected. The applied accelerograms to the system are scaled according to the Sefidroud dam site DBE response spectrum. To determine the initial conditions before occurring earthquake, a series of detailed static analyses are done under the effect of dam body weight, hydrostatic pressure, uplift pressure and ambient temperature. Seismic loading due to longitudinal and vertical components of earthquake is applied and the nonlinear behavior of dam under various factors such as different seismic loading scenarios and different properties of dam body and also foundation materials is investigated. The results of analyses show that the dam body downstream kink, heel, toe and buttress web are sensitive and vulnerable zones. The results also demonstrate that the compressive stresses in the dam body are usually much less than the compressive strength of concrete. Therefore, the possibility of compressive failure is almost zero. But the conditions of tensile and shear stresses are different and large stresses may occur at the mentioned zones and considerable tensile and shear damages to the dam body are possible. According to the results of analyses, it is apparent that when the ratio of dam body modulus to that of the foundation (called softness modulus) is small, i.e. when the foundation modulus is high and near to that of dam body, the construction of concrete buttress dams at highly seismic zones may cause local failure and unfavorable situations for the tensile stresses at the kink, the heel and the toe of the dam body. Therefore, adaptation of this dam type in such sites should be carefully studied and in these circumstances, the modulus of the concrete of dam body should be kept more than usual practice. Furthermore, the shear damage at the dam-foundation contact surface is highly dependent to the applied earthquake type, but increasing the softness modulus could reduce this type of damage. The compressive strength of concrete has no effect on the shear damage at the dam-foundation contact surface