3D modeling and ground motion scaling for the prediction of seismic demands on the gravity dams


Thesis Type: Postgraduate

Institution Of The Thesis: Orta Doğu Teknik Üniversitesi, Faculty of Engineering, Department of Civil Engineering, Turkey

Approval Date: 2016

Student: MILAD BYBORDIANI

Supervisor: YALIN ARICI

Abstract:

Seismic behavior of gravity dams has long been evaluated using the classical two dimensional modeling approach and time history analyses the assumptions for which have been rarely challenged. Formulated for the gravity dams built in wide-canyons, 2D modeling is utilized extensively for almost all concrete dams due to the established procedures as well as the expected computational costs of a three dimensional model. However, a significant number of roller compacted concrete (RCC) dams, characterized as such systems, do not conform to the basic assumptions of these methods by violating the conditions on canyon dimensions and joint-spacing/details. Based on the premise that the 2D modeling assumption is overstretched for practical purposes in a variety of settings, the first purpose of this study is to critically evaluate the use of 2D modeling for the prediction of the seismic demands on these systems. Using a rigorous dam-foundation-reservoir interaction (DFRI) approach, the difference between the 2 and 3D response for gravity dams were investigated in the frequency and time domain for a range of canyon widths and foundation to dam moduli ratios. The results of the analyses show the significant variance in the dam response for different ground motions. In the light of this data, the second purpose of this study is to investigate the selection and the scaling of the ground motions usually required for the reduction of this variance in the determination of the seismic demands on gravity dams. In this regard, the existing ground motion scaling techniques are evaluated for determining the efficiency and accuracy of the scaling technique for predicting the target demands for concrete gravity dams. A large ensemble of near fault ground motions were used in order to consider the effect of the soil-structure interaction (SSI) on the motion selection for concrete gravity dams. The required number of ground motions for the consistent and efficient analyses of such systems was investigated. The results of the study show that the conventional approach for the modeling of gravity dams, including 2D modeling and common scaling procedures optimized for buildings, can significantly mislead the designers on the demands on these systems.