The Use of 3D Modeling for the Prediction of the Seismic Demands on the Gravity Dams

Abstract eng:
Seismic behavior of gravity dams has long been evaluated using a representative 2D monolith for the system. Formulated for the gravity dams built in wide-canyons, the assumption is nevertheless 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 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 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 robust SSI approach, the difference between the 2 and 3D response for gravity dams are investigated first in the frequency domain. Rigorous frequency domain solutions for both cases were used in order to compare the frequency response functions for the crest response quantities. In the 3D configuration, monolithic models with no construction joints as well as models composed of independent monoliths were used as the two ideal cases in order to investigate the behavior of the 3D model. The effect of the narrowness of the canyon as well as the foundation rigidity was evaluated for a range of canyon widths and foundation moduli and the differences between the natural frequencies and the damping ratios between the two modeling approaches were presented. As the engineering decision parameters on such systems are based on time domain parameters such as the stresses and displacements, next, a time domain comparison between the responses of 2 and 3D models were obtained using 70 different ground motions. The maximum crest displacement and toe stress values for the 2 and 3D models were compared for different canyon widths and foundation moduli. The scatter of the differences between the 2 and 3D model results were presented for different ground motions in order to show the possible bias introduced into the analysis results due to the modeling approach. The results of the study show that even for relatively wide canyons, the 2D analysis can lead to misleading predictions.

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