Comparison of Modal Analysis and Time History Finite Element Method for Analysis of Concrete Gravity Dams: A Case Study of Gilgel Gibe III Dam

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Addis Ababa University


The Gilgel Gibe III Dam is a 243 m-tall roller-compacted concrete dam with a hydroelectric power plant On the Omo-Gibe River in Ethiopia. Concrete gravity dams are stable structures that can withstand external stresses with the help of their weight, shape, and strength. Stresses affect these complex structures, which is developed due to the action of dead loads, reservoir and tail water loads, uplift pressure, earth pressure, and silt pressure. Because it is frequently long and straight, a concrete gravity dam can be conceived of as a plane strain structure. The analysis is carried out using two-dimensional (2D) numerical modeling since the majority of the loads operating on the dam are distributed along the same plane. The design is based on rigid body analysis with "no tensile stress permitted" standards. Nonlinearity in shape, material, and the boundary condition of dams, foundations, reservoirs, and dynamic systems, however, may now be accounted for using finite element analysis methods. This study examine assessments on a roller compacted concrete dam (RCC) and analyze the effects of seismic loading using 2D numerical modeling in ANSYS software with comparisons between modal analysis and time history finite element analysis are compared in the case of the Gilgel Gibe III RCC dam. The distribution of stress under static and dynamic loading situations is also evaluated. The analysis of the Gigel Gibe III dam is used as a control case, and the number of mode shapes required for each modal approach to produce an accurate analysis is determined. For the findings, the study provides the time histories of a few significant parameters including directional deformation, the maximum principal stress and minimum principal stress at the dam heel and the horizontal component of displacement at the dam crest. For comparison's sake, the direct method's outcome is also displayed in each graph. The results of each analysis method will also be compared to show how they differ in their correlation concerning the design of gravity dams. In this work, the researcher visualizes stress distributions and dam displacement patterns using ANSYS software. The study's overall conclusion demonstrates that both analysis methods (time history and modal) have their effective areas like modal in terms of time while the time history method includes non-linearity in the dam structure.



Finite Element Method, Concrete Gravity Dams, Gilgel Gibe III Dam, Modal Analysis