Comparison of Coupled and Decoupled Modal Approaches in Seismic Analysis of Concrete Gravity Dams in Time Domain

In spite of the fact that different methods are available for dynamic analysis of concrete dams, researchers are still studying other more efficient alternatives. Modal approach can highly improve the efficiency of analysis, since the response of dam must be generally calculated for several earthquake ground motion records. In this paper, two different methods have been considered for dynamic analysis of concrete gravity dams including dam-reservoir interaction (with pressure degrees of freedom for fluid domain). The coupled modal approach utilizes the coupled modes of the system for the analysis, and the decoupled modal approach employs the decoupled modes of system for this purpose. Calculation of the coupled modes involves some complications, due to its corresponding unsymmetrical eigen-problem induced. However, dynamic equation of motion can be solved very efficiently by using this method. In decoupled approach, the symmetry in the assumed corresponding eigen-problem results in faster calculation of modes. The equation of motion can also be solved with reasonable speed in this method.
In the last part of this paper, analysis of a typical dam-reservoir system is performed by both methods mentioned above. Both of these approaches have shown to be very efficient and have their own advantages at point of view of the calculation cost and memory allocations.