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Effects of Cracking on Damping of Concrete Dams

by Peter R. Barrett, ANATECH Research Corp, La Jolla, United States,
H. Foadian, ANATECH Research Corp, La Jolla, United States,
Y. R. Rashid, ANATECH Research Corp, La Jolla, United States,



Document Type: Proceeding Paper

Part of: Lifeline Earthquake Engineering

Abstract:

A methodology is presented for dynamic analysis of concrete dams that considers the nonlinear stiffness and energy absorption effects of cracking. A cracking-consistent damping model is developed for treating damping in concrete structures subjected to time-dependent cracking under dynamic loading. This model treats viscous damping within the mathematical framework of the well-established smeared cracking constitutive model. The resulting stress-strain rate equations reflect spatially varying and anisotropically oriented damping relations that depend on the distribution and orientations of the local cracks. Furthermore, the magnitude of damping is made to depend on the time-dependent status and width of the crack opening, thereby simulating observed behavior where damping is seen to vary with the level of damage in the structure. This detailed treatment of damping disposes of the unrealistic uniform damping assumptions generally adopted in nonlinear time history analysis of concrete structures. The new model accounts for energy dissipation at the local level in a similar manner to the modeling of plasticity in metal structures where hysteretic energy losses are modeled directly at the location of the affected material, rather than as a smeared effect uniformly distributed over the entire structure. The application of this model to the dynamic analysis of concrete gravity dams shows that using large uniform damping values (greater than 5%) for assessing the seismic resistance of gravity dams could lead to nonconservative results. The immediate benefit of the model is to provide an acceptable method to represent the nonlinear dynamic mechanical properties of concrete structures. For example, once properly verified against experimental data, this model can be appropriately used as a basis for establishing new guidelines for selecting appropriate damping ratios for use in linear analysis procedures.

Subject Headings: Concrete dams | Damping | Gravity dams | Cracking | Structural models | Dynamic models | Mathematical models

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