where the primed quantities correspond to the dynamic constitutive parameters. The increased material resistance due to inertia and viscous effects under dynamically applied loads has been explicitly considered in the dynamic equilibrium equations. Reviewing the literature on the dynamic fracture behaviour of concrete, a 20 percent dynamic magnification of the apparent tensile strength is considered adequate for seismic analyses of concrete dams. Under dynamic loading, the material parameter U0 in equation 5.25 is replaced by the corresponding dynamic value, U0. At the instant of softening initiation under a dynamically applied load, the principal stress, a1, and the principal strain, £1, are designated by a0 and ^0, respectively, as shown in Figure 5.10-b.

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