SEISMIC DESIGN AND PERFORMANCE CRITERIA FOR
LARGE DAMS AND METHODS OF DYNAMIC ANALYSIS
Dr. MARTIN WIELAND
Chairman, ICOLD Committee on Seismic Aspects of Dam Design,
Poyry Switzerland Ltd.,
The seismic design criteria for large dams and safety-critical elements, such as spillways and low level outlets that must function after a strong earthquake, are discussed. The ground motion parameters of the Safety Evaluation Earthquake (SEE) and Operating Basis Earthquake (OBE) are given. The general seismic performance criteria for large storage dams are presented, which today are mainly specified in terms of displacements for the worst ground motion to be expected at a dam site. For concrete dams the compressive stresses shall not exceed the compressive strength of concrete, which may be a limiting factor for slender arch dams. For the OBE the seismic performance criteria are given in terms of stresses and deformations. For the hydro-mechanical components of gated spillways and low level outlets mainly stress analyses will be needed in order to show that the equipment can be operated and will not be jammed by inelastic deformations. The seismic design criteria are given for dams with different risk classification and the corresponding methods of dynamic analyses for OBE and SEE ground motions are presented. In the past all seismic analyses were carried out by the pseudo-static method, which, today, is considered outdated and obsolete and shall no longer be used for large dams located in seismic areas. Today, the seismic stress and deformation analyses are carried out by linear dynamic analyses and dynamic stability analyses by rigid body analyses such as the Newmark sliding block method, which is standard practice for the slope stability analysis of embankment dams. New types of dams such as dams with thin impermeable membranes require a higher degree of accuracy in the analysis of deformation as these membranes may be vulnerable to dam deformations. Moreover, under the effect of strong earthquakes, dams may experience inelastic deformations, which are acceptable as long as the reservoir can be retained safely after the earthquake. A thorough understanding of the inelastic and nonlinear seismic phenomena, which are expected during strong ground shaking, is the prerequisite for any nonlinear seismic analysis of dams. The appropriate methods of dynamic analyses are discussed.
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