UNCERTAINTIES IN SEISMIC ANALYSIS OF CONCRETE and EMBANKMENT DAMS

In the dynamic analysis and seismic safety assessment of concrete and embankment dams, various uncertainties have to be taken into account. The following uncertainties have to be considered: occurrence of earthquake (return period); characteristics of strong ground shaking (peak ground acceleration, response spectrum, duration of strong ground shaking); spatial variation of ground motion at dam site; superposition of static and dynamic load cases; uncertainties in dam-reservoirfoundation model (two-dimensional model, geometry of dam and reservoir), dynamic soil-structure interaction effects; dam-reservoir interaction effects (shape of reservoir, compressibility of water, wave absorption in reservoir, wave height, etc.); dynamic material properties (soil, concrete, foundation rock); dynamic (tensile) strength properties of concrete, soil and rock; -joints in concrete and rock; effect of pore pressure in joints, pore pressure build up in soils; structural damping; type of numerical analysis (time domain analysis, response spectrum analysis, linear analysis, nonlinear analysis, analysis of zones (corners) with stress concentrations, etc.); compilation of results of time history analyses (use of maximum response quantities); uncertainties in performance criteria (allowable stresses, stability safety factors, etc.) for assessing the results of dynamic (and static) analyses. These different factors are discussed in a qualitative manner and they are grouped together as follows: • Uncertainties in seismic hazard and seismic input • Uncertainties in structural modeling • Uncertainties in material properties • Uncertainties/errors in dynamic analysis • Uncertainties in compilation of numerical results • Uncertainties in performance criteria These uncertainties have to be accounted for in sensitivity analyses. To avoid a large number of analyses, engineering judgment and conservative assumptions are needed, and used in practice. The seismic behavior of dams under strong ground shaking is rather unpredictable as each large dam is a prototype located at a unique site. Therefore, generalization of results from one site to another site is hardly satisfactory. It has to be expected that each strong earthquake that causes damage to dams will bring up new features of the seismic behavior of dams, which may have been overlooked or which have been considered to be of lesser importance.