DYNAMIC RESPONSE OF DAMS AND OTHER STRUCTURES TO DIFFERENTIAL GROUND MOTIONS.

Effect of the finite travel time is to produce what is called a pseudo-static displacement, in addition to the normal dynamic displacement. Whereas the latter is caused by inertia, the former is caused by the differential movement of the structure-foundation interface, and its determination requires the deformed shape of the structure to be calculated for a given unit displacement of each point, or region, of the interface connection. The computational problem is therefore appreciably greater than in the conventional seismic analysis, where all interface points have the same acceleration. These additional, pseudo-static displacements are time-dependent, and cause stresses to be added to the dynamic stresses which derive from inertia. First in this paper, the magnitude of these stresses for a single bay and a three-bay portal frame, with speeds of propagation equal to infinity, 4000 m/s and 2000 m/s, is considered. The infinite speed gives the conventional case, where there are no pseudo-static effects, but its use with the more general computer program described in this paper presents many useful checks. An embankment dam and a gravity dam are then considered, with the same three speeds of propagation in each case.