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Dynamic Behavior of Rocking Stuctures Allowed to Uplift.
Psycharis, I. N.
National Science Foundation, Washington, DC., January 1982, 238 p.
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This report studies the phenomenon of uplifting, the partial separation of the base of a structure from its foundation, which occurs during the ground-shaking of an earthquake. The cases of both a rigid superstructure and a multistory building are considered. Two foundation models which permit uplift are used: the Winkler foundation model, and the much simpler, two-spring foundation model. Simple approximate methods for calculation of the apparent fundamental period of the rocking systems are developed and simplified methods of analysis are proposed. Because uplift leads to a softer vibrating system which behaves nonlinearly, the apparent fundamental resonant frequency of the uplifting system is always less than the fundamental resonant frequency of both the soil-structure interacting system, in which lift-off is not allowed, and the superstructure itself. The second and higher resonant frequencies of the superstructure are not significantly affected by the lift-off. For damped foundations, the ratio of critical damping associated with the apparent fundamental mode decreases with the amount of lift-off.
Dynamic response; Buildings; Theses; Earthquake engineering; Earthquakes; Foundations; Soil structure interactions; Vibration damping; Uplift pressure; Mathematical models; Dynamic structural analysis