Scaled shake table tests: free-standing structures with varying footprint geometry

Aleece Barnard, Khalid Saifullah, & Christine Wittich

Published November 6, 2019, SCEC Contribution #10044

Rocking is a primary response mode observed during earthquake excitation for a rigid free-standing structure resting on a rigid surface with sufficient friction to prevent sliding. Reducing this problem to two dimensions the equations of motion are piecewise with respect to orientation non-linear with respect to geometry and lump energy dissipation at point impacts. While many researchers have studied rocking systems in various contexts very few have investigated the response of geometrically complex freestanding structures. A handful of studies have emphasized the importance of considering the three-dimensional geometry yet a reduction to two-dimensions dominates most analyses of rocking structures. Many two-dimensional analyses have highlighted the most influential geometric parameters surface conditions and earthquake intensity measures on the dynamic response of a free-standing structure starting from rest. Consideration of these factors framed the research question and design of the experimental program and model structures. To this end this study aims to systematically study the impact of complex footprint geometries or basal interfaces on the body's response and failure. Similar to previous studies the focus is limited to rigid structures resting on a rigid foundation. Several 3D models were subjected to 50 distinct horizontal ground motions via shake table testing. The in-plane and out-of-place response as well as modal observations are presented for each test. The results are analyzed statistically and correlations are presented between the rate of overturning and the three-dimensional geometry of the system. Results from the study show that the rate of overturning is nearly consistent despite varying interface geometry. While failure rates were mostly similar response modes varied widely which affects the rate of failure to some degree. Results from this study may apply to natural geological structures particularly fragile precarious boulder systems which exhibit rigid body response modes when subjected to ground excitations. Their existence can provide physical constraints on historical and future seismicity for a site.

Barnard, A., Saifullah, K., & Wittich, C. (2019, 11). Scaled shake table tests: free-standing structures with varying footprint geometry. Poster Presentation at American Geophysical Union.