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Impact of interface geometry on the seismic response of freestanding structures

Khalid Saifullah, Aleece Barnard, & Christine Wittich

Submitted September 1, 2020, SCEC Contribution #10045

Freestanding structures constitute a wide range of structures ranging from classical multi-drum columns in historic structures to modern rocking wall systems and electrical transformers. This class of structures includes critical systems to support the post-earthquake functionality of buildings, irreplaceable cultural heritage structures, and precariously balanced rock systems (PBRs) which are necessary for accurate seismic hazard characterization at long return periods. Therefore, accurate prediction of the seismic response of freestanding structures is imperative. Despite the seeming simplicity of freestanding structures, the seismic response is known to be highly nonlinear with respect to geometry and extremely sensitive to small changes in the geometry, orientation, and ground excitation. Owing to this complexity, geometrically-simplified models are often utilized in predictive studies, with substantial simplification occurring at the base of the freestanding structure (the interface). In an effort to better predict the performance of freestanding structures under earthquake loads, this study aims to evaluate the impact of the interface geometry on the overturning rates for freestanding structures. A preliminary numerical study on naturally occurring freestanding precariously balanced rocks was conducted to briefly learn about the impact of footprint geometry. This was followed by an experimental investigation, in which 3D printed models of various prototype freestanding structures incorporating a range of interface/footprint geometries were used in a scaled shake table testing campaign. The critical geometric parameters in two-dimensions (rocking radius and critical angle) were held constant throughout the testing to isolate the impact of the interface shape. A large number of near-fault ground motions representing a range of intensities were used as excitation in the shake table tests. The experimental results are compared with analytical analyses on simplified 2D model to elucidate the conservativeness of results for overturning behavior. The rate of overturning was assessed probabilistically through seismic fragility curves and is shown to be fairly consistent for the majority of the interface shapes tested, though key differences emerged for shapes with sharp angular irregularities in the interface.

Saifullah, K., Barnard, A., & Wittich, C. (2020, 09). Impact of interface geometry on the seismic response of freestanding structures. Oral Presentation at 17th World Conference on Earthquake Engineering.