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Characterizing the Rotational Components of Earthquake Ground Motion
Rotational components of ground motion, torsion about the vertical axis and rocking about orthogonal horizontal axes, are not routinely considered in seismic design because such components have not been recorded and their effects on the response of structures are unknown. Rotational recordings are unavailable because instruments are by-and-large unavailable. The characterization of rotational components of ground motion is a focus of this thesis. Because recordings of rotational components are unavailable, procedures are derived to extract rotational components of ground motion from recorded translational data. Two categories of procedures are proposed: Single Station Procedure (SSP) and Multiple Station Procedure (MSP). Single station procedures employ translational data recorded at a single station. Multiple station procedures employ three-component translational acceleration time series recorded in an array of closely spaced but spatially distributed accelerographs.
One of the newly developed MSPs, the Surface Distribution Method (SDM), enables the development of a design procedure for dense seismic arrays, whose primary purpose is to extract rotational ground motions. Design criteria are proposed to determine the length of the array, the number of recording stations and their spatial distribution.
Seismic codes and standards currently recommend the shifting of the center of mass to account for the effect of accidental torsion. This method does not produce the desired result, which is an increase in torsional response with an increase in accidental eccentricity. An improved definition of accidental eccentricity is proposed and studied for a wide range of one-story elastic systems, and nonlinear isolation systems. A preliminary investigation of the effect of rotational ground motions on the response of structures is performed. The rotational excitations are calculated using the SDM. The sample structures are a chimney subjected to horizontal and rocking ground motions, and four-story, base-isolated and fixed-based buildings subjected to horizontal, torsional and rocking ground motions. Rotational components of ground motion significantly affect the response of these structures.