Accidental Eccentricity to Account for Torsional Ground Motion in Seismic Design Using Response History Analysis

In the absence of any rational framework in the seismic standards, shifting of the floor centre of mass (CM) on either side to account for the accidental torsion in time history analysis has now become a standard practice. Shifting of CM leads to the alteration of eigen properties of the building, thereby resulting in an interesting paradox: increasing the accidental eccentricity may also lead to an underestimation of design member force resultants. This paper aims to quantify the accidental eccentricity contributed by torsional ground motion when using the response history analysis. A wide range of one-storey torsionally coupled buildings subject to a suite of seismic events comprising the recorded orthogonal pair of horizontal accelerograms and the recorded torsional accelerogram are considered. The proposal for accounting for the torsional ground motion in response history analysis is to derive an artificial torsional component from the scaling of the recorded horizontal pair through a function of accidental eccentricity. Accidental eccentricity required, in a median sense, to account for the torsional ground motion, arguably the most dominant contributor of accidental torsion, is first evaluated considering the directionality effect of a horizontal pair of seismic excitations. Recommendations for accidental eccentricity are then proposed that are contingent on a pair of system parameters: the uncoupled torsional period and radius of gyration. Implementation of the proposed accidental eccentricity to a multistorey building is also illustrated. The example problem considered here shows that the proposed accidental eccentricity successfully captures displacement and drift demands at all floor levels.