Accidental Eccentricity to Account for Torsional Ground Motion in Equivalent Lateral Force Procedure

ABSTRACT Equivalent‐lateral‐force (ELF) procedure of seismic design considers accidental torsion by applying the design lateral force profile at an offset equal to accidental eccentricity from the CM. This paper evaluates the required accidental eccentricity in a median sense to account for the torsional ground motion, arguably the most dominant contributor of accidental torsion. The proposed recommendation of accidental eccentricity is based on the analysis of 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. Demands from the simultaneous incidence of horizontal pair and torsional ground motions are met with those from the base shear of bidirectional horizontal excitations applied conforming to the required accidental eccentricity. The resulting accidental eccentricity depends on the triplet of uncoupled horizontal and torsional periods if the natural eccentricities are small, and otherwise, contingent on the natural eccentricities as well. Further, accidental eccentricity is observed to vary linearly with the radius of gyration beyond a threshold. A system with a threshold radius of gyration is defined as the reference system. Accidental eccentricity of any system with an arbitrary radius of gyration may be obtained using an amplification or deamplification of that recommended for the reference system. The proposal also considers all possible directionality of the horizontal pair given the same recorded torsional accelerogram. One example is also included, demonstrating the computation of system properties that are required to implement the proposed recommendation of accidental eccentricity in a multistorey building.