Criticality of Controlling Seismic Torsional Response in Plan Unsymmetric Buildings

Abstract eng:
Influence of critical system parameters is studied on lateral-torsional coupling of buildings with unsymmetric stiffness in plan. Elastic analyses of idealised single storey building (with two degrees of freedom, namely one translation and one rotation) were performed under set of earthquake ground motions; lateral-torsional response of buildings is sensitive to both structural and ground motion characteristics. Under earthquake shaking, (1) Dynamic response amplifications are in the ranges 1–20, depending on whether the ground motion is of far-field type or of near-field type (both fault-normal and fault parallel component), respectively; these results are for buildings with eccentricity in the range 0.01B–0.40B, where B is lateral dimension of building measured perpendicular to direction of earthquake; and (2) Usual code provisions (though conservative for small eccentricity buildings) may not be applicable to large eccentricity buildings. In lightly eccentric buildings, the behaviour of building is different when τ<1.0 and τ>1.0, where τ is ratio of torsional and translational natural periods of the idealised two degree of freedom model. Influence observed are summarised for far-field (usually with high frequency content only) and near-field ground motions (usually with dominant low frequency content also) on short and long period large eccentricity buildings: (a) Buildings with τ<1.0 Response of short period buildings is amplified on both flexible and stiff sides; short period buildings sustain more amplification. (b) Buildings with τ>1.0 Response of short period buildings is amplified (though lesser than that when τ<1) on flexible and stiff sides; under some ground motions, flexible side response is significantly higher. And, response of long period buildings is amplified on stiff side and de-amplified on flexible side. Lateral-torsional coupling is present in both short and long period buildings; the effect is more critical in buildings with large stiffness eccentricity in plan. Hence, seismic design codes should place an upper limit on torsional eccentricity.

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