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The behaviour of symmetric buildings subjected to spatially varying base excitations is studied. The spatial variation of ground motion is modelled by an empricial coherency loss function. A parametric study on the effects of coherency loss and phase shift between multiple input motions on base shear, torque and column shear forces is carried out. The adequacy of seismic code provisions on accidental eccentricity is discussed. It is found that the torsional response depends strongly on the ratio of torsional to lateral vibration frequencies W gq / w x Code provisions on accidental eccentricity are adequate for torsionally stiff structures in accounting for the spatially varying multiple excitations. For torsionally flexible structures, the accidental eccentricity provisions underestimate the multiple excitation effect on torque. It is also found that the effects of spatially varying multiple excitations on column shear forces are insignificant if the structure is flexible or its dimension is small. However, if the structure is stiff or its dimension is large such that its response is mainly quasistatic, considering only the uniform excitation-induced dynamic response greatly underestimates column shear forces. |
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