Efectos torsionales en estructuras sobre suelo blando

In this work the torsional vibrations of structures on soft soil caused by the combined effects of structural asymmetry and base rotation are evaluated. A simplified soil-structure system with five degrees of freedom is analyzed. The foundation is considered to be embedded into a uniform layer resting on an elastic half-space, under the incidence of inclined SH waves. Instead of matching the maximum torsional moments due to the independent effects of structural asymmetry and base rotation, a new approach for computing the design eccentricity is applied. Specifically, torsional eccentricity coefficients compatible with the peak coupled lateral-torsional response are obtained, using an ensemble of narrow-band earthquake motions typical of soft-soil sites in Mexico City. It is shown that these coefficients are not constant along the principal axes, but they depend on the position of the resistant element. Hence, it is necessary to compute them without assuming a linear variation, as is done nowadays. The torsional eccentricity coefficients can significantly differ from the currently codified values, particularly for torsionally flexible structures for which both coefficients may be negative. This implies that the lateral displacement is reduced by the effects of torsion.