la-Colina a)Multistory models are studied to assess design recommendations for torsionally unbalanced multistory buildings. Structural systems are assumed as shear beams with nonlinear lateral-resisting elements oriented along two orthogonal directions and subjected to a bidirectional earthquake ground motion. Five-story rigid-diaphragm models with mass or stiffness eccentricity are considered in the study. Accidental eccentricity is not included in the modeling and, therefore, its related recommendations are not assessed. Design recommendations studied here are based on the simple static procedure and include two values and an expression for the amplification factor ␣ to compute story eccentricities, three values of the torsion-shear reduction factor ␦, an additional lateral force F t to be applied at the building top level, and a minimal story eccentricity for the building. Design recommendations to control ductility demands of torsionally unbalanced multistory buildings are given.

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SUMMARYAn ensemble of earthquake records is used to carry out non-linear analyses of simple torsionally unbalanced systems considering both resisting elements and earthquake components along two perpendicular directions. These fully bidirectional analyses are focused to study the e!ect of the following factors: (i) seismicforce reduction factor; (ii) factors and used to compute the design eccentricity; (iii) initial lateral period; and (iv) initial sti!ness eccentricity. Results indicate that the ampli"cation factor can be speci"ed as a function of the force reduction factor, the lateral uncoupled period, and the sti!ness eccentricity. It is concluded that the coe$cient depends on the lateral period, the sti!ness eccentricity, and the geometrical eccentricity. It was observed that negative shears caused by torsion should be neglected in the design of the sti! element, particularly in the case of systems with large sti!ness eccentricity. Results suggest that additional studies should be performed to verify the assumed (partial) equivalence between unidirectional (resisting elements and earthquake components along one direction only) and fully bidirectional analyses to study building torsion problems.

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