Accidental and natural torsion in buildings.

摘要

Because of the torsional vibration of a building during an earthquake, the displacement demands on the different resisting planes of the structure may increase relative to those of a similar system with no torsion. It is the global objective of this work to develop procedures that would enable engineers to predict such increase in building response resulting from accidental and natural torsion.; This work on the effects of accidental and natural torsion is divided into two parts. In part I, the increase in building response due to accidental torsion is evaluated with the objective of developing an improved procedure to account for these effects in building analysis. Such procedure is based on an individual study of the increase in response due to different sources of accidental torsion, such as stiffness uncertainty and rotational excitation at the base. Also, a critical evaluation of the use of accidental-torsion provisions in code-static and dynamic analyses of single and multistory systems is presented. It is concluded that the new procedure has several advantages over the code-specified static and dynamic analysis procedures to include accidental torsion, such as: (1) the elimination of cumbersome static or three-dimensional dynamic analyses to account for accidental torsion effects in building design, and (2) the inclusion of the effects of all sources of accidental torsion, as opposed to current seismic codes, which consider only those effects that can be represented by a constant accidental eccentricity.; In part II, the nonlinear torsional response of single and multistory asymmetric buildings is considered with the objective of developing a simple conceptual framework, for the use of scientists and engineers, to understand the earthquake performance of different asymmetric structural configurations, and to develop a simplified method for nonlinear analysis of asymmetric buildings. It is concluded that the understanding of the story shear and torque response histories in conjunction with the story shear and torque ultimate surface for each building story is sufficient for a good conceptual design of an asymmetric structure. As a result of this research, several design guidelines for asymmetric systems are proposed, which may be used, for instance, to spread yielding uniformly among resisting planes in very asymmetric structures. Furthermore, the simplified nonlinear model developed, which is based on a single super-element per building story, should be very useful in practice because it provides an accurate and quick procedure to estimate the inelastic deformation demands in these structures. Buildings analyzed using the simplified procedure developed and designed using the conceptual guidelines described are expected to be cost-effective, safer, and more efficient seismically than designs based on current linear code-static or dynamic analyses.