Application of Eccentricity Concept to Multistory Buildings

Abstract

<p>This thesis presents a study in extending the eccentricity concept to multistory buildings to evaluate their torsional behavior under static and dynamic lateral loads. The eccentricity concept enables the separation of horizontal loadings into lateral and torsional components thereby enhancing the understanding of the structural behaviour of multistory buildings. To achieve this, formal definition for the center of rigidity is established for eccentric and irregular multistory buildings in general. The centers of rigidity are shown to be load centers at the floors and they should be reference points from which eccentricities are measured. A procedure is given to locate the rigidity centers with the aid of a plane frame program. The centers of twist are then defined. They are convenient points of reference for the generalized floor displacements and will lead to uncoupled equations of lateral and torsional equilibrium. The centers of rigidity and centers of twist are in general load dependent and not the same set of points for multistory buildings. Only for buildings with proportional framing that they become the same set of points. A method for lateral load analysis of symmetric and eccentric setback structures is presented next. The eccentricity concept and the displacement compatible load concept are employed by the proposed method. This is a non-trivial application of the eccentricity concept in analysis of irregular multistory buildings. Finally, an analytical investigation into the static and dynamic behaviour of uniform eccentric wall-frame buildings is carried out to examine the complexities of wall-frame interaction under eccentric loadings. A class of uniform eccentric wall-frame buildings had been identified which will have centers of rigidity along a vertical line, thereby satisfied the seismic provision requirement of NBCC 1985. The adequacy of the code procedures is evaluated for this class of structures. It is shown that for other classes of eccentric wall-frame multistory buildings, it is difficult to apply the concept of eccentricity and code procedures to obtain reasonable estimates of the torsional effect. For eccentric wall-frame structures in general, therefore, dynamic analysis remains to be the most reliable method to distribute the torsional effect at the present time.</p>