Behavior of Structures under Loads Causing Torsion

The results of several analyses of a 24-story structure are presented. The structure is composed of two series of orthogonal bents; the bents consist either of steel rigid frames or of steel rigid frames coupled to reinforced concrete shear walls. The analysis used to predict the response of the structure is an approximate elastic-plastic technique which first lumps the members of each bent into an equivalent structural model. The results presented illustrate the change in behavior which accompanies an increase in the eccentricity of lateral load or an increase in asymmetry of structural arrangement. The effect of increasing the torsional resistance of the vertical element or shifting the positions of the stiffer bents, is illustrated. A reduction in ultimate strength is predicted (below that for the symmetrical case) whenever the secondary moments are effectively increased. This may be achieved by increasing the eccentricity of lateral or vertical loads or by changing the structural arrangement and ranges up to 66% in the extreme case.