A Modification to the Subassemblage Method of Designing Unbraced Multi-story Frames

A new method of designing girders and columns of unbraced multi-story frames under the combined action of vertical and horizontal loads has been recently developed.The method is based on the concept of sway subassemblages and takes into account the elastic-plastic behavior of girders, the instability of columns, and second order effects, caused by PD moments, on the structure. It employs a semi-graphical solution using specially prepared charts, and its final aim is to obtain the complete horizontal load-lateral deflection curve for each story of the frame.This method has been thoroughly described in Ref. 1, and Ref. 2 is an abstract of the method. Its bases appear in Ref. 3, and the charts required have been published in Ref. 4. The purpose of this paper is to present a modification to the original method, which allows us to obtain the horizontal load-sway deflection curve (QD curve) for each story of the frame without employing the charts of Ref. 4 and reducing, at the same time, the numerical work required to solve the problem. The final results are very close to those obtained by application of the original method, and the small differences between both methods are not significant in most practical cases. The basic assumptions relative to the behavior of the structure are maintained (see Ref. 1, p. 4), but the condition of uniformly distributed gravity loads on the girders is deleted, because solutions can be obtained for any type of vertical loads.