Design of tapered member portal frames

Non-uniform steel frames, incorporating tapered or haunched members, have proved to be economical solutions for warehouses and factory buildings over a wide range of spans. Design rules, however, deal only with uniform member frames and therefore attempts have been made to convert the non-uniform frame into an equivalent uniform frame. One such method appears in the AISC Specification for the Design of Steel Buildings(l) but it is relatively complex and of limited application. A simpler and more general procedure, based on a parametric study and using a limit state interaction equation, is presented. In the study, elastic instability computer programs were used and changes were made to the values of the principal variables. The results were subjected to an analysis of variance and equations of best fit were obtained for the conversion to an equivalent uniform frame. At this stage of the research program only completely symmetrical taper member frames have been investigated. The proposed conversion enables the designer to use all the currently available design aids dealing with the effective length factors of uniform members. The method is illustrated by two numerical examples.