Effect of 3-D Modeling on Design of Pre-Engineered Steel Buildings

Abstract

Professional engineers in Pre-engineered buildings (PEB) generally adopt two dimensional (2-D) modeling of frame i.e. plane frame modeling in transverse direction and profess for optimal design using elastic method. In practice, 2-D modeling of main frame is done for transverse frame while in longitudinal direction truss analogy is adopted mainly for analysis of bracing. In 2-D analysis majority of cases only major axis moment is considered and the minor axis moment (having sometimes significant effect on design) is generally ignored. This minor axis moment can be of less magnitude if the bracings are of X type designed as compression as well as tensile members. However in practice these bracings are designed as tension only members, leading to significant effect of minor axis moments. It is also reported that if the ratio of deflections at knee joint in direction of ridge from second order analysis (Δ2) versus first order (Δ1), is 1.5 or more, second order analysis is must, however most of the cases in practice this factor is ignored. This study is aimed to check and quantify the effect due to minor axis moment, i.e. My, in braced frames at knee joint. In absence of second order analysis, as mandated by codes, an approximate practical approach is presented for assessment of the minor axis moment, My, in terms of major axis frame moment, Mz. It is observed that the ratio of building height to width is significant in determining ratio My/Mz. Keywords: 2-D analysis, 3-D analysis, Cross bracing, Minor axis Moments,, Pre-engineered buildings, PEB,.