Geometric Factors Influencing Structural Ductility in Compact I-Shaped Beams

Abstract

Local and global instabilities are usually treated as separate phenomena in the design of steel beams in buildings. In the past, such a notional decoupling of these local and global manifestations was necessary in the interest of creating useful and practical design provisions for steel specifications. Currently, great emphasis is being placed on ensuring ductile and predictable member level behavior in the interest of guaranteeing adequate systemwide response. The current practice of decoupling localized and global instabilities when determining member response is inadequate if structural ductility is to be correctly predicted. The research outlined herein uses experimentally verified nonlinear finite-element modeling techniques to identify two distinct inelastic modal manifestations that occur in compact I-shaped beams at failure. The existence of the two modes is supported by experimental evidence obtained from the literature, as well as other corroborating finite-element studies in the literature. A bifurcation point in the equilibrium path of I-shaped beams, at a load level close to the plastic capacity of the cross section, appears to mark the transition point between the so-called Mode 1 and Mode 2 inelastic buckling manifestations. Simple geometric parameters such as flange slenderness, web slenderness, and unbraced length, among others, are incapable alone of predicting the transition from one mode to another. In addition, predictive techniques contained in the literature and aimed at quantifying steel I-beam rotation capacity are not applicable to a wide enough range of beam geometries so as to be useful in design practice. Similarly, these techniques do not consider the important influences of certain features in the steel constitutive response for a particular steel grade. In order for a robust method to be identified as useful in predicting steel I-beam ductility, it is believed that one must consider cross-sectional geometry, unbraced length, and certain important material parameters such as yield strength, yield ratio, whether or not a yield plateau is present, and strain-hardening modulus.