Chai H. Yoo

Flexure and Shear Interaction in Steel I-Girders

AASHTO LRFD and AISC specifications have adopted Basler’s interaction equation, which was formulated for noncompact sections without considering shear buckling. AASHTO LRFD specifications, however, have completely neglected the interaction effect of bending on shear strength since the 3rd edition in 2004. AISC LRFD specifications followed suit with the 2005 edition. This study was aimed at investigating interaction behaviors in noncomposite I-girders and developing new interaction equations without considering shear buckling. Also, a simple and universally applicable methodology was proposed for web panels whose failures were associated with shear buckling. The interaction should not be ignored when web failures are governed by yielding because of combined bending and shear. Otherwise, the shear capacity of the web panel could be grossly overestimated, thereby compromising the safety of the girder.

Longitudinal Stiffeners in Concrete-Filled Tubes

To enhance the strength and constructability of rectangular (or trapezoidal) tubular compression members, reinforced or unreinforced concrete may be filled in the tube. Longitudinal stiffeners are often attached to increase the local buckling strength of the thin-walled skin. The effect of important design parameters on the minimum required stiffener moment of inertia was investigated numerically in this study by examining the residual stress distributions, initial imperfections, and elastic and inelastic buckling stresses of a number of hypothetical models. Because the thin-walled panel can only buckle (bulge) out from the concrete core, the buckling mode shape of a panel with multiple stiffeners resembles a waffle slab. A series of parametric studies was performed to characterize and quantify the analytically collected data. A new equation for the minimum required moment of inertia for the longitudinal stiffeners was derived. Through the evaluation of a few selected case studies and a design example, the validity and reliability of the proposed equation was demonstrated.

Bending Strength of a Horizontally Curved Composite I-Girder Bridge

AbstractHorizontally curved I-girders are subjected to combined vertical bending and torsion under gravity loads alone. The torsional behavior of open I-shaped girders is commonly and conveniently transformed to self-equilibrating lateral bending of flanges. The interaction of vertical bending and lateral flange bending reduces the vertical moment–carrying capacity of the section. The interaction equations for predicting the nominal bending strength of horizontally curved I-girders of compact-flange sections subjected to vertical moment and torsion have been derived. Singly symmetric composite and noncomposite I-shaped cross sections in the positive and negative moment zones, respectively, are considered. These strength criteria are based purely on the static equilibrium of the cross section with no secondary amplification considered. The limitations and applicability of the derived equations toward the design use are demonstrated and analyzed.

Lateral-Torsional Buckling

This chapter derives differential equation for lateral-torsional buckling. If transverse loads do not pass through the shear center, they induce torsion. In order to avoid this additional torsional moment in the flexural members, it is customary to use flexural members of at least singly symmetric sections so that the transverse loads pass through the plane of the web. In addition, the reason buckling occurs in a beam at all, is that the compression flange or the extreme edge of the compression side of a narrow rectangular beam, which behaves like a column resting on an elastic foundation, becomes unstable. If the flexural rigidity of the beam with respect to the plane of the bending is many times greater than the rigidity of the lateral bending, the beam may buckle and collapse long before the bending stresses reach the yield point. Along with this, designing simplification for lateral torsional buckling is also explained; it determines the critical loading for beams with several different boundary conditions and loading configurations.

Federal Highway Administration's Horizontally Curved Steel Bridge Research Project: An Update

Since 1992, FHWA has had a major concentrated research project in the area of horizontally curved steel bridges, the objective of which is to conduct research to better define the fundamental behavior of such bridges. The project involves theoretical work leading to the development of refined predictor equations and verification of those equations through linear and nonlinear analysis and experimental testing of I-girder components. The overall experimental program involves testing of a series of full-scale bending and shear curved steel I-girder components and subsequently a full-size bridge. The development and refinement of predictor equations are summarized, and the work leading to the first series of experimental tests, which involve testing of full-scale bending components, is described.

Effects of Distortion on Strength of Curved I-Shaped Bridge Girders

The curved I-shaped plate girders used in bridges with curved alignment are subjected to forces that cause significant distortion of the cross section during construction and during application of live loads after the deck has hardened. Furthermore, the addition of curvature reduces the vertical bending stiffness, increases deflection nonlinearities, and changes stability characteristics of behavior. Although the design equations of the AASHTO Guide Specifications for Horizontally Curved Highway Bridges are formulated to address these behavioral issues, design and construction engineers often are not familiar with the difficulties curvature introduces and do not understand the relationship between distortion and deflection amplification with the design equations. Analytical research conducted as part of the FHWA Curved Steel Bridge Research Project was used to highlight and describe the effects of curvature on the strength and stability of curved I-girder bridge superstructures. Issues described include the following: (a) effects of cross-frame and diaphragm spacing on system behavior, (b) effects of curvature on the lateral-torsional stability of curved I-shaped beams, (c) effects of warping stresses on flange buckling, (d) effects of curvature on web behavior, and (e) effects of curvature on initiation and propagation of yield stresses in the girders of curved I-girder frames.