Practical Implementation of Stability Bracing Strength and Stiffness Guidelines for Steel I-Girder Bridges

Abstract

Traditionally, cross frames for straight steel I-girder bridges have been designed with consideration of little more than wind loads and criteria for individual member slenderness. While this practice has usually resulted in acceptable designs, the lack of quantification of design loads has been disconcerting to some engineers, and some have questioned if this practice is sufficient. Recent research has advanced the state of the art in bridge engineering, particularly in the area of cross-frame design. NCHRP Report 725 provides key insights and practical guidance for the analysis of straight steel I-girder bridges with moderately to significantly skewed supports, specifically in the area of calculation of cross-frame forces; this work recommends the use of refined structural analysis methods for these bridges, which directly calculate the forces in individual cross-frame members. Meanwhile, research by Yura and Helwig has produced guidelines for assessing the minimum strength and stiffness requirements for bracing members such as the cross frames of straight steel I-girder bridges with little or no skew, for which simplified line-girder analysis methods, which do not produce any assessment of cross-frame member forces, are commonly used. However, specific guidance is lacking with regard to practical implementation of these guidelines within the context of composite steel I-girder bridge design performed under the provisions of the AASHTO load and resistance factor design specifications for bridge design. This paper recommends appropriate load factors and load combinations for use with these guidelines and provides a discussion of recommendations for their implementation in positive- and negative-moment regions of multiple-span continuous steel I-girder bridges.