Lean-On Bracing Refined Design Methods and Field Assessment

Abstract

Cross-frames are critical for the stability of steel bridges during construction and play a significant role in completed bridges. Historically, brace locations have been regions of fatigue concerns, and each brace requires significant handling and processing during fabrication. The braces represent one of the most expensive bridge components per unit weight. Therefore, there are significant benefits to minimizing the number of cross-frames in a bridge in terms of economics and structural performance.

Lean-on bracing concepts replace select cross-frames in certain bracing lines with top and bottom struts, which allow a single cross-frame to brace several girders as a method of minimizing the number of cross-frames in a bridge. Lean-on concepts were developed for the Texas Department of Transportation (TxDOT) in the early 2000s. Previous studies developed design guidelines, but recent applications of lean-on bracing in TxDOT bridge designs demonstrated the need for improved efficiency and clarity.

The stiffness and strength of a given line of bracing are functions of the number and location of cross-frames in the line, as well as the specific cross-frame geometry (X-, K-, or Z-frames). While previous lean-on bracing equations were applicable to systems with one X- or Z-shaped cross-frame positioned in an exterior bay, derivations and model validation have been completed to extend the application of the design guidance. Derived equations with simplified design expressions for lean-on brace stiffness and strength will be discussed in terms of stability implications, with consideration for future application.