Refined Stringer Ratings for Girder–Stringer Floor Beam Bridges

The Tennessee Department of Transportation has charge over 31 bridges using steel girder–stringer–floor beam construction. While no distress in the stringers of these bridges has been observed during inspection, load rating calculations have indicated insufficient capacity. To reconcile this apparent contradiction, the research described herein investigated the behavior and capacity of stringers in girder–stringer–floor beam bridges by: (1) conducting a literature review of the current state-of-practice and emerging research regarding the capacity and behavior of girder–stringer–floor beam bridges; (2) experimentally measuring strain under known loading at various locations and identifying characteristic behavior in the stringers and girders of the Billy Nance Memorial Highway Bridge under live load; and (3) conducting line girder analysis of continuous stringer spans using SIMON software and identifying potential sources for the refinement of load ratings. Upon evaluation of the diagnostic load testing results, it can be inferred that the stringers in the girder–stringer–floor beam bridge, SR-114 over the Tennessee River, have a higher load rating than initially anticipated. While this is due, in part, to the fact that some of the stringers act semi-compositely with the concrete bridge deck (as demonstrated by the measured strain profile at the member cross sections), composite behavior was not relied upon in refining load ratings for stringers. The stringers do not have top flange shear studs, and not all stringers exhibited composite behavior. Live load demand on the stringers was observed to be less than that which would be predicted using the lever rule for live load distribution. Other sources of undue conservatism in typical stringer rating calculations were identified as well: lateral–torsional buckling factor computation, critical stress calculation, and the provision for web plastification in the design equations.

Engineers who provide load ratings for girder–stringer–floor beam bridges with rolled-shape stringers may find the results of this study useful in refining rating factors for these bridge elements. Sources of refinement include a potential reduction in the demand on stringers through diagnostic load testing and a potential increase in resistance through the application of more accurate $C_b\text{-}\text{factors}$, critical stress calculations, and web plastification for compact, rolled-shape stringers.