National Concrete Pavement Technology Center

Project description

To assess the strength of the Bibb Graves Concrete Arch Bridge, the Alabama Department of Transportation sponsored an investigation by Auburn University. In one of the spans, the arches are experiencing severe longitudinal cracking from Alkali-Silica Reactivity (ASR). The significance of ASR on the load carrying capacity of the bridge is unknown for two reasons. First, the effects of ASR on cross section capacity are not well understood. Second, a load rating for the bridge has never been performed. As an initial step for planning maintenance activities for the bridge, a load rating is performed in this project. Ratings are provided for the vertical columns, vertical tension hangers, transverse floor beams, and arch ribs. Rating factors were calculated for components of the bridge based on the as-designed condition. Ratings were provided for eight standard trucks and the American Association of State Highway and Transportation Officials (AASHTO) lane load. Evaluation of the impact of ASR on the ratings was outside the scope of the project. An operating rating factor significantly less than 1.0 was calculated in preliminary ratings of one arch. The low rating was attributed primarily to slenderness effects calculated using the moment magnifier from AASHTO Standard Specifications for Highway Bridges (2002). An investigation was conducted to identify a more accurate method for including slenderness effects in the arch rating calculations. An analysis that combines an elastic second-order analysis for dead loads and a linear analysis for moving live loads was found to be very practical and sufficiently accurate. This combined analysis approach resulted in significant increases in the arch ratings beyond those found by using the AASHTO moment magnifier method. Sensitivity analyses were performed to assess the effects of ASR-expansion on the bridge components’ internal forces and arch vertical deflection. Effects of changes in concrete strength and modulus of elasticity on member forces, deflection, and ratings were examined. Load ratings of the Bibb Graves Concrete Arch Bridge resulted in multiple operating rating factors less than 1.0. The lowest operating rating based on the as-designed condition is 0.86 for shear in an arch rib. This rating factor was increased to be greater than 1.0 by using a higher concrete compressive strength that was justified by concrete core test results. The final load ratings are controlled by the positive bending capacity of the transverse floor beams. The controlling operating rating factors are 0.93 and 0.97 for the standard tri-axle truck and concrete truck, respectively.