pRE-sTRESSED aDJACENT NON-cOMPOSITE bridgeS
In December 2005, on Lakeview Drive over I-70 in Washington County, PA, a prestressed non-composite exterior beam and barrier from an overpass bridge collapsed onto the roadway below. Fortunately, no one was seriously hurt, and since this incident, there have been numerous steps taken to ensure that this structure type is carefully monitored through NBIS inspections.
Currently, LDG inspects 72 of these structures on an annual basis, using specific inspection criteria and descriptions for prestressed non-composite adjacent box beams. Most of these bridges have a superstructure rating of 4 (poor condition) or less, which identifies the bridge as structurally deficient. Over the past decade, many have been rehabilitated or replaced across the state. However, there are still many structures currently in service that could be rehabilitated before severe deterioration occurs and significant weight restrictions or closure is required.
What can be done to rehabilitate these bridges? One option is to remove the bituminous pavement over the prestressed beams and construct a new reinforced concrete deck in its place. The concrete deck integrates the beams as one unit, increases the structure’s capacity by removing the independent beam action, preserves the beams by eliminating water infiltration through the bituminous pavement, and minimizes the chances of a catastrophic beam failure (similar to the I-70 incident).
If nothing is done to maintain these structures, water will continue to penetrate into the beams, accelerating deterioration (as shown in the photo above). Deterioration is evident by hairline to open cracks on the bottom of the interior beams with visible areas of efflorescence and rust staining. Other signs of distress include loss of beam camber and failed shear keys between the beam joints (water stains). Non-composite adjacent box beam bridges that exhibit independent beam action with cracks and signs of distress will require significant weight restrictions and possible closure. Once a non-composite adjacent box beam structure is load posted, there are few options to remove the weight restriction other than a superstructure replacement or composite deck reconstruction. Even then, a composite deck reconstruction may not be a viable solution based on the overall condition of the beams.
The photos below show the progression of deterioration in the same fascia beam from 2011 to 2014.
Fortunately, this can be prevented. Historical construction costs indicate that a non-composite adjacent box beam rehabilitation costs less than half as much as a superstructure replacement for this structure type.