No. 4 BRIDGE: From coal to colossal

Nov. 12, 2013

There are a few abandoned coal mines around Brownsville, Pa. Work also has stopped on the Monongahela River Bridge, but you can feel the energy in the air now more than ever.

There are a few abandoned coal mines around Brownsville, Pa. Work also has stopped on the Monongahela River Bridge, but you can feel the energy in the air now more than ever.

The deep valley of the Monongahela River required a bridge nearly 200 ft tall and a structure length of 3,000 ft. The high-strength piers, 30 ft by 23 ft, were created with 15-ft-tall jump forms that were advanced upward as the concrete lifts below were sufficiently cured. The piers consist of 5,500-psi concrete with some locations requiring Type II cement for moderate sulfate resistance. Added protection was warranted because the site is near an old mining quarry with sulfates in the soil. The approach piers farther inland were designed as C-shaped twin-wall columns that used the octagonal design split in two, leaving an open side. This design created flexibility to mitigate the long-term creep and shrinkage of the superstructure while providing the strength and stability needed during cantilever construction without special support framework.

The abandoned coal mines created complicated restrictions on pier locations. Extensive mine grouting and stabilization, which was minimized by the elimination of two piers, was required prior to deep foundation construction. This work kept the integrity of the mines and the surrounding area intact and provided for efficient foundations.

“Designing the pier was one of the most challenging parts of the project, because not only did we have to try to match that shape on the existing footprint but also because it’s tall. These are tall piers, and you have all of the load considerations because they are so tall,” Jay Rohleder Jr., senior vice president at FIGG, told Roads & Bridges.

The superstructure box girders feature low-permeability 6,000-psi concrete with variable depths between 12 ft toward the midspan and 27 ft 2 in. at the piers. Some portions of the concrete superstructure were poured during the winter, requiring more attention to curing methods. These consisted of using wet burlap and a heating element, as well as three curing blankets to reach 3,500 psi.

In all, 51,000 cu yd of concrete, 7 million lb of reinforcing steel and 3 million lb of post-tensioning strand were used in the project.

The bridge is part of an extensive expansion to the Mon/Fayette Expressway that supports efforts by the National Road Heritage Park. The goal was to make nearby Rte. 40 less of a transportation artery and more of a local traffic corridor and tourist destination. The Monongahela Bridge improves access, addresses future capacity requirements and pulls traffic, including trucks, off of Rte. 40. R&B

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