Reconstructing an Historic Bridge to its Former Glory

Built in 1904, the Little Muncy Creek Pratt Truss served Moreland Township for more than a century. The truss provided a critical waterway crossing for this small, rural community of residents and farmers in northeastern Pennsylvania. It’s also a contributing resource to the Smith/Wallis Gristmill Historic District and a significant component of the area’s history.

Unfortunately in 2009, the truss superstructure was given a National Bridge Inspection Standards (NBIS) rating of 2-“critical condition” and the crossing was posted for a five-ton weight limit. This was a concern for the local community, because it required emergency services, school buses, and local deliveries to use a detour.

When it came time for the Pennsylvania Department of Transportation (PennDOT) to update the truss, they wanted a bridge that maintained the historic integrity, aesthetic appeal, and function of the 1904 Pratt truss, but with increased load capacity, load path redundancy, and low maintenance.

The new truss is a replica of the original 1904 design to maintain the historic integrity of the local area.

The new truss is a replica of the original 1904 design to maintain the historic integrity of the local area.

The Design and Reconstruction

Our team investigated the potential reuse of the existing members to retain the structure’s historic nature, but due to widespread deterioration, this method was cost prohibitive and provided limited improvement of the structure. We determined the best alternative would be complete reconstruction of the pin-connected Pratt truss utilizing modern materials, while maintaining the historic nature of the bridge. The only element of the original bridge that was able to be salvaged was the lattice railing.

The original truss had widespread deterioration to its members, and only the lattice railing was salvageable.

The original truss had widespread deterioration to its members, and only the lattice railing was salvageable.

Some of the details we used to improve the capacity and provide redundancy included catch plates, additional counters, and increased sizes of critical members. We also mitigated future maintenance concerns by galvanizing all members, adding grease fittings to the pin connections, filling the grid deck with concrete over the abutment seats and over the beam flanges, and adding bent curb plates to direct runoff away from the main truss bearings. Our structural engineers also developed details to reduce vibrations of the diagonal members and open grid deck.

The reconstructed truss bearing retained the original aesthetics.

The reconstructed truss bearing retained the original aesthetics.

Challenges Faced

We faced a few unexpected challenges during this project. After it was decided to salvage the lattice work and had proceeded to final design, a truck drove across the bridge and got hung up on the lattice. It tore off a considerable amount and damaged the lattice to the point that it couldn’t be salvaged. We designed details to recreate the damaged portion of the lattice in time for the bid package to be advertised.

To build the new bridge, we mimicked the original fabrication drawings that were used to build the bridge. During a site visit to determine the geometry, we realized that the skew of the bridge didn’t match the fabrication drawings. Our bridge engineers quickly addressed the skew challenge by completing detailed field measurements, reanalyzing the structure design, and modifying our drawings so the project would not be delayed.

New Bridge, Old Design

This new bridge is essential to the public safety, health, and welfare of the local rural community and is expected to function for at least another 113 years, while keeping the historical integrity of the area.

The Little Muncy Bridge is one of approximately 30 state-owned Pratt trusses that still carries traffic in Pennsylvania, and only four of those are pin-connected according to research conducted in 2017. Little Muncy is also the only skewed bridge of the four, making it very unique. Getting to work on this project was a once in a lifetime career opportunity. Little Muncy might be the last standing Pratt truss that’s I-bar connected 50 to 100 years from now.

Salvaged by a private citizen, the original bridge plate was reinstalled.

Salvaged by a private citizen, the original bridge plate was reinstalled.

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  • Curtis Sanno
    Curtis Sanno
 
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