Metal truss bridges are an important part of Pennsylvania’s engineering and technological history and stand as a legacy to the state’s iron and steel industry.

However, the loss of over half of the historic metal truss bridges in Pennsylvania since the 2001 Statewide Historic Bridge Inventory spurred the need to reevaluate the National Register eligibility of the remaining population.

In 2016-2018, a committee composed of staff from both the Pennsylvania Department of Transportation (PennDOT) and the PA State Historic Preservation Office (SHPO) was formed to lead the revaluation. As part of the effort, inconsistencies in the 2001 application of National Register Criterion C were identified and addressed. More current scholarship, including a point system for evaluating eligibility, was also introduced.

Long metal bridge over Allegheny River

The Parker Bridge carries PA-368 over the Allegheny River between Armstrong and Clarion County was determined eligible under the newly developed point system for evaluating bridges for the National Register of Historic Places. This 4-span, rivet-connected Parker thru truss, was built in 1934 and receives points for being a multi-span bridge, expressing exceptional span length, and being built at a 6.32% grade which was an important achievement at the time of its construction. Photo courtesy of PennDOT.

The previous Statewide Historic Bridge Inventory presented inconsistencies in National Register eligibility evaluations. The evaluations included varying levels of detail regarding the important character defining features. For example, features and characteristics of early bridges such as unique floorbeam hangers or the work or a prominent builder, were called out, while later bridges were simply described as “traditionally composed” or a “late example”.

In addition, for those bridge types and designs considered “common in the county and region,” only the most complete examples were considered significant. In consideration of the dwindling population of metal truss bridges, only a few early or complete examples still remain in 2018. Finally, those bridges already considered National Register eligible as contributing resources to historic districts, were not assessed for individual eligibility under Criterion C for engineering significance in the 2001 survey.

Metal truss bridge over Monongahela River

The Elizabeth Bridge carries PA-51 over the Monongahela River in Allegheny County. In the previous Statewide Historic Bridge Inventory, this 1951 Warren thru truss was described as traditionally composed for the period with no innovative or distinctive details that distinguish it from the many other post-WWII major bridges in the region. However, this metal truss bridge is an unusual design and one of only a few metal truss bridges with a suspended deck in the state. In addition, this bridge receives points for being multi-span and having exceptional span and overall length, resulting in enough points to elevate it to the status of National Register eligible. Photo courtesy of PennDOT.

In an effort to objectively look at the remaining population of metal truss bridges, a point system was developed to provide a consistence application of the National Register Criteria for evaluation than what had been used in the past.

The point system awards points to all bridges with distinctive characteristics, special features, or innovations, as well as early and rare bridges. The system subtracts points for loss of distinctive characteristics or lack of integrity of materials, design or workmanship. Those bridges that maintain a minimum threshold of points are considered eligible for the National Register.

An important feature of the point system is that a bridge can be reevaluated at any time if new information should become available.

1905 short metal truss bridge over water

Heffners Bridge carries Garman Road over Sacony Creek in Berks County. This 1905 pin-connected Pratt thru truss was considered traditionally composed with no innovative or distinctive details in the 2001 survey. In 2018, in the context of the remaining population of this metal truss bridge type, the bridge was found to reflect early use of rolled metal truss members retaining distinctive connections that represent the transition from all pinned to riveted connections; the bridge received enough points to be determined eligible. Photo courtesy of PennDOT.

Staff applied the point system to all metal truss bridges in PennDOT’s Bridge Management System, including those previously determined eligible. Using the points system, 12 of the previously eligible bridges were unable to meet the minimum threshold of points and are not eligible, while 51 previously not eligible bridges are determined eligible.

Metal truss bridge over water

Minersville Bridge carries 4th Avenue over the Conemaugh River in Cambria County. This 1914 rivet-connected Baltimore thru truss is a contributing resource in the Cambria City Historic District and was therefore not assessed for individual eligibility in the 2001 survey. In 2018, Baltimore trusses have become rare, and in the remaining population, this metal truss bridge is considered an early example. The bridge also exhibits an unusual feature; the deck of the bridge is constructed at a noticeable incline inside the truss structure instead of running parallel to the chords. The Minersville Bridge received enough points to be determined individually eligible. Photo courtesy of PennDOT.

The methodology for the reevaluation of the metal truss bridges, including a complete breakdown of the point system and a list of the eligible metal truss bridges, can be found on PennDOT’s website. The point system provides a consistent and replicable approach to determining the eligibility of a bridge, regardless of its type, design, materials, feature, or age and it can be applied to other bridge types and designs.

Stay tuned, as PennDOT and SHPO staff plan to use the point system in the reevaluation of the open spandrel concrete arch bridges in the coming year!

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