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.
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.
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.
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.
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!
Want to learn more about historic metal truss bridges in Pennsylvania? Sign up for our bridge newsletter now to receive the first installment this fall!