York PedBridge to the Past:

A Management Plan for Pennsylvania’s Historic Metal Truss Bridges

Evaluating, Assessing and Prioritizing Historic Metal Truss Bridges

Perhaps the most important elements of this plan are careful considerations of both the heritage value and preservation potential of each bridge.  These are the elements that help guide planning, design and management decisions affecting the bridges.  The heritage value of each bridge is derived from its National Register evaluation and whether and how it meets the preservation benchmark established in this plan. The preservation potential is also informed by the preservation assessments conducted for each bridge.

National Register Evaluations and Status

As noted in the management summary, the National Register status of metal truss bridges is one of the most critical elements for the evaluation of their heritage value. A determination of National Register eligibility under one or more of the four National Register Criteria is based on two separate but related evaluations: significance and integrity.

National Register evaluations of Pennsylvania’s metal truss bridges were completed in 2001 and a reevaluation was completed in 2008. These evaluations produced a list of over 180 National Register eligible or listed bridges.

In the first half of 2017, inventoried metal truss bridges that were determined not eligible as individual structures for their engineering significance in the last round of evaluations (2008) underwent a re-evaluation of their National Register eligibility and the results are being reviewed.   The reevaluation resulted in a number of additional bridges being determined eligible for the National Register. The list of re-evaluated bridges is now available, and public comment on the reevaluation is welcome. You can provide your comments to Kara Russell at PennDOT or Tyra Guyton at Pa SHPO. Newly eligible bridges will also be integrated into the transportation planning process.

Of course, conditions change, and this management plan recognizes that inevitability. Bridges may be removed and replaced, or rehabilitated, or moved for adaptive reuse. New information and better contexts may cause evaluations to evolve. Local or regional land use and transportation networks may change and affect the context of a bridge.  This means that like all populations of historic structures, these bridges will continue to undergo periodic re-evaluations of National Register eligibility. The management plan calls for re-evaluations at ten year intervals, so the next anticipated re-evaluation effort would occur in 2026. Obviously, if new information regarding the individual eligibility of any particular bridge becomes available, re-evaluation of that bridge could take place sooner.

Another important consideration in evaluating the National Register status of bridges is their potential to be contributing elements to a larger National Register eligible or listed Historic District.  Some of the historic metal truss bridges have already been evaluated and determined eligible as components of a National Register Historic District, even though in some cases they did not meet National Register criteria for eligibility as individual structures.  Others have never been evaluated for inclusion in a historic district.  In still other cases, the landscapes and communities around these bridges have never been formally evaluated to determine if a National Register Eligible Historic District is present.  Normally, an evaluation and determination of a potential historic district would occur as part of the Section 106 of the National Historic Preservation Act (Section 106) process as part of a project involving the bridge. 

As a planning tool for these bridges, the PA SHPO conducted an inventory of both existing and potentially eligible historic districts in proximity to truss bridges has been developed.  They have also completed preliminary evaluations of the potential each bridge might have to contribute to these districts.  It must be noted that the potential districts defined in the inventory have never had formal determinations of National Register eligibility

You can download the PA SHPO’s report on  potential, eligible and/or listed historic districts around historic trusses HERE.  The final list of those districts will be posted here as soon as the tabulation is complete.


 Preservation Priority

Section 106 and its implementing regulations provide for equal consideration of all eligible or listed bridges.  That said, any effective resource planning effort for historic truss bridges has to account for the reality that each of Pennsylvania’s eligible and listed bridges has a heritage value relative to the state’s other eligible and listed bridges.  That reality is acknowledged in other state-wide preservation plans (Vermont, OhioIndiana ) and in planning and preservation guidance (NPS Preservation Planning Guidance).   The effort to prioritize the heritage value of bridges is critical to their effective integration into the transportation planning process. 

A clear and empirical path to the evaluation of preservation priority has to be based on measurable and relevant criteria. The criteria and benchmarks employed in this plan include a consideration of the various bridge designs (e.g. Pratt, Warren, Bowstring, etc.) and their rarity, context, unique or distinguishing technological and design features, and their condition. Our preservation priority evaluation divides the bridges into three priority categories: moderate, high and exceptional. The criteria that define each category are discussed below.

A point system was designed to evaluate truss bridges under Criterion C for eligibility for listing in the National Register.  The following characteristic were used in the assignment of points: 

 

Characteristics

 

  • Built before 1900
  • Constructed using wrought/cast iron
  • Uncommon and/or distinctive type and design or the only known example in the state
  • Early example in the state or region as defined by each type and design
  • Earliest example of a state standard plan
  • Rare in a region (as defined by PennDOT district) - three or less truss bridges of the same type and design
  • Exceptional span length compared to bridges of the same type and design
  • Exceptional overall length compared to bridges of the same type and design
  • Important or unusual special features or innovations that show evolution, variation, and/or transition of a type and design
  • Outstanding technological achievements
  • High artistic value
  • Work of a Master (Important or prolific designer, builder, and/or engineer)

 

 

Category

Characteristics

Exceptional

Exceptional bridges exhibit an average of 8 characteristics. (Range from 5 to 11 characteristics).

High

High bridges exhibit an average of 5 characteristics (Range from 4 to 7 characteristics). Some bridges exhibit more characteristics than average but lack integrity to qualify as exceptional.

Moderate

Moderate bridges exhibit an average of 3 characteristics (Range from 2 to 6 characteristics).  Some bridges exhibit more characteristics than average but lack integrity to qualify as exceptional or high. 

 

Notes:

Exceptional bridges exhibit a combination of most of the following characteristics.  These bridges are usually the earliest bridges of each type and design.  Many utilize wrought or cast iron in their construction.  Most often these bridges are uncommon truss designs or the last remaining bridges of a type and design.  They display multiple details that are rare or represent the era of experimentation.  Many exhibit exceptionally long spans and/or overall bridge length.  Most of these bridges are regionally important as the earliest bridge in the region and/or one of the last remaining bridge of a type and design in the region.  Many of these bridges have high artistic value and are built by prolific or important builders.   A few common bridges that are unaltered or complete examples may also be included in this category.

 

High bridges exhibit a combination of many but not most of the following characteristics.  These bridges are usually early bridges but not necessarily the earliest of a type and design.  A few may utilize wrought or cast iron in their construction but most do not.  Some are uncommon truss designs but lack other characteristics and/or integrity to be classified as exceptional.  They often display details that are important in the evolution from the era of experimentation to the standardization of the truss design.  Some exhibit exceptionally long spans and/or overall bridge length.  Some of these bridges are regionally important as the earliest bridge in the region and/or one of the last remaining bridge of a type and design in the region.  Many of these bridges have high artistic value and are built by prolific or important builders in the state.  Some bridges express characteristics of a bridge categorized as exceptional but lack sufficient integrity to be classified as such. 

 

Moderate bridges exhibit a combination of only a few of the following characteristics.  A few bridges are early bridges but lack other characteristics of exceptional and high bridges.  Most of these bridges do not utilize wrought or cast iron in their construction.  A few are uncommon truss designs but lack other characteristics and/or integrity to be classified as exceptional or high.  They may display a detail that is important in the evolution from the era of experimentation to the standardization of the truss design and a few may display multiple details.  A few exhibit exceptionally long spans and/or overall bridge length but lack other characteristics to be classified as high or exceptional.  A few may be regionally important as the earliest bridge in the region and/or one of the last remaining bridge of a type and design in the region.  A few of these bridges have high artistic value and are built by prolific or important builder in the state.  Some bridges express characteristics of a bridge categorized as high but lack sufficient integrity to be classified as such. 

As of this writing (March, 2017), the bridge population is being evaluated against the prioritization matrix, and each bridge is being assigned to one of the three categories.  We expect to complete that task by summer 2017 and will post the results here.

Preservation Assessments

Of the eligible and listed bridges whose status was determined before 2016, 85 were identified as good candidate bridges for integration into the formal transportation planning process at PennDOT.  The remaining bridges were not good candidates for planning for a variety of reasons. Some had already progressed beyond planning to design for rehabilitation or replacement. Some had already been rehabilitated, moved, or replaced. Some were railroad bridges. A list of the National Register listed or eligible bridges that are being integrated into the transportation planning  segment of this plan can be found HERE. Preservation assessments are being prepared for these bridges.

Individualized preservation assessments were prepared for metal truss bridges that are expected to be integrated into the Departments planning process. In most cases, these assessments include an evaluation of each bridge’s ability to meet the 15 ton benchmark and still retain its historic significance. The ability to meet a minimum load carrying capacity of 15 tons is generally considered the minimum load carrying capacity for rehabilitated structures that continue to serve most vehicular traffic, although some bridges that can’t meet this benchmark can still be viable as pedestrian facilities. A limited but important caveat to this 15 ton benchmark lies in the original design capacities of some bridges. Since a few truss bridges were originally designed with a load capacity below15 tons, those bridges with lower designed capacities were evaluated for their ability to meet their original designed weight limits while retaining their historic integrity.

The assessments contain useful baseline information about each bridge including location, ownership, bridge type and design, year built, dates of alterations and/or rehabilitations, width/length, number of spans, roadway classification and type of service, preservation priority , character defining features (those features that enable a bridge to convey its engineering significance) , setting description, average daily traffic, observed crash history, safety features, proximity of alternate routes, a summary of structural geometry, hydraulics, condition rating, load ratings, and a summary of structural deficiencies. The assessments also include options that could address structural deficiencies including whether the problems  can be addressed through maintenance or would require rehabilitation.  The assessments are intended to help guide and inform planning organizations as they decide whether and how to program bridge projects on their Transportation Improvement Program (TIP) and Long Range Transportation Plan (LRTP).  You can download copies of the completed assessments below.

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Completed Bridge Preservation Assessments

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