Wherever you travel in Pennsylvania, you are likely to cross a historic bridge. These bridges are an important part of the cultural landscape and a link to Pennsylvania’s transportation and engineering history. Eventually these bridges need some level of work to continue providing a safe passage, but what is the best way to execute this work without diminishing the bridges’ historic character? By consulting and applying the Secretary of the Interior’s Standards for the Treatment of Historic Properties (the Standards) for guidance.
The Secretary of the Interior is responsible for establishing standards for national preservation programs. These standards offer common sense preservation principles that promote best practices for the maintenance, replacement, and repair of historic materials and the design of new additions and alternative uses for historic properties.
The Standards are broad concepts meant to be applied to a wide variety of property types. To provide guidance on the treatment of bridges, the Pennsylvania State Historic Preservation Office (PA SHPO) has developed more specific guidance on the consideration and application of the Standards to historic bridges in Pennsylvania.
The character defining features of this stone arch bridge that carries Price Road over Unami Creek in Marlborough Township in Montgomery County are the voussoirs, arch barrels, and stone construction, including the cut stone and the coursing. Photo from PennDOT BMS2 Inspection Files, April 2016.
Why are the Standards important?
It seems that modern materials and construction methods would make for a stronger bridge, but that is not always the case. Modern materials and methods can alter the character of a bridge and in some cases, accelerate physical deterioration or cause damage to the bridge.
In addition, the Standards have guided Federal agencies in carrying out their responsibilities under Section 106 of the National Historic Preservation Act. Section 106 requires Federal agencies to consider the effect projects they sponsor might have on historic properties (resources listed in or eligible for listing in the National Register of Historic Places). If a bridge rehabilitation project can be designed to meet the Standards, then it is possible to avoid or minimize an adverse effect under Section 106.
By considering historic character and the Standards alongside project needs, a bridge can retain its historic character, and the Federal agency can save time and money.
The Standards offer four approaches to the treatment of historic properties:
Treatment of historic bridges usually falls under the Standards for Preservation or the Standards for Rehabilitation. For bridges, preservation includes routine maintenance and repair of the bridge with an emphasis on the retention of historic materials.
These heavy timber trusses are among the character defining features of the Burkholder Covered Bridge over Buffalo Creek in Somerset County. Photo from PennDOT BMS2 Inspection Files, June 2016.
Standards for Rehabilitation
Rehabilitation allows for alterations or additions to a bridge to meet continuing transportation or new uses. Rehabilitation also requires consideration of the retention of historic material but it allows more flexibility for the replacement of deteriorated or missing features, while taking into consideration economic and technical feasibility.
Because the goal of the Standards is to preserve historic material and the resource’s historic character, it’s important to understand why a bridge is significant and what features and materials are considered important to conveying significance, also known as character defining features.
Most bridge in Pennsylvania possess individual National Register significance under Criterion C in the area of Engineering. These bridges embody distinctive characteristics of a type, period, or method of construction; are the work of a noted engineer, engineering firm or bridge company; or possess high artistic value.
The character defining features of historic bridges varies by the type and design of the bridge but are generally those features that convey the structure’s type and construction era, such as design features, materials, craftsmanship, connections among members, and decorative detailing. Often the National Register significance and character defining features of a historic bridge will be called out in the documentation that resulted in the structure’s listing or determination of National Register eligibility.
There are ten Standards for Rehabilitation. Those most relevant to bridge rehabilitation include retaining and preserving the historic character of a property include:
- Standard 2, preservation of distinctive features and finishes that characterize a historic property
- Standard 5, preservation of construction techniques or examples of craftsmanship that characterize a historic property
- Standard 6, preference for repair of deteriorated historic features over replacement
The upper chords of the Ross Fording metal truss bridge in Chester County are Phoenix columns. These columns reflect an important engineering technology and care should be taken to protect these features. Photo from PennDOT BMS2 Inspection Files, July 2014.
Guidelines for Rehabilitating Historic Properties
The Guidelines for the Treatment of Historic Properties are more specific guidance for the application of the broad Standards. These Guidelines create a hierarchy for conducting work from the least to the greatest level of intervention. The Guidelines specific to rehabilitating historic properties include approaches, treatments, and techniques that are consistent with the Standards and include:
Identify, Retain and Preserve
To begin, all character defining features of the historic bridge should be identified. Measures should be taken to protect and maintain these features. Protection can include posting a roadway weight restriction or using a headache bar to stop traffic that is taller than the allowable clearance.
Repair
If character defining features need repair, it should be undertaken in a manner with the least intervention, using recognized preservation methods such as splicing, consolidating, straightening, or reinforcing through additional material.
Replace
If a character defining features cannot be repaired, then replacement is allowed. It is preferred to replace the material in-kind using the original material with the same visual qualities. If in-kind replacement is not possible, then a substitute material can be used, however, it must convey the same visual appearance as the original material. Care should be taken to only use substitute materials when necessary, like when a bridge must be stronger to accommodate use, since substitute materials can reduce the historic character of a resource. In cases where a significant feature in the bridge’s historic appearance is missing, the features should be accurately replicated based on adequate historical, pictorial, and physical documentation.
Design for Missing Historic Features
If this documentation of the historic appearance of the bridge is missing, it is acceptable to create a new feature that considers the size, scale, and material of the bridge, and is compatible with the remaining character defining features of the bridge; however, the new work should be visually differentiated from the original so that a false historical appearance is not created.
This clearance device protects the Mill Road Covered Bridge in Bedford County from being crossed by vehicles that are taller than the portal opening. Photo from PennDOT BMS2 Inspection Files, February 2017.
Historic Bridge Rehabilitation Considerations
A range of alternatives need to be considered by the Federal agency as part of Section 106 with the goal of meeting purpose and need while accommodating historic preservation concerns.
For historic bridge projects, this includes consideration of rehabilitation for continued vehicular use. Since every bridge and project is different, the Standards and Guidelines must be applied on a case-by-case basis.
The following questions were designed to guide engineers and historians in the decision-making process. Investigation of these question can also be part of the documentation showing due diligence in the consideration of rehabilitation options. They are:
- Why does the bridge have National Register significance?
- What are the key aspects of integrity that allow the bridge to convey its significance?
- What are the character defining features that need to be retained for the bridge to convey its significance including distinctive engineering and stylistic features, finishes, construction techniques, and examples of craftsmanship?
- Does the bridge have historic alterations (more than 50 years old) that contribute to the overall significance of the bridge (Standard 4)?
- Are there levels of importance among the character defining features? If so, what are they? (More significant/distinctive features should receive greater levels of consideration for preservation or rehabilitation.)
- Can the character defining features of the bridge be preserved while accommodating the project purpose and need and safety requirements (Standard 1)?
- If it is not possible to repair the character defining features of the bridge, can you replicate historic materials, methods, and construction techniques without affecting the historic character of the bridge (Standard 5)?
- Can the new work on the character defining features match the old work in terms of size, design, color, texture, architectural detailing, and other visual qualities (Standard 6)?
- For missing features that will be replicated, is there documentary, physical or pictorial evidence (Standard 6)?
- Do new features, such as lighting, railing, or other decorative elements, give a false sense of the bridge’s history (Standard 3)?
- If new work is required, such as strengthening or reinforcement, can it be designed not to compromise the historic engineering significance of the bridge (Standard 10)?
- Can the new work be hidden from view?
- Are new features, such as guide rails, differentiated from the old and compatible in terms of massing, size, and scale (Standard 9)?
- Would a lay person viewing the bridge after it is rehabilitation be able to understand its original use, design, function, materials, engineering, and/or association (Standard 2)?
The Pine Creek bridge in Lycoming County is a rare lenticular truss. An early Inappropriate repair led to the bearings being encased in concrete which caused the bottom chord to buckle over time. In a later rehab, the damaged bottom chord was replaced with high-strength steel, an example of the use of substitute material. Nathan Holth, photographer, Courtesy of HistoricBridges.org., 2016.
If rehabilitation is chosen as the preferred alternative, the successful execution of plans and specifications developed in accordance to the Standards is crucial in order to avoid an adverse effect to the historic bridge Elements critical to a successful execution include comprehensive plans and specifications include: the use of qualified construction personnel with demonstrated experience working with the relevant historic material; open and consistent communication between engineers; historians; and construction personnel and construction monitoring.
Through careful planning and collaboration, application of the Standards can result in high-quality bridge rehabilitations that enable Pennsylvania travelers to continue to enjoy these historic bridges.
