April 2008

PENNDOT ENGINEERING DISTRICT 6-0
STONE ARCH BRIDGE MANAGEMENT PLAN


INTRODUCTION

The Greater Philadelphia Region has North America’s largest population of stone arch highway bridges. A total of 125 bridges 20 feet in length or longer[1] is found in Bucks, Chester, Delaware, Montgomery, and Philadelphia counties, which constitute the Pennsylvania Department of Transportation’s (PennDOT’s) Engineering District 6-0. The bridges are enduring cultural icons, but they also represent a complex challenge to effective and safe transportation planning and implementation. Although remarkably durable, many are narrow or single-lane structures constructed for traffic volumes, weights, and speeds prior to motorized vehicles. Visibility can be a problem for approaching traffic. This combination of factors can result in difficult safety and mobility problems. In addition, stone arch bridges, and bridges in general, can be expensive to maintain and repair. Because of the high number of older bridges in general in PennDOT District 6-0, available maintenance and repair funds are currently spread very thin in the Greater Philadelphia Region.

The purposes of Project Keystone are to develop a Management Plan and maintenance procedures in order to preserve at least some of the stone arch bridges in the Greater Philadelphia Region. To accomplish this, PennDOT and its partnering agencies have:

  • Developed criteria for ranking the stone arch bridges in the Greater Philadelphia Region according to their suitability for long-term preservation[2];
  • Developed a program and procedures to maintain and repair those bridges while preserving their historic character; and
  • Designed a system for monitoring and updating the prioritization and the Plan.

The goal is to produce sound management recommendations for each bridge and integrate them into the PennDOT’s Transportation Improvement Program (TIP) and maintenance planning and procedures.

RANKING THE BRIDGES

A wide range of factors affects a bridge’s potential longevity. A bridge’s condition, the traffic it carries, what it crosses, its size, its past maintenance, the land use around the bridge, its importance to the local, regional, and national community, and the value the public places upon it all contribute to the length of time that the bridge will exist.

A bridge’s potential longevity will affect its potential for preservation. To achieve a comparative means of assessing the preservation potential of the 125 stone arch bridges in PennDOT District 6-0, a matrix of pertinent factors was developed. The matrix is comprised of seven primary variables:

  1. Condition;
  2. Transportation;
  3. Waterway Adequacy;
  4. Rehabilitation or Replacement Costs;
  5. Anticipated Development Pressure;
  6. Historical, Recreational, and Cultural Values; and
  7. Public Input.

The Condition Code assesses the condition of the bridge’s superstructure (the arch, spandrels, and roadway) and substructure, using information from the bi-annual bridge inspection reports.

The Transportation Code contains six components: Bridge Operation Status, which notes whether a structure is open, posted with weight restrictions, or closed; Functional Classification, which tells the type of roadway on which the bridge is located (for example, Principal Arterial or Minor Collector); Average Daily Traffic (ADT), which measures how much traffic a bridge carries on a typical day; Average Daily Truck Traffic (ADTT), which measures the same thing for trucks (which create more of a strain on a bridge); Deck Geometry Appraisal, which is a measure of the adequacy of the bridge’s deck to carry the traffic; and Approach Roadway Alignment Appraisal, which assesses how the alignments of the roadways approaching the bridge relate to the general highway alignment.

The Waterway Adequacy Code rates the frequency with which a bridge floods; because the arch is the main structural element, correcting an inadequate waterway opening is a difficult problem to fix. The Cost of Rehabilitation or Replacement Code consists of a formula that roughly estimates the cost of rehabilitating or replacing a bridge. The Anticipated Development Pressure Code uses information provided by the Delaware Valley Regional Planning Commission (DVRPC) and by the individual counties on planned growth areas. A bridge within a growth area is more likely to face greater development pressure and, in turn, higher levels of traffic in the future.

The Historical, Recreational, and Cultural Values Code notes whether a bridge is listed or eligible for listing in the National Register of Historic Places or located in a park, natural areas, or greenways. National Register listing or eligibility is a measure of historical significance; location in a park, natural area, or greenway presumes an aesthetic component to the bridge. The Public Input Code measures the public interest in a particular bridge, based on the number of comments, correspondence, and petitions received.

Each variable was assigned a numeric value and then standardized for cross-comparison purposes. The values were then totaled, producing a rank ordering of the structures from 1 to 125, with 1 being the highest rated bridge and 125 the lowest. High-ranked bridges generally scored high or very high in all or most of the seven variables on which the bridges were evaluated. Low-ranked bridges, by contrast, generally scored low or very low in most variables.

After the bridges were ranked, the results were reviewed by a committee composed of representatives from PennDOT District 6-0, PennDOT Central Office, and a representative from the Federal Highway Administration (FHWA), which reviewed the rankings and made preservation recommendations based on the results. Not surprisingly, high-ranked bridges were recommended as stronger candidates for long-term preservation, while low-ranked bridges were not. These recommendations were presented to both PennDOT management and PennDOT’s partnering agencies in the project (the FHWA, the United States Army Corps of Engineers, and the Pennsylvania Historical and Museum Commission [PHMC]), who are currently reviewing the rankings and recommendations.

OTHER COMPONENTS OF THE PLAN

Stone Arch Bridge Maintenance Manual

The Stone Arch Bridge Maintenance Manual is a stand-alone publication providing guidance on maintaining, repairing, rehabilitating, and restoring stone arch bridges. Its purpose is to ensure that historic features of the bridges are not damaged or destroyed in the course of performing maintenance and repairs. The manual emphasizes routine maintenance as the most cost-effective way to preserve stone arch bridges. It also provides procedures for repairing all components of stone arch bridges. Although developed for Project Keystone, it will be used by PennDOT districts throughout the Commonwealth. A draft version is currently being completed.

Other Measures to Assist in Preserving Stone Arch Bridges

The plan recommends other steps to increase safety and improve traffic operational quality, which can help prolong the life of stone arch bridges. These include improving signing at the approaches to stone arch bridges with substandard widths; improving the horizontal and vertical alignment of the approaches to increase sight distances and limit impact vehicle damage; and working with the DVRPC and local governments to limit development in the vicinity of a stone arch bridge through land-use restrictions to moderate the amount of traffic carried by a bridge.

Context-Sensitive Solutions

The Management Plan emphasizes that when engineering and safety concerns require that a stone arch bridge must be replaced, context-sensitive bridge designs should be explored for the replacement structures. Context-sensitive solutions represent a proactive approach to transportation planning, design, and implementation that looks at the broad context streets and roads play in enhancing communities and natural environments, whether they are urban, suburban or rural, scenic, or historic. The intent is to address safety, mobility, and community impacts, while preserving scenic, aesthetic, historic, and environmentally sensitive areas, and community-valued resources. Context-sensitive solutions are a collaborative process that engages stakeholders early on and continuously throughout the process, embraces flexibility in design, minimizes impacts on the community and surrounding environment, and achieves safety through reducing risks.

Status of the Management Plan

The Final Management Plan was posted in April 2008 and can be accessed by clicking the Management Plan button.  If you have any comments or feedback, please forward them by clicking Your Comments/Contact Us

[1]The Federal-Aid Highway Act of 1968 mandated that each state institute a bridge inspection program for bridges 20 feet and over within the Federal-Aid Highway System. The Surface Transportation Act of 1978 extended the bridge inspection program to all bridges 20 feet and over. The original 1968 legislation was prompted by the collapse of the Silver Bridge connecting Point Pleasant, West Virginia, and Gallipolis, Ohio, across the Ohio River in 1967. The Management Plan focuses on bridges subject to bridge inspection and management.

[2] Long-term preservation is defined in the Management Plan as committing to the bridge for a period of 25 to 30 years. Depending on factors such as the materials used to construct the bridge, the care and maintenance it previously received, and present and future structural and traffic conditions, recommending a bridge for long-term preservation could mean that a bridge will have a lifespan that extends beyond 25 to 30 additional years.