By: Eric Deloney
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Because Pennsylvania was one of the first settled areas of the United States, it should come as little surprise that it possesses one of the most interesting collections of historic bridges of any state. Its ever-expanding population and consequent transportation requirements made the Keystone State a pioneer in transportation innovation, particularly in the design of bridges. Following the American Revolution, Pennsylvania embarked on infrastructure improvements that would not be complete until the Commonwealth was knit together by a series of turnpikes, canals, railroads, and highways. And all needed bridges.
Builders and engineers-and now historians and historic preservationists-classify bridges by design types, identified as rigid beam, arch, truss, cantilever, suspension, and moveable span. Bridges are constructed of many kinds of materials-wood, stone, cast and wrought iron and what we use today, concrete and steel. Pennsylvania has examples of all of them. The Commonwealth's geography, like that of any state, plays a decisive role in shaping not only the transportation patterns but also the appearance and style of bridges. The western and central portions of the state are defined by a crescent of ridges and valleys coursed by rivers and small streams. More than half the Commonwealth's topography is marked by the Appalachian Mountain Range arching north and east. The range is interrupted by furrowed valleys and rivers that have always challenged the turnpike, canal, railroad, and interstate highway builders.
In the eastern part of Pennsylvania, the major waterways, the Delaware and Susquehanna Rivers, flow south. Settlement moved westward, and both rivers hindered the flow of commerce until bridges could be built. Many communities-Pittsburgh, Philadelphia, and Harrisburg, for instance-are defined by water at one of their edges, also requiring bridges. Such topography instigated technological innovation.
Pennsylvania Claims Many "Firsts"--and the Nation's Oldest Bridge
Although the majority of Pennsylvania's bridges date from the twentieth century, some of the older routes have bridges built in the early colonial years. America's oldest stone arch bridge, constructed in 1697, still carries Frankford Avenue over Pennypack Creek on U.S. Route 13 in Philadelphia, and at least five eighteenth-century stone arch bridges remain.
Pennsylvania boasts many milestones-and, naturally, "firsts"-in American bridge-building technology. The isolation of its western counties prompted a Fayette County judge, James Finley (1756-1828), to invent America's first suspension bridge in 1796. Finley built a seventy-foot, chain-link suspension bridge over Jacob's Creek, near Uniontown, in 1801. On November 7, 2008, the Pennsylvania Historical and Museum Commission (PHMC) dedicated an official state historical marker commemorating the bicentennial of the first U.S. suspension bridge patent granted to Finley in Uniontown. Neither this nor any of the nearly forty bridges he constructed between 1808, when his patent was granted, and 1816, when wire cable suspension bridges made his iron chains obsolete, survives. His design incorporated many of the principles of the modern suspension bridge: a catenary cable with hangers holding a relatively level deck whose rigidity was maintained by longitudinal stiffening trusses and transverse deck beams.
Pennsylvania claims the first wire-cable suspension bridge in the world, as well. This bridge was developed by Josiah White and Erskine Hazard, ironmasters and owners of a rolling mill and wire plant in Philadelphia, who built a small suspension bridge, in 1816, across the Schuylkill River so their workers could walk to the factory. Pennsylvanian Charles Ellet Jr. (1810-1862) constructed the first wire-cable suspension bridge used for general public transportation over the Schuylkill at Fairmount, in 1842.
America's oldest surviving suspension bridge spans the Delaware River between New York and Pennsylvania. It originally served as an aqueduct carrying the Delaware and Hudson Canal over the Delaware River at Lackawaxen, the only kind of its type in the world. It was designed by John A. Roebling (1806-1869), of Brooklyn Bridge fame, who arrived in the United States in 1831 and established an agricultural community later named Saxonburg, in Butler County. Educated as an engineer in Europe, Roebling secured a position with the Pennsylvania Canal. The most challenging terrain along its length was the Allegheny Mountains in western Pennsylvania. To surmount the mountain's steep slopes, the Mainline Canal built the Allegheny Portage Railroad, a series of incline planes and the first railroad tunnel in America. Allegheny Portage Railroad is a national historic site maintained and interpreted by the National Park Service. Hemp ropes, used to haul the canal boats over the summit, often broke, resulting in fatalities. Roebling suggested substituting wire cable for hemp. He began manufacturing wire cable in Saxonburg. Success on the portage railroad launched Roebling on a career that initiated the American wire-rope industry. Roebling is best known for his suspension bridges, but his wire works, relocated from Saxonburg to Trenton, New Jersey, provided wire cable for all types of industrial development.
An All-Iron Vehicular Bridge Still in Service From 1839
The first all-iron vehicular bridge, erected in 1839, replaced one of Finley's suspension bridges and remains in service. Located on the old Cumberland Road, now U.S. Route 40, it spans Dunlaps Creek in Brownsville. Accretions have obliterated the public's view of it, but behind them lies one of the most significant engineering artifacts in the country. Designed by Captain Richard Delafield of the U.S. Army Corps of Engineers, it spans eighty feet, rises eight feet, and includes one hundred and sixty-seven tons of iron cast into two hundred and fifty individual pieces. The segmental arch consists of five parallel iron tubular ribs, each made of nine identical segments bolted together. It is an anomaly in American bridge design because, although they proliferated in Europe, few cast-iron arches were built in this country. Instead, American builders opted for a bridge design based on parallel chord trusses, a form evolved from the timber trusses of the wooden bridges of the 1840s.
Pennsylvania also lays claim to both the first and the oldest all-iron railroad bridge still in service. The first, Richard Osbourne's Manayunk Bridge on the Philadelphia and Reading Railroad line, was built about 1845. (One of its cast and wrought iron trusses is on display at the Smithsonian Institution in Washington, D.C.) The oldest iron railroad bridge still in service, designed also by Osbourne, is the Reading Railroad's Hall's Station Bridge, erected in 1846, that now connects a private farm to a branch of the former Reading, just east of Williamsport.
A more recent Pennsylvania technological innovation is America's first pre-stressed concrete girder bridge, a type that has become standard for freeway and interstate highway design. The deck rested on thirteen girders spanning one hundred and sixty feet, each of which was pre-stressed by tightening four wire cables embedded in concrete. It was designed in 1951 for the Philadelphia Department of Public Works by a Belgian engineer, Gustav Magnel, to carry Walnut Lane over Lincoln Drive in Philadelphia. Deteriorating concrete forced its replacement in 1985.
Wooden covered bridges are usually the first type that comes to mind when people talk about historic bridges. They are dark and mysterious, quaint and romantic. Their settings along pastoral streams provoke nostalgia, as do their material and texture. Many covered bridges are also outstanding examples of wood construction technology. The United States has more covered bridges than any other nation. Pennsylvania currently claims one hundred and ninety-seven covered bridges, a loss of twenty-three since 2000. The attrition rate is alarming. Although some losses have been caused by flooding, many are lost to arson. Pennsylvania falls within the national average of three to five covered bridges lost each year to wanton vandalism, nothing less than a tragedy.
The "Permanent Bridge" in Philadelphia, opened in 1805, was so named because it was the first permanent bridge over the Schuylkill River; previous spans had been washed away. It was also the first covered bridge. Timothy Palmer (1751-1821) decided to cover the timber members to protect them from rotting and the idea caught on.
By the 1820s, trade had been developed to such an extent that ferries on wider rivers became bottlenecks to commerce. Pennsylvania responded with a series of world-class covered bridges by Palmer, Theodore Burr (1771-1822), and Lewis Wernwag (1769-1843). This trio pushed timber bridge technology to its limits. Burr's 360-foot single span over the Susquehanna River at McCall's Ferry was one of the longest. Although none of these big-river bridges survive, their descendants, designed and built by local craftsmen emulating the masters, remain today.
Pennsylvanians truly appreciate their covered bridges, and numerous covered bridge groups stress their value and rarity. The Pennsylvania Department of Transportation (PennDOT) provides state assistance for covered bridges through funding legislated by former state Senator Robert C. Jubelier, whose district included Bedford County, the county with the most covered bridges. As a result, Pennsylvania's covered bridges have been structurally evaluated and priorities set for long-term preservation. But much of the actual restoration remains to be undertaken.
In the lexicon of bridges, a truss is a rigid framework of wooden beams or metal parts designed to support a bridge. The many varieties of metal truss bridges are not easily recognizable by the layperson, yet they are technologically significant and are one of America's greatest contributions to bridge technology. Manufactured by bridge-fabricating companies located predominantly in the eastern and midwestern United States, the ubiquitous metal truss solved the need for thousands of inexpensive and efficiently manufactured bridges. Metal trusses are characterized by prefabricated parts made from standard mass-produced shapes and materials. They were easily and inexpensively erected using unskilled labor. Agents representing bridge-fabricating companies would traverse the countryside, meeting with local officials to hawk their product line with claims of low cost, high durability, and ease of erection. These bridges were called "catalogue bridges" because they were sold to township supervisors and county commissioners from catalogues. Many of these trusses were based on specific, proprietary patents. No other country experimented or built as many metal trusses as America did during the late nineteenth and early twentieth centuries, when hundreds of patents for iron bridges were granted. While many came from trained engineers, others were drawn up by crafters, millwrights, and mechanics. These unschooled "apple-tree engineers" tried to make improvements, but most of their designs were impractical. While metal trusses continued to be fabricated into the twentieth century, many built in the twentieth century are in use today.
Because the heart of the iron and steel industry was in Pennsylvania, many iron-bridge manufacturing companies operated here. While Andrew Carnegie's Keystone Bridge Company in Pittsburgh and the Reeves brothers' Phoenix Bridge Works in Phoenixville were two of the largest, dozens of smaller firms existed. None of these survive, but their products still dot the countryside.
Around 1900, concrete began to compete with iron as the material engineers preferred. Reinforced concrete had been developed in nineteenth-century Europe. Concrete, like stone and cast iron, has good compression properties to bear heavy weights, but is essentially worthless under tension. Adding iron or steel rods eliminated this deficiency. Iron, steel, and cement have similar coefficients of expansion and contraction, so they work well together. Competing reinforced-concrete bridge systems were developed and patented simultaneously by two American Midwestern engineers, Daniel Luten and James Barnard Marsh, and an Austrian engineer, Joseph Melan. Bridges based on these patents began to appear in Pennsylvania at the turn of the century. The oldest concrete bridge in Pennsylvania is a 1902 three-span, closed-spandrel arch on Scharffs Road, over Tulpehocken Creek, two miles west of Womelsdorf. Berks County engineers and commissioners were willing to experiment with this new structure and material for good reason: concrete didn't rust like metal or burn like wood and required little maintenance.
Larger and grander than the little country spans are the early concrete bridges in Philadelphia. The oldest, Walnut Lane Bridge over Wissahickon Creek, is distinguished because it uses no reinforcing whatsoever and is one of the few mass-poured concrete bridges in America. Its appearance closely resembles the Pont Adolphe in Luxembourg, the world's largest stone arch bridge. When completed in 1908, the Walnut Lane Bridge was the world's largest concrete arch.
Pennsylvania also has the distinction of having the world's largest reinforced concrete bridge in terms of mass and weight: the Delaware, Lackawanna and Western Railroad's Tunkhannock Viaduct at Nicholson. Completed in 1915, this monolithic arched structure, measuring twenty-two hundred and thirty feet in length, was built with one hundred and sixty-seven thousand cubic yards of concrete and five hundred and seventy tons of reinforcing steel. It rises two hundred and forty feet above a little stream, awing both visitors and residents.
Preserving Historic Bridges
Many people realize Pennsylvania's historic bridges need to be preserved. While the massive concrete structures will long remain standing and cannot be moved, smaller concrete spans and the ubiquitous metal truss bridges are threatened by the highway improvement and bridge replacements programs of the last forty years. Not every metal truss or concrete arch can be retained in service, but certainly representative examples of each of these types and of each manufacturer are worthy of preservation. While the best preservation is continued use-engineers constantly improve techniques to strengthen old bridges without altering their appearance or character-there are alternatives for bridges that cannot accommodate today's vehicular traffic. A number of states, including the Commonwealth of Pennsylvania, have saved metal truss bridges by relocating them to less traveled roads and adapting them for pedestrian and bicycle traffic. For example, national, state, and city parks have trails, backcountry roads, and service roads that don't require full-service bridges. Moving old metal trusses to these roads preserves the scale and park-like qualities that are otherwise impossible to save if modern concrete culverts or steel girders are used.
The selection of iron bridges that has been saved in Pennsylvania is impressive. The Smithfield Street Bridge in Pittsburgh, a two-span vehicular truss that has served as a gateway to the city since it was constructed in 1883, has been meticulously rehabilitated for full service. Its truss design was developed in Europe and first used in America by Gustav Lindenthal, a German engineer who later became one of the nation's foremost bridge specialists. As part of its rehabilitation, the bridge is lighted at night, providing Pittsburghers a glorious spectacle.
In the 1980s, several state agencies worked together to save one of the Commonwealth's major vehicular trusses. Manufactured by the Berlin Iron Bridge Company of Connecticut, the Waterville Bridge, with a three-hundred-and-forty-foot span, had outlived its usefulness in Lycoming County. Because it was recognized as one of the largest vehicular truss bridges in the state and manufactured by a noted company, a special effort was made to save it. Through coordinated efforts of several state agencies, PHMC, PennDOT, and the Department of Conservation and Natural Resources (DCNR), it was determined it could be given a new life on the Appalachian Trail-if it could be moved. State engineers decided to dismantle and transport it over more than one hundred and twenty hilly miles of narrow roads to Swatara State Park in Lebanon County. Today, the reassembled bridge is a tribute to Victorian era engineering and an excellent example of historic preservation through state government initiative.
Partners in Preservation: PHMC and PennDOT
Although many examples of outstanding engineering works in the Commonwealth have been spared, more needs to be done. Fortunately, help is on the way from a source best equipped to deal with bridges, PennDOT. In 1982, PennDOT and PHMC sponsored a statewide inventory of bridges on the federal and state-aided road systems. One hundred and eighty bridges were deemed eligible for either the state or federal registers of historic places. In 1996, PennDOT and the Pennsylvania Division, Federal Highway Administration, in cooperation with PHMC, launched an evaluation of all pre-1957 bridges, which includes county and municipality-owned structures, to identify and record even more historic bridges.
In 1997 and 1998, PennDOT and PHMC sponsored documentation of fifty-two historic bridges by the Historic American Engineering Record (HAER). An agency of the National Park Service, HAER compiles a national archive of the engineering and industrial achievements that made America a foremost technological nation. HAER drawings, photographs, and histories articulate the historical and technological significance of selected bridges. In 2002, HAER conducted an additional bridge documentation project. PennDOT is working on a management plan for metal truss bridges on a district-by-district basis. To date, PennDOT has realized that the most "preservable" are locally-owned truss bridges because they carry the least amount of traffic.
PennDOT's stone arch bridge management plan, known also as "Project Keystone," was completed in spring 2008. The management plan commits PennDOT to the long-term preservation of select stone arch bridges and creates a "reserve pool" for additional bridges should an example in the "preserve" category be lost through disaster or major unanticipated change in local use. The plan includes provisions for a task force to meet with every Transportation Improvement Program (TIP) update to review the status of bridges in the plan. It also includes recommendations for local bridge owners. An uncompleted component of the plan is the development of a programmatic agreement which will provide some streamlining for the section 106 process for the bridges identified and included in the plan. A metal truss bridge management plan effort is currently underway. A consultant is developing historical contexts and ranking bridges according to their potential for preservation (based upon a range of criteria) on a PennDOT district-by-district basis. PennDOT anticipates the completion of the ranking portion of the management plan effort by December 2008. Following the rankings, PennDOT will conduct internal meetings to discuss the results to gauge the degree to which a management plan is feasible, particularly should the higher ranking bridges be locally owned.
In 1991, HAER, in conjunction with the West Virginia University Institute for the History of Technology and Industrial Archaeology, field quartered at Lehigh University in Bethlehem, conducted the first Cast and Wrought Iron Bridges Recording Project.
Many factors are considered before a bridge is saved. Most important, aside from safety issues, is funding. Until recently, only limited funding was available, but in 1987 Congress declared it within the national interest to encourage the rehabilitation and preservation of bridges significant in American history, architecture, engineering, and culture. Four years later, Congress passed the Intermodal Surface Transportation and Efficiency Act (ISTEA), a new approach for engineers and planners developing transportation alternatives such as buses, light railways, and bicycles. It charged officials to include the public in the decision-making process and to fund transportation "enhancements." Eligible projects included planting wild flowers, building bike trails, and rehabilitating historic bridges. This six-year program as renewed by the Transportation Equity Act for the Twenty-first Century (TEA-21), one of the largest transportation funding measures in history. TEA-21 provides more money for paving potholes, building new bridges, and saving historic bridges.
Historically, the interstate highway era in the United States is over. Eisenhower's system marked fifty years of service in 2006, becoming eligible for the National Register of Historic Places. Although the Federal Highway Administration, in cooperation with the states, developed a list of approximately one hundred bridges as "exceptional" features that are considered historic, bridge historians are just beginning to get their arms around this new generation of bridges. As research progresses, others will be identified. With as fine an interstate system as any country in the world, there is no reason to expand on the scale achieved between the 1950s and the present; the major issue is maintenance. Naturally, new bridges will continue to be built, but nothing on the scale of the past. Although billions of dollars are available for transportation improvements, there simply isn't enough to replace the country's estimated one hundred and sixty thousand of approximately six hundred thousand bridges designated as structurally deficient or functionally obsolete by FHWA (2008). Bridge enthusiasts hope that representative examples of America's estimated 27,000 historic bridges will be among those maintained in continued vehicular service, the preferred preservation option, or adaptively reused as hiking, biking or pedestrian bridges.
Interest in recreational driving continues as Americans and foreign visitors alike are rediscovering the wonders of picturesque backcountry roads. Every state has adopted a scenic highways and byways program. Historic bridges are integral to these road systems. In addition to scenic byways, the nation is developing new trails that provide additional opportunities for recreation and serve to link communities together. With the nationalization of rail systems over the last thirty years, rails-to-trails programs have blossomed. These bike and walking trails offer reuse opportunities for bridges that cannot be maintained on primary and secondary road systems.
No matter how promising these projections for bridge preservation may sound, the only way they will succeed is through public involvement. It is an old American proverb that a traveler should respect the bridge that carries him or her over dangerous areas. Today's bridge preservation efforts need public involvement so that future generations can travel over the intellectual bridge connecting technical thinking of the past with bridge-building principles of today.
To learn more about historic bridges and to become better informed, a new historic bridge section has been posted on the American Association of State Highway and Transportation Officials' Center for Environmental Excellence Web site. A new section on historic bridges can be found in the "Overview" to the Historic Preservation/Cultural Resources and is entitled "Bridge Preservation."
Eric DeLony served as chief of the Historic American Engineering Record (HAER) from 1986 until his retirement in 2003. He had been with the program since its inception in 1969. As the first professional architect of this National Park Service program, he created benchmarks in drafting standards for HAER with his drawings of the Gas Holder House in Troy, New York. His early drawings have been models for subsequent architects working on HAER summer projects. Through his writing and speeches, he helped stimulate public awareness and appreciation of the value of historic engineering and industrial resources. The author currently lives in Santa Fe, New Mexico, where he continues to help individuals, groups, organizations and engineers appreciate and preserve historic engineering, industrial and technological resources, especially historic bridges.
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Pennsylvania Historical and Museum Commission and the Pennsylvania Department of Transportation. Historic Highway Bridges in Pennsylvania. Harrisburg: Pennsylvania Historical and Museum Commission and the Pennsylvania Department of Transportation, 1986.
_______. Historic Highway Bridges in Pennsylvania: Preservation Guide. Harrisburg: Pennsylvania Historical and Museum Commission and the Pennsylvania Department of Transportation, 1987.
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Eric DeLony's "Bridging the Past for the Future" originally appeared as the cover story of the Winter 2000 edition of Pennsylvania Heritage. Back issues of Pennsylvania Heritage magazine