STORIES FROM 'THE CHRONICLE'
Elegant Exercises in Engineering
By Lawrence Biemiller
Lower Alloways Creek Township, New Jersey -- Even before he reaches the turn for New Bridge Road, Patrick Harshbarger spots the bridge itself, or the top of it -- a handsome metal truss, topped with classical urns, that he can just see over tall marsh grasses swaying in the winter wind. RESTRICTED BRIDGE AHEAD, warns a sign, WEIGHT LIMIT 4 TONS. Another sign follows -- SPEED LIMIT 15. And still another -- ONE LANE BRIDGE. "They must have reopened it," Mr. Harshbarger says. "The last time I was down here it was closed." Now the bridge itself is before us -- a small swing span whose entire weight is carried on an improbable-looking superstructure of lattice-box beams and riveted connectors.
In reality the New Bridge is anything but: Mr. Harshbarger dates it to about 1905. Its age makes it notable as a survivor, given that most of its contemporaries have long since been replaced. But it's the bridge's design, as elegant as it is utilitarian, that endears it to anyone with an eye for the masterworks of our grandparents' America -- all- Pullman trains, Mississippi sternwheelers, the Lincoln Highway, the Sears catalogue.
"Covered bridges have this great following," Mr. Harshbarger says, pulling slowly through the bridge to park on the shoulder beyond. "But metal trusses don't yet have that same popular following, although they're disappearing rapidly -- just as covered bridges disappeared 50 years earlier." Which is where people like Mr. Harshbarger come in. Now 33, he's a Ph.D. candidate in a history-of-technology program sponsored jointly by the University of Delaware and the Hagley Museum and Library. His thesis will be a history of the "ordinary" road bridges of the Delaware Valley. "Most of these bridges are an engineering legacy," he says. "What's so sad is that so few engineers really appreciate that." Engineers and preservation-minded historians -- so often "bitter enemies," as he puts it -- have to work together if this legacy is to be protected.
The New Bridge crosses what was once considered a navigable stream. The bridge pivots -- or used to pivot, anyway -- on a heavy center bearing set on a low pier in the middle of the river. Directly above the bearing is the tallest part of the superstructure; from the side the bridge looks like a bell curve made out of interlocking triangles -- it rises sharply, then gently, then sharply; it levels off; it falls sharply, gently, sharply on the other side. The triangles, by the way, are the secret -- it's the rigidity of the triangle, in tension or compression, that makes trusses work, and in all sorts of unlikely-looking configurations.
Although it is fixed in place now, Mr. Harshbarger says the bridge used to be cranked open by hand. Without a moment's reluctance he climbs over the railing and down onto the low pier to show off the center pivot. When the bridge was opened, the whole weight of the bridge was carried on the pivot, he says, while the ends were supported by the truss above. When the bridge is closed, the ends are fixed in place -- originally with a locking mechanism, now with permanent hardware -- and they help support the "live load," the load that traffic adds to the weight of the structure itself.
Walking back to the car, Mr. Harshbarger says he is particularly glad to see the New Bridge reopened. It's not far from a nuclear power plant, and he had worried that officials would find its one lane inadequate to their evacuation plans. Many older bridges are threatened because their carrying capacity is limited, he says, although others are endangered because they have awkward approaches or have been poorly maintained.
"Nowadays maintenance crews don't always know how a bridge like this is supposed to work," he says an hour later, standing on a turn-of-the-century Pratt pony truss in New Garden Township, Pa. "Look -- someone came along with a blowtorch and welded all these parts together. Now the bridge can't function the way it was designed to." The diagonal rods he's pointing at, intended to pick up the extra weight of heavy traffic, are welded tight to vertical posts beside them, making them useless. Truss bridges, Mr. Harshbarger says, are meant to be flexible -- that's how they distribute loads evenly and carry them back to the piers. An old bridge that has a little spring to it when a car crosses is nothing to worry about, he says; an old bridge that doesn't move could be a problem.
Now a bridge enthusiast, Mr. Harshbarger once worried about bridges a lot. "In the late 60's there was a horrible bridge disaster over the Ohio River," he says. "The Silver Bridge collapsed with cars lined up on it. My grandmother had been across the bridge twice that day. Until I was in college I would get sweaty palms if I had to, say, drive across the Delaware Memorial Bridge."
As an undergraduate at Brown University, Mr. Harshbarger lived hard by the old Fox Point railroad bridge, a Scherzer Rolling Lift -- "really heavy duty," he says -- that has loomed eerily open ever since the tracks were torn up. More importantly, he became interested in the history of technology, so much so that after he finished college he took a job at a Rhode Island historic site called Slater Mill. Later, after acquiring a master's in public administration, he decided he was still interested in history and found a summer job studying old bridges for the Historic American Engineering Record, which is part of the National Park Service. Now, in addition to preparing for his Ph.D. exams, he works as a historian for A.G. Lichtenstein and Associates, a bridge-engineering company that prepares historic-bridge surveys for state highway departments. "It's the best job in the world," he says. "Someone's paying me to drive around taking pictures of bridges."
It's true that Mr. Harshbarger's impromptu guided tour of "ordinary" Delaware Valley bridges includes all types -- a four-arch, 1826 stone bridge in Chester County, Pa.; a 1925 one-leaf Strauss drawbridge with an overhead counterweight in Salem, N.J.; a 1942 reinforced-concrete bow arch on old State Route 7 in New Castle, Del. Mr. Harshbarger visits two different kinds of covered bridges, a Town lattice and a Burr truss, and even a 1938 through-girder span with Art Moderne pylons. But the metal truss bridges -- versatile, inexpensive, easy to erect, gloriously all-American -- the truss bridges are clearly his favorites.
"Because of the industrial expansion there was a great market for bridges that could be built quickly and cheaply," he says, standing on an 85-foot-long pony truss that spans Octoraro Creek between Chester and Lancaster Counties. Manufactured by the Phoenixville Bridge Co., the bridge's main structural members are Phoenix columns -- flanged 90-degree sections of piping joined into tubes that are both strong and light.
"Between the Civil War and 1880, a whole slew of really talented engineers worked for Phoenixville," Mr. Harshbarger says. Truss-bridge components were made to order in factories and shipped to the bridge site, often along with a foreman to oversee construction. "Smaller bridges were usually assembled within a week," he says, whereas a masonry bridge might take months or years to construct. The truss bridge was far from perfect, however: The pins that hold it together rotate -- that's why the bridges are flexible -- but over time rotating can cause pins to fail. Pin fractures, Mr. Harshbarger adds, are invisible to the untrained eye.
The bridges are wondrous nonetheless -- their triangles cannot help but intrigue. Glance at any arched bridge and you know instinctively how it works; glimpse the Brooklyn Bridge from your cab and it makes perfect sense. But the plainest truss seems to defy explanation as easily as it defies gravity: You can stare at the New Bridge for an hour and still come away scratching your head.
If you're going to stare for an hour, though, take Mr. Harshbarger's advice and spend the time at the 1869 Bollman truss bridge in Savage, Md., off I-95 a little south of Baltimore. Designed by Wendel Bollman for the Baltimore and Ohio Railroad, its two dark-red spans are among the most exquisite in the nation. Quartets of handsome end posts support the top chords; eye bars radiate from the end posts' heads to the bottom of each intermediate descender; the Little Patuxent gurgles below. That such an elegant exercise in engineering could stand up to the brute weight of a steam locomotive, to the pounding of its drivers, to the endless thudding of coal-hopper wheels against rail joints -- it's not going too far to count the Bollman truss among America's most beautiful achievements. Or to thank Patrick Harshbarger for reminding us that it's there.
Copyright © 1995 by The Chronicle of Higher Education. Published March 10, 1995.