An Old Bridge Spans Lessons in History and Engineering

With one hand clutching at a railing and a safety harness cinched around his waist, Jai B. Kim inches along a pair of camber rods on the downstream side of a rusty, wood-floored span known as Henszey's Wrought-Iron Arch Bridge. Mr. Kim, a

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professor of civil engineering at Bucknell University who often works to save old bridges, is pulling away brittle vines and poking a screwdriver at stonework that hides the ends of the camber rods, the end of the downstream arch, and the end of the metal chord that pulls the arch into a taut bow.

"What I want to know is, How is this bottom chord connected to this guy here?" Mr. Kim says thoughtfully, tapping the arch beside him. He is six feet or so above Ontelaunee Creek, which meanders past old farms and dense thickets in a sunny valley here. Already two Bucknell students, Ryan Fasnacht and David Horton, are chipping away at the top of the abutment, hoping to remove nonessential stones that hide the arch's end piece. Two more, David Evans and J. R. Baughman, are checking notebooks full of measurements and drawings. "This is where I'm worried," Mr. Kim says. "J. R., we're going to have to draw this up."

All four students are civil-engineering majors, but the senior project Mr. Kim has asked them to work on here is unlike any they've faced in class: They're trying to calculate how much weight the 132-year-old bridge will be able to carry safely once it has been rehabilitated and re-erected on the campus of Central Pennsylvania College, a 600-student proprietary institution near Harrisburg. Last December they submitted a feasibility study that persuaded the college's board to go ahead with the plan, under which the bridge will carry pedestrians over a stream separating the existing campus from a new technology building. Now Mr. Kim and his students are writing specifications that Central Penn officials will distribute to contractors interested in bidding on the job.

"Hey, Evans!" calls Mr. Horton, sledgehammer in hand. "Get your muscles over here, guy. We gotta rip that out." He nods down at a stone covering one end of the upstream arch. As long as rocks hide the bridge's ends, no one can say how the arches, the bottom chords, and the camber rods are connected.

"If anything fails, it will fail at the connection," Mr. Kim says a few minutes later. He's on the other end of the bridge now, standing on a ladder that's sinking into the mud as he squints into the darkness where arch, chord, and rods plunge into stonework. He reaches a hand up under the metal. "What the hell -- " he mutters.

What makes the students' analysis especially challenging is the bridge's unusual design, attributed to Joseph G. Henszey of Philadelphia. Ninety-two feet long and 18 feet wide, the bridge is what's called a bowstring truss -- "bowstring" because the two arches are tied with metal chords the way hunting bows are tied with string. Unique to this bridge is the addition of the camber rods -- "camber" means "slightly arched" -- below the bottom chords. The camber rods form two shallow upside-down arches beneath each side of the bridge. Although the floor beams are chiefly carried by vertical members hanging from the main arches, heavy loads can also be supported from below by the camber rods, Mr. Kim says.

All of which means the students have a lot of parts to analyze. On their first visits -- Wanamakers is about an hour and a half from Bucknell by car -- they measured everything they could find to measure: the big main arches, the small diagonal rods running underneath the wooden flooring, even the turnbuckles on the camber rods. Then they plugged their measurements into structural-analysis software, which initially insisted that the bridge was unstable, Mr. Fasnacht says. The problem turned out to be that the bridge's parts work together in several different ways to assure against collapse. "It has a lot of redundancy," he says. "It has so much redundancy that we had to simplify to run an analysis. It's unique in its complexity."

Its history is just as interesting. The bridge that now stretches over Ontelaunee Creek here is half of a two-span structure assembled by the Continental Bridge Company in 1869 in Slatington, Pa., some 15 miles northeast of here. In 1900, the Lehigh County Commissioners decided to replace that bridge, and one of the spans was brought here to augment the local network of farm roads. Sometime before 1914, highway crews built a stone pier under the center of the bridge -- a pier that an engineer who examined the span in 1932 said was "reversing all the stresses in the top chord and web members." The pier was removed and the bridge served until 1986, when state crews closed it and built barricades of vertical I-beams at each end.

It has not been maintained since, but neither has it been forgotten. It is described in several books, including Landmark American Bridges, by Eric DeLony, chief of the National Park Service's Historic American Engineering Record (Little, Brown & Company, 1992). And when Central Penn's president, Todd A. Milano, got in touch with the Pennsylvania Department of Transportation to ask whether it had any old spans available, Engineering District 5 gave Henszey's bridge a glowing review. Kara Russell, a cultural-resource specialist with PennDOT, printed out a list of unused spans for Mr. Milano and highlighted Henszey's.

"It's definitely a nationally significant bridge," says Ms. Russell, who adds that the transportation agency has "about 10 bridges a year that we're seeking new homes for." But she says Henszey's is the first span in years that has attracted a taker. When it did, the state put it on the market, specifying that the buyer would have to restore the bridge. Central Penn's bid of $22 -- Mr. Milano's lucky number -- was the winner.

Mr. Milano says he considered building a new bridge, but the idea of something old appealed to him. "When you're the college president you can get away with a few crazy things," he says. "I think of Central Penn as a bridge -- a bridge from high school to a career." It was also Mr. Milano's idea to call Bucknell's well-known engineering department for help with the project. The students' feasibility study estimated that the cost of building a new bridge would be only slightly less than the cost of moving Henszey's bridge 70 miles and rehabilitating it; in either case they figured, the college would spend roughly $260,000.

Mr. Kim looks at it another way. Bridges and aqueducts from antiquity have survived 2,000 years, but in the United States 150-year-old bridges are rare. Highway engineers strive to make roads and bridges indistinguishable to drivers, he says, because drivers don't want to slow down for any reason. "Nowadays young engineers have this philosophy that old is bad," he says. "But if we cannot trace back what we have accomplished, we are just like animals."

Mr. Kim, who has taught at Bucknell since 1966, is a prominent bridge consultant who developed the Bucknell System, a means of strengthening the boxy metal-truss bridges that were once ubiquitous on American roads. Unlike Henszey's bridge, most metal-truss bridges are held together with large pins that fit through holes in horizontal and vertical members. But engineers have no way of examining or testing the pins -- the structures' weakest points -- on older bridges. The Bucknell System involves building new arches within the existing bridge to relieve stress on the pin connections. Some preservationists disapprove of the Bucknell System because the arches alter the appearance of the bridge, but Mr. Kim says altering the bridge is preferable to replacing it.

Over all, he says, "I lose more battles than I win" when it comes to saving bridges. But so far Henszey's bridge looks like a winner. Rigging contractors have told Mr. Kim's students that moving the bridge won't be difficult, and otherwise the project is going smoothly. The one unsolved riddle now is how well everything is connected at each end.

"Fulcrum! Fulcrum!" Mr. Baughman cries as Mr. Fasnacht and Mr. Horton struggle with the crowbar to move a large stone above the fourth end piece. Mr. Horton's hands are blistered and bleeding; he and Mr. Fasnacht will discover later that they've been in poison ivy somewhere. But right now they're finally uncovering a connection they can reach all the way around. Mr. Kim lies down and stretches a hand in to feel the sides of the joint. Then he looks up and grins. "OK, somebody else. I say this is the trunk of the elephant."

Each student takes a turn. The consensus is that a wedge driven through a hole in the bottom chord connects it to the arch -- an important discovery, because the hole in the chord means it is weaker than their calculations had assumed. Within minutes, though, they're sketching a plan to reinforce both the bottom chords with cables that will loop around new end pieces.

"We work well as a cohesive unit," Mr. Baughman says in the van on the way back to Bucknell. "Fas and Evans are more the bridge guys," he adds, while he and Mr. Horton are "the foundation guys," working on plans for the bridge's new approaches and abutments. All four have jobs lined up after graduation -- Mr. Baughman with PennDOT, the others with big construction companies.

Mr. Kim, as the only licensed engineer in the group, is the person whose signature counts on the paperwork. He double-checks everything, but he's clearly proud of how much his students have accomplished. "This is a really big project for these guys," Mr. Kim says with a smile. "When I do a bridge like this, I charge a lot."