Surveying History, One Bridge at a Time

The trick is to measure as much as possible, count everything you can, take lots of photographs, and draw every detail -- all without slipping down a ladder

ALSO SEE: The students' covered-bridge drawings

or sliding off a rock into the gurgling Cold River. For seven industrious, cheerful students documenting an 1880 covered bridge on a dirt road here, this summer is all about getting the numbers right, and keeping straight which numbers belong to which parts of which drawing.

Just now two of the students are underneath the wooden span, called the Brown Bridge, with a brightly marked six-foot scale stick they're using to record how high the bridge stands above the sun-dappled riverbed. Two more are taking turns climbing a lightweight telescoping ladder inside, getting final dimensions for the roof supports and figuring out how they're joined to the tops of the walls. Every so often a car rumbles across, its occupants peering out at the team members in their bright-orange safety vests. Three other students are a few yards upstream, setting up surveying equipment that will be used to measure how much the 112-foot-long bridge sags under a live load -- in this case, a county road crew's nine-ton truck, due just before lunch.

			Will Dickinson and Vuong Dang review drawings outside the Brown Bridge. (Photographs by Lawrence Biemiller)

The goal is to produce accurate and complete architectural drawings, accompanied by engineering analyses and historical information, that will become part of the National Park Service's Historic American Engineering Record (http://www.cr.nps.gov/habshaer/haer/). "We're documenting the present state and trying to determine the original state," says the team's leader, Naomi Hernandez, who is a graduate student in industrial design at the Pratt Institute, in New York. Team members are architecture and engineering students, along with some interns from overseas, who are hired as summer employees of the park service. Working with full-time employees of the engineering record, they will produce six sheets of drawings for each of the four covered bridges they're visiting this summer -- here and in Indiana, Ohio, and Pennsylvania. In coming summers, the three-year project will document as many as 20 more spans.

This morning's work began a bit before 9, when Ms. Hernandez convened an impromptu meeting in the middle of Upper Cold River Road, right behind the team's cluttered U.S. Government van ("Covered Bridge Country -- Putnam County, Indiana," says a bumper sticker taped to the back window). The students unfolded drawings on oversized clipboards and discussed what numbers they still needed to get.

			The Brown Bridge is a Town lattice truss. Diagonal members are connected by pegs called "treenails."

The team already knew how many diagonals are in the bridge's two lattice trusses, and how the pieces of wood that make up the bridge's main upper and lower members are fastened together. Vuong Dang, who is studying architecture at the University of Arkansas at Fayetteville, had already climbed up on the roof to count and measure the rows of slate (15 on each side). Dave Groff, an architecture student at the University of Maryland at College Park, was holding a large sheet of graph paper with pencil drawings of one end of the bridge and of its cross-section, and with assorted dimensions recorded in red and black ink.

"Did you measure on the inside how wide each of these is?" asked Ms. Hernandez, pointing to a drawing on another student's clipboard. Someone had. "Did you get that end cap?" asked Mr. Groff. Someone did. "On the section, I need the angle of the truss," said Kimberly Clauer, who graduated in June from the University of Wisconsin at Milwaukee.

Once the meeting broke up, team members loaded the pockets of their safety vests with pencils and plumb lines and carried their clipboards down to the bridge. Much of the work is straightforward counting or measuring or drawing, but each of this summer's bridges represents a different structural type, and each conceals secrets behind its sheathing and within its joinery. "Whatever you can't see is difficult," says Ms. Hernandez, a five-summer veteran of engineering-record field trips. Abutments, for instance, are troublesome when the ends of the wooden members are hidden in contemporary concrete. "You know it's going in, but you don't know how far," she says. On the final drawings, elements about which the team has had to speculate are drawn with dotted lines.

			Naomi Hernandez, Mr. Dickinson, and Charu Chaudhry inspect the Lincoln Bridge, a bowstring-truss design.

Between the four bridges they've drawn and others they've visited on their own -- "Our big day was 13 bridges in Parke County, Ind.," says Ms. Clauer -- team members have seen six types of covered-bridge truss systems. "You really get into it," says Ms. Hernandez. "We've learned a lot figuring out the way the pieces go together."

The first span the team drew, the Pine Grove Bridge over Octoraro Creek, in Pennsylvania, is what's known as a Burr arch truss. Its inventor, Theodore Burr, combined wooden arches with standard trusses to produce a design that was at once popular and somewhat mysterious -- the arches prevent the bridges from sagging as much as they might otherwise, but over the years they have confounded engineers attempting to figure out how the bridges really work. The Pine Grove's two spans proved difficult for the students to measure accurately, because the spans are no longer precisely straight -- they seem to bow slightly upstream. That also makes them hard to draw, because the computer software the students are using assumes that straight lines will be, well, straight.

The next two bridges the students visited depend on plainer trusses that were also popular for covered bridges. The Eldean Bridge, just north of Troy, Ohio, is a Long truss, named for Stephen Long, and the Pine Bluff Bridge, outside of Bainbridge, Ind., is a Howe truss, named for William Howe. The Eldean Bridge proved interesting because the angled joints of its trusses are fitted with wedges that can be hammered in to tighten the structure. Francesca da Porta, a Ph.D. student in engineering at the University of Padua, in Italy, is one of two engineers working with the team. She says that building a bridge with wooden trusses is a challenge because it's hard to create strong connections between wooden members that are inclined to pull away from one another. (Connections where the members push against each other are easy.) Essentially, the wedges on the Eldean Bridge strengthen those joints by pushing them taut. The Pine Bluff Bridge, meanwhile, has vertical iron rods, which can be tightened with big wrenches to pull the structure tight.

			Mr. Dang and Mr. Dickinson beneath the Taftsville Bridge, a two-span crossing that combines arches and trusses.

The 122-year-old bridge here is a Town lattice truss, named after Ithiel Town, who patented the system. It's the simplest of covered-bridge forms: A plain lattice of wooden members supports the span. Dylan Lamar, an engineering undergraduate at Fayetteville who is the project's other engineer, says the Town lattice relies on smaller pieces of wood that would have been cheaper to buy. And the members are connected not with hard-to-shape joints but with simple pegs known as treenails (pronounced "trunnels"). "It's easier to construct," he says. "It doesn't require skilled workers, just drills and treenails."

The bridge appears to be in beautiful condition. In June, some of its lattice members were replaced, along with all of its siding, but part of the beauty of wooden bridges is that new components can be substituted for old ones. Roofs and siding serve to keep the structural members dry and are renewed as necessary; new siding is easy and relatively cheap, but new trusses cost real money. Will Dickinson, who just graduated from the University of Pennsylvania, is perhaps the team's most serious bridge fan. "You can tell the bridges with better siding -- board-and-batten or tongue-in-groove -- because they're in better shape," he says. Simpler kinds of siding, he notes, have more cracks, which admit moisture and weaken structural members.

When the county truck shows up, Mr. Lamar gets a chance to find out just how strong the Brown Bridge is. He and Mr. Dang measure the deflection with plumb lines and a laser as the truck stops at several points on the bridge. Later, Mr. Lamar calculates that the structure sagged less than a quarter of an inch.

			The Taftsville Bridge, which dates to 1836, has been renovated several times. At one point, triangular metal braces were added to help square the structure.

Lunch consists of peanut-butter-and-jelly sandwiches from a cooler in the back of the van. In the middle of the afternoon, Ms. Clauer and Charu Chaudhry, a young architect from New Delhi, struggle with strings and levels in an attempt to measure the angle of the truss for Ms. Clauer's section drawing. Then, finally, the team members pack up their clipboards and ladders. An hour later, they reconvene in the parking lot of their motel for a 45-minute drive to Woodstock, Vt. There more covered bridges await -- the Taftsville Bridge, an arch truss, and the Lincoln Bridge, a bowstring truss, which is an arch-based variation they haven't seen before. "A seventh type!" Ms. Hernandez exclaims as they jiggle its tie rods and climb its arches.

For good measure, they visit three metal bridges, too. "We eat bridge, we sleep bridge," says Ms. Chaudhry, but it doesn't sound like a complaint. After dinner they find another Town lattice on a darkened road -- "Yeah," says Ms. Clauer, "we've done a few by headlight." It's their 29th covered bridge of the summer.