Engineering Bridges

Andrew Smeltz
September 01, 1998

The bridge we went to see was built in 1899. Its arch only clears passing cars by a few feet. Made of huge sandstone blocks, cut and chiseled by hand, lifted and placed by monstrous steam cranes, it seems to be hunkered down over the road for the duration. Soot and exhaust blacken its facade, and in places stones have cracked and mortar has fallen away, but it's still functional.

drawing of bridges with labels

Jim Bintrim named its different parts. "This is the barrel," he said, pointing to the tunnel of the bridge over the road. "The arch itself supports the bridge, and the keystone is in the middle of the arch. To the sides are the retaining walls—they basically hold back the earth."

We walked under the bridge, and I noticed some cracks in the stone. One crack, maybe an eighth of an inch wide and six feet long, extended through several stones. Where the crack continued between the stones mortar had fallen away. The crack shifted after each block and narrowed to a hairline fracture.

"Will that affect the strength of the bridge?," I asked.

"Maybe," said Bintrim. "But the location of the crack is important. Here in the middle of the barrel, I don't know. If it were out farther, it might be an indication that the walls were pulling away from the rest of the bridge.

"In some places the mortar's been repointed," he added. Between the stones the cracked and eroded mortar had been refilled. The bridge didn't look too shabby. While Bintrim and I stood under it a train rumbled by overhead.

Bintrim is a graduate student in civil engineering. He studies masonry arch bridges with Tom Boothby, an assistant professor of architectural engineering at Penn State. Since Boothby was out of town, he sent Bintrim to see a sample with me. He left directions and a map taped to his office door. Neither of us had been to the bridge before.

In his directions, Boothby said we should get on 45 West and turn right onto PA 453. We never found 453. We left State College and drove for about half an hour on route 45, following Spruce Creek and the ridge of Tussey Mountain. We passed field after field, old farms falling behind us as we drove west. Some of the farms dated back one hundred years or more. The barns were dug into hillsides; masons had built them with stone foundations. Eventually, we assumed we had missed the other highway. We turned around and drove back, scrutinizing every road sign along the way. A mangy red fox lurched in the middle of a field of corn stubble. to watch the sick fox. When we reached the town of Pine Grove Mills, we asked a gas station attendant. He didn't even know where Spruce Creek was. (He wasn't a fisherman.) So we bought a map of Centre County, discarded Boothby's directions, and compared the new map with the map

bridge over green grass and brown water

Boothby had given us. We hadn't driven far enough. Boothby and Bintrim have driven all over Pennsylvania, New Jersey, and Ohio testing the strength of masonry arch bridges. One of Boothby's goals is to establish a practical method for determining how much weight these bridges can handle.

Boothby was on time for our first appointment, but because I was there early, waiting outside his office, he apologized for being late. He asked me to take a seat. "I'm going to get some coffee," he said. "Would you like some?"

"No thanks," I said, and he was gone.

Books, papers, and maps cluttered his office. Near the door sat a metallic box with electric gauges. Next to that was a chunk of concrete, and tumbled in a corner were a child's building blocks. On the wall opposite his desk were books, shelves of books that towered at least ten or 12 feet into the air. A ladder meant to reach the highest ones—Arch Bridges, Les Cathédrales de France, and Structural Analysis—leaned against a filing cabinet. On a low shelf, next to his desk, were vinyls—Sergeant Pepper's Lonely Hearts Club, The Doors, The Grateful Dead. (He had dug the albums out of his basement for the office Halloween party, he told me later.) Tacked to his bulletin board was a postcard with pictures of stone bridges in Scotland.

Boothby returned with a cup of coffee and apologized for the clutter. He stirred his coffee, and began talking about masonry arch bridges.

"Masonry arch" includes both stone and brick bridges. These bridges typically fail in two ways, by hinging and by sliding. A wedge-shaped gap between two blocks he called hinging. Masonry arch bridges can handle huge loads—if they are well distributed. Problems happen when a heavy load hits in one place. To demonstrate how sliding occurs in masonry arch bridges, he pulled out a little arch made of wooden blocks. With his forefinger he pressed down on one block: The block slowly slid out of place, sticking out below the arch. Boothby talked as he pushed, not paying attention to the block. The arch fell. Wooden blocks clattered across the desk. He smiled and said, "Well, yes, that's what happens."

The walls between the arch and the sides of a bridge, called spandrel walls, might also fail, tumbling outward and allowing the arch to collapse. "That's what happened to the Rockville railroad bridge in Harrisburg," said Boothby. "One of its spandrel walls just blew out." The Rockville railroad bridge crossed the Susquehanna River west of Harrisburg, Pa. It collapsed on August 19, 1997, and four cars loaded with coal fell into the Susquehanna. According to Boothby, Conrail had repairs scheduled to begin only days later.

bridge over road

Boothby and Bintrim use two types of tools to test a masonry arch bridge: linear variable differential transformers (LVDTs) and seismometers. The LVDTs are spring-loaded gadgets that measure how the bridge shifts or moves under weight. They work best when trucks are driven slowly over the bridge. If a stone moves even five microns (each micron is a thousandth of a millimeter) the LVDTs will send an electrical signal to a computer that records the data. Seismometers, the same instruments used to measure earthquakes, record the way the bridge vibrates. But since a truck moving at a slow crawl causes few vibrations, the seismometers record only limited data.

Bintrim has built a new tool to get more information out of the seismometers. He calls it the hammer or, more properly, a drop weight. Adopted from a technique used to study bridges in the United Kingdom, the hammer has a rectangular frame about five and a half feet tall. A crank winch on the frame pulls 150 pounds of weight up a steel pipe in the center of the contraption. When Bintrim releases the winch, the weight drops on the bridge with a dull thud. The hammer puts the seismometers to work.

Under the weight of trucks and the force of the hammer, the bridges actually move. But while steel or concrete bridges can move on the order of centimeters, masonry arch bridges of similar spans move only fractions of a millimeter. Boothby leaned forward and folded his hands on his desk. He figured in his head and then opened a book to check. "I would say that one millimeter is the most movement I've seen in a masonry arch bridge," he said, "and the reason for the larger movements was not the length of the span, but the poor condition of the bridge." Boothby and Bintrim have driven all over Pennsylvania, New Jersey, and Ohio testing the strength of masonry arch bridges.One of Boothby's goals is to establish a practical method for determining how much

After recording the bridge's movements, Boothby and Bintrim compare the results from the LVDTs and seismometers with what a computer predicts. The computer runs what is called a finite element model. Basically, Boothby says, "it's a simplified bridge that predicts the response of the structure to loads beyond what we can test." Boothby feeds known information about the bridge (such as dimensions and type of building material) into the computer, and the model simulates what happens when the bridge is put under stress (like when a truck drives across). In his tests, the finite element model worked reasonably well, but there were a few bugs. The model worked fine with forces bearing straight down on the bridge. It fell short, however, of predicting forces pushing out on the abutments and spandrel walls—the kind of thing that caused the Rockville bridge to collapse.

Construction of masonry arch bridges practically stopped 80 years ago when engineers turned to steel and concrete. Boothby, however, is looking forward to a renaissance. "Masonry arch bridges have proven durability," he said. A few steel or concrete bridges might last 50 or 75 years, but many masonry arch bridges last at least a century.

Among his research goals he places "assessment, preservation, the development of sensitive repair techniques, and an understanding of the masonry arch bridge in engineering history. I was first interested in medieval architecture," he noted, "but since there isn't any in the U.S. I chose stone bridges."

bridge with train going over it

Medieval cathedrals and masonry arch bridges have more than the crafts of masonry and stonework in common. Time and products of our modern culture have steadily deteriorated the facades of Nôtre Dame de Paris and other cathedrals. In Europe, scaffolds surround cathedrals like steel exoskeletons, and the crane has become an everpresent part of the skyline. Time and wind and rain wear away at the stones. Gradually, over the course of millennia the monuments would crumble, but the scaffolds and cranes aren't there because of the inexorable assault of the elements. Smog and acid rain have accomplished in a hundred years what might have taken thousands.

Stone bridges also suffer from the advance of the industrial era. Here, the problem isn't erosion; most bridges in the U.S. have only been around 100 or 200 years. (Construction on Nôtre Dame, by comparison, began in 1163.) The problem with masonry arch bridges is that most are too narrow. They often cause a steep incline or dip in the road. Built last century, these bridges were meant to carry a horse and carriage, not 18-wheelers and commuter traffic.

"You can't say that in every case preservation is more important than transportation," says Boothby. "Sometimes you have to tear down a bridge and replace it, but sometimes you can go around it or limit traffic." Many bridges already have signs that limit the weight of trucks traversing them or the number of vehicles that may cross at a time. But the question has to be answered: What is the primary objective of this bridge? In most cases it is efficient transportation. Other bridges are more important for their historical significance.

Boothby's work in Princeton, New Jersey, on a bridge that was too narrow, might help preserve the bridge by having it listed as a historic landmark. The builders had added to its sides when widening it; now the sides are separating from the original structure.

On the other hand, Boothby himself wants to tear down and replace stone arch bridge in Chester County, PA. The bridge looks like a stone wall built of field-stones, except for the semi-circular arch that a small stream runs through. County men built the bridge in 1916. Maple trees and a couple of oaks shade the stream and the bridge in the summer. Grassy banks lead down to the water. If you lean against the bridge and touch the rough stones, you can feel the cracks in the spandrel walls. You can stick your fingers inside them and brush out some grout. The bridge needs serious repairs. Boothby proposes to build a new one of brick rather than the more costly stone. The contractor would install instruments in the bridge to monitor stresses and strains, especially any displacements in the spandrel walls. The brick arch won't be a reminder of Chester County's past, but it will provide a safe stream crossing in a more elegant way than the usual steel or concrete slab. And it will help Boothby and his students understand more about how masonry arch bridges handle heavy loads.

Jim Bintrim is an M.S. student in the department of civil engineering, the College of Engineering, 105 Sackett Building, University Park, PA 16802; 814-865-2340; jwb121@psu.edu. Thomas E. Boothby, Ph.D., is assistant professor of architectural engineering and a research associate in Penn State's Pennsylvania Transportation Institute; 104 Engineering Unit A, University Park, PA 16802; 814-863-2082; tebarc@engr.psu.edu. His research on masonry arch bridges is currently sponsored by the Chester County, PA, County Engineer; the Princeton, NJ, Township Engineer; the Hunterdon County, NJ, Planning Board; Pennoni and Associates, Inc. (the Adams County, PA, Engineer); and the National Science Foundation. Much of his past work was sponsored by the Ohio Department of Transportation.

Last Updated September 01, 1998