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Burned Bridge Offers Clues to Howe Truss Behavior

By Kevin Wilcox

A charred railroad bridge gets a second life, providing researchers with insights into the behavior of posttensioned wood structures.

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The Moose Brook Bridge was essentially destroyed by arson in 2004. Only some iron and steel components could be salvaged. © Dario Gasparini

February 24, 2015—The massive wooden trusses of the Moose Brook Bridge are back home in Gorham, New Hampshire, following a remarkable journey that began with arson in 2004 and will end in a display at the Gorham Historical Society & Railroad Museum on Railroad Street later this year.

In between, the trusses spent two years on the campus of Case Western Reserve University, where Dario Gasparini, Ph.D., M.ASCE, a professor of civil engineering, and his students conducted structural testing that provides new insights into the structural behavior of Howe pony trusses.

Pony trusses are a less common arrangement in which the deck is at the bottom chord of the truss and there is no overhead lateral bracing between the trusses. Instead, the trusses are braced independently with outriggers. The pony trusses of the Moose Brook Bridge were covered in a wood cladding.

"The main aspect that captivates me about these Howe trusses is that posttensioning technology was used in the 1830s to the 1860s by these builders," Gasparini says. "We now think posttensioning technology is something related to concrete, but in fact, these wood Howe bridges were posttensioned. That has always fascinated me."

The arson largely destroyed the Moose Brook Bridge, a railroad structure built in 1918 along a 30 mi branch line connecting the New Hampshire towns of Whitefield and Berlin. The line was serving as a trail at the time of the fire. Because of the rarity of the bridge form, the Historic American Engineering Record (HAER) program of the National Park Service (NPS), led by Christopher Marston, a HAER architect, documented the bridge in drawings that are now in the HAER collection at the Library of Congress.

Tim Andrews, the owner of Barns & Bridges of New England, a contractor firm based in in Gilford, New Hampshire, worked on the restoration of the trusses, which was extensive. The wood was charred beyond salvage and many of the iron components were damaged as well. Gasparini consulted with Andrews on the project.

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The truss was re-created and tested on the campus of Case Western Reserve University. Among those who worked on the project are, from left, Kane Riggenbach, A.M.ASCE, a graduate student in civil engineering; Tim Andrews, the owner of Barns and Bridges of New England; and Dario Gasparini, Ph.D., M.ASCE, a professor of civil engineering. © Dario Gasparini

"I've had this fascination with Howe trusses for a long time," Gasparini says. "I was in a good position to know exactly how the pieces went together. Actually, there was a little bit of forensic work also done by Tim Andrews. He saw shadows of where the wood bore on the iron, so he was able to tell the sizes of the pieces that were originally used for the truss."

The cast-iron elements had to be specially repaired using a technique known as brazing. Additionally, some elements that were missing, such as nuts, had to be re-created.

Gasparini obtained a grant from the Federal Highway Administration, managed by HAER, to bring the trusses to Case Western for two years of structural testing after the restoration.

"I wanted to posttension the truss using modern materials to see if I could achieve a permanent posttensioned state, just like in posttensioned concrete construction," Gasparini says. "That was really the main objective. The other objective was to see how wood viscosity and hygroscopicity -its ability to absorb and desorb moisture-affected the stress state."

To accomplish this, modern posttensioning bars like those typically used in concrete were installed and tightened using hydraulic jacks. The team tensioned the bars to 80 kips in the center and 40 kips at both ends. "We used 80 kips because this force caused a nominal compressive stress in the smallest wood counter-diagonals-equal to about 1,000 psi," Gasparini says.

This is a much different process than when the bridge was built in 1918. At that time, workers placed large nuts onto threaded iron rods as much as 4 in. in diameter. Multiple workers then tightened them using a large wrench. This would achieve approximately 5 to 10 kips of tensile force in the bars.

What hasn't changed in the past 97 years is the behavior of wood under posttensioning. Because wood is viscous, the stress state decreases. The team found that this decrease occurs largely in the first four months. Just as railroad workers retightened the rods on the original bridge in 1918, the research team retightened the bars after one year.

The team attached strain gauges to the steel bars and temperature and moisture sensors in the wood, and then placed the trusses outside, covered by a tent, for two years.

"[We monitored] 40 channels of data for over two years. The data really told us about the life of the bridge in an outdoor environment over time," Gasparini says. The data includes the effects of a period of five days of heavy rain as Hurricane Sandy barreled up the New Jersey coastline.

The research indicates that engineers can achieve a permanent prestressed state in wood structures. The team also learned that the viscous behavior of the wood decreases prestress primarily in the first four months, and after that, any additional decrease is very small. The hygroscopic behavior of wood and any thermal cycles also cause relatively small effects on the prestress state.

Although first wrought iron and later steel have made wooden Howe-truss bridges largely obsolete for most transportation applications, the trusses can be quite effective as roof supports in timber-framed buildings. "Structures such as lodges often have exposed heavy-timber framing for aesthetic purposes. Typically designers now use gusset plates and bolts to tie the members together," Gasparini says. "Howe-truss technology is really simpler than that. It's much more evocative of 19 th -century framing, and much cleaner than using gusset plates and bolts, which are really more suitable for steel than for wood."

There are approximately 140 historical Howe-truss bridges in existence, a small fraction of what was once a very common type of heavy railroad bridge design.

"The Howe truss enabled industrialization of bridge fabrication in the United States," Gasparini says. "All of the pieces could be premanufactured. All of the pieces could be precut, preassembled if you wanted to, and then shipped to the site. In about two days you could erect a 200 foot bridge."

Gasparini and his team experienced this speed in reverse when they disassembled both trusses in a single day last fall and shipped them to New Hampshire, where Andrews will reassemble them, likely this spring. When the bridge is rebuilt, however, some of the wood cladding will be replaced with clear material so museum patrons can see the trusses. "It's a really nice story; it is nice that this bridge was salvaged," Gasparini says. "It came from Gorham and now it's going back at Gorham."


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