ORONO — The design for Maine’s first long-span timber bridges will be put to the test this spring at the University of Maine. Two new timber bridges that are slated for construction by fall 1993 could launch a forest-products export industry potentially worth $600 million a year.

The 45-foot wooden bridges will replace aging bridges in Byron in Oxford County and North Yarmouth in Cumberland County. Bridge construction is being funded with $390,000 in federal money recently appropriated for the Maine Timber Bridge Initiative, a joint effort of UM, the Maine forestry industry, and the DOT.

“This is an exciting time for us. This has been three years in the making, and now finally we are embarking on our plan to build long-span bridges using Maine designs and Maine timber species,” noted Habib Dagher, UM associate professor of civil engineering and the principal designer of the wooden truss-supported bridge.

Federal funding for the two new bridges, announced by the state’s Congressional delegation early this month, marks the beginning of a five-year program to build up to 16 additional timber bridges in Maine. Each would be equipped with a sophisticated monitoring system designed with the help of Vincent Caccese, UM associate professor of mechanical engineering.

Dagher and Caccese designed the state’s first short-span timber bridge, which was built last year in Gray. They will test their newest truss-bridge design with simulated truck loads in a UM testing facility before construction begins on the actual bridges.

The two new bridges will be about twice as long as the Gray bridge and will use a system of wooden supporting trusses. A model will be stressed to failure — or intentionally collapsed — in the UM testing facility, which will include a 4-foot thick concrete “strong floor” measuring 60 feet by 25 feet.

Once tested and constructed, the new bridges will be monitored for the next four years. Remote sensors attached to the bridges can send information over telephone lines to a computer at UM.

Maine now has about 270 rural bridges that need replacing because they are either functionally obsolete or structurally deficient. All of them, according to Dagher, could be replaced with timber bridges using Maine’s most abundant natural resource, native timber.

Modern timber bridges have a life expectancy of about 70 years, compared to concrete bridge decks in Maine, which require major repair work after 40 years, Dagher said.

Barry Goodell, who heads UM’s Wood Science and Technology Program and is a member of the timber-bridge research team, is studying the best method of protecting the Maine timber species with wood preservative. Although wood is resistant to the corroding effects of de-icing road salt, wood preservative is critical because wood will decay unless it’s adequately treated, Goodell said.

The forest industry has supported the timber-bridge initiative to create new jobs and market opportunities for Maine forest products. Future bridge construction will be videotaped to educate the industry on timber-bridge design and construction.

The timber bridges could be prefabricated in Maine and then exported, said Dagher. About 200 rural bridges collapse each year in the United States, and the federal government estimates that 40 percent of the country’s 575,000 bridges now need major repair work.

Many such bridges are on rural roads ineligible for federal aid for bridge construction. The relatively low cost of wooden-bridge construction, plus recent advances in timber-bridge design and wood preservation, make the new timber bridges an attractive solution for many rural areas. But, Dagher cautioned, timber bridges are not the best solution for every bridge site. They are best suited for low-volume rural roads and spans less than 50 feet in length, he said.

The state’s first modern timber bridge, which was designed at UM and built from native timber, is back on “pilot control” after a day of successful on-site tests late this summer. The first on-site test of the Gray bridge, built last year to replace an aging concrete bridge, produced better-than-expected results, according to Dagher. Researchers measured bridge movement of only one-half inch at mid-span after trucks carrying 44,500 pounds on the back axle — 10 percent more than their normal load — were positioned on the bridge.