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Many of the 1,400 timber bridges in Minnesota need to be improved to meet present day standards. When the desired service level can be attained by widening a bridge six feet or less, a retrofit can be done by placing a second, wider, transverse deck onto the existing deck and substructure. Bridge components must be carefully inspected prior to a retrofit project. Bridge #6641 in Sibley County was retrofitted. First the bituminous surface was removed. A longitudinal beam supported the extended deck. Grout was poured and leveled and then nail-laminated panels were laid transversely. A bituminous surface was laid over the full width of the new deck. The cost of the project was $51,632. (Replacing the bridge was estimated to take 2-3 years and cost $215,000). The county quantified the strength change and load distribution characteristics of the retrofitted bridge deck. Static and dynamic tests were performed both before and after the retrofit. The tests show that adding a second deck effectively decreased the static deflections and improved the transverse load distribution. Nail-laminated timber bridge #2642, also in Sibley County, was retrofitted in 1992 and was load tested again in 1995. All dynamic deflections were lower than those of the post-retrofit tests in 1992. This improvement can be explained in part by the drying of the moisture that was introduced into the bridge deck during grouting. Subsequent drying would add stability to the bridge deck. A retrofitted timber bridge is expected to last an additional 20-40 years.

Approximately 35% of Minnesota's 1,373 bridges are reaching the end of their expected service life. Many of these timber bridges are located in rural, low traffic settings that do not justify the expense of replacement. Extending the life of these bridges will not only save communities money, but will ensure continuous bridge service. This study reviewed both new and traditional techniques for inspecting decaying wood on timber bridges. Newer, nondestructive technologies include stress wave timber or Resistograph drill and infrared thermography. These methods offer many advantages, but they are still expensive and time consuming. Traditional inspection methods might provide the best alternative. The researchers recommend inspecting older bridges every three years and newer bridges every five years, with more careful evaluation of areas that show wood decay. The researchers conclude that the best way to pass on knowledge about timber bridge inspection is through hands-on seminars.

This summary offers an overview of on a research study that assessed the magnitude of premature asphalt deterioration on timber bridges; identified the primary mechanisms responsible for wear surface deterioration; and suggested methods for improving asphalt pavement performance on timber bridges. The study revealed that approximately 50 percent of counties experience some problems with premature reduced serviceability of the asphalt pavement wear surfaces that cover their timber bridges. The summary looks at possible pavement failure mechanisms and presents the following proposed solutions for controlling timber bridge asphalt pavement cracking: asphalt pavement saw & seal, asphalt pavement fabric or material underlay, removal of extruded oil-type preservative before surfacing, conditioning of bridge timbers to the expected equilibrium moisture content before bridge installation, and tightening of timber decks through maintenance practices.

Timber in nail-laminated and stress-laminated bridges is often installed with moisture contents (MC) near the fiber saturation point. Post-installation moisture loss induces shrinkage in the timber components, which results in loosening of component fasteners. This research project sought to establish the equilibrium moisture content (EMC) of timber bridges in Minnesota. Researchers took seasonal MC measurements on six nail-laminated timber bridges to determine annual MC variations and moisture gradients in individual bridge components: three bridges from northern Minnesota in St. Louis County and three from southern Minnesota in Sibley County. An electrical resistance meter measured moisture content, with oven-dry and toluene distillation methods of MC determination as controls. The study found the average MC of bridge components in St. Louis County was 2 percent-11 percent higher than bridge components in Sibley. The study determined the average MC at a three-inch depth on three of the major bridge components as: • deck laminations 18 percent (Sibley) to 28 percent (St. Louis) • transverse stiffener beams 14 percent (Sibley) to 18 percent (St. Louis) • deck supports 17 percent (Sibley) to 27 percent (SI. Louis) . The results indicate that the regional microclimate may greatly affect MC. Results from this research will allow MC specifications to be determined before bridge installation, helping minimize post-installation moisture-related problems and optimize design calculations. In addition, results will provide necessary data for ongoing research on transverse load-sharing characteristics of longitudinally nail-laminated timber bridges. Finally, this information will provide a basis for inspecting MC in timber bridges.

Asphalt wear surfaces cover 1,378 of Minnesota's timber bridges. This study assessed the magnitude of premature asphalt deterioration on timber bridges; identified the primary mechanisms responsible for wear surface deterioration; and suggested methods for improving asphalt pavement performance on timber bridges. Research methods included surveys, meetings with several county engineers and tours of their timber bridges, interviews with both asphalt and timber bridge industry professionals, and literature reviews. The study revealed that approximately 50 percent of counties experience some problems with premature reduced serviceability of the asphalt pavement wear surfaces that cover their timber bridges. Possible pavement failure mechanisms include low-temperature cracking, reflective cracking from deck fault lines found at deck panel joint lines and deck lamination separations, asphalt fatigue fracturing, and asphalt de-bonding due to oil preservatives interference. The report presents the following proposed solutions for controlling timber bridge asphalt pavement cracking: asphalt pavement saw & seal, asphalt pavement fabric or material underlay, removal of extruded oil-type preservative before surfacing, conditioning of bridge timbers to the expected equilibrium moisture content before bridge installation, and tightening of timber decks through maintenance practices.