Moisture Content Variations of Nail-Laminated Timber Bridges in a Northern Climate

Abstract

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 (St. 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.