Dynamic Field Performance of Glued Laminated Timber Bridges (05-2177)**
Brent Matthew Phares, Iowa State University
Jake Bigelow, Iowa State University
Terry J. Wipf, Iowa State University
Mike Ritter, USDA Forest Service
Douglas L. Wood, Institute for Transportation at Iowa State University

To better utilize and develop timber structures in transportation, the United States government implemented several national programs starting in the early 1990’s to address the needs of the timber industry. One need was to investigate the dynamic field performance of timber bridges due to vehicular loading. The American Association of State Highway and Transportation Officials (AASHTO) recommend a dynamic load allowance (DLA) of 0.165 for timber bridges. To investigate this codified value, research was needed to determine the dynamic characteristics of timber bridges and to study their dynamic performance. To fulfill this research need, five glued-laminated girder bridges and four longitudinal glued-laminated panel bridges were selected for testing. The testing involved loading the nine structures to obtain dynamic deflection, acceleration, and overall condition state of the bridges. In general, the nine bridges tested were found to have fundamental frequencies between 5 Hz and 11Hz as well as a dynamic load allowance less than 0.25. The bridges found to have dynamic amplifications above specified code values were also found to have physical characteristics (i.e., rough entrances) that likely caused the higher dynamic amplification values.