Institute for Transportation

In order to better utilize and develop timber structures in transportation, the United States government implemented several national programs starting in the early 1990‰Ûªs. One specific need identified was investigating the dynamic field performance of timber bridges due to vehicular loading. Currently, the American Association of State Highway and Transportation Officials (AASHTO) recommend a dynamic load allowance of 0.165 for timber bridges. Previously however, AASHTO did not include a load allowance for timber bridges. To quantify the appropriate code values, research was needed to determine the dynamic characteristics of timber bridges and to study their dynamic performance with respect to time and bridge condition. To fulfill this research need, five glued-laminated timber girder bridges and four longitudinal glued-laminated timber panel bridges were selected for testing. The testing involved loading the structures to obtain dynamic response data. The information collected relates to the dynamic deflection, acceleration, and overall condition assessment for all nine bridges. The results of the individual bridges are also compared with each other to determine the validity of the current AASHTO recommendations and to develop better design standards. 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 caused the excessive dynamic amplification values.