To use and develop timber structures in transportation better, the U.S. government implemented several national programs starting in the early 1990s to address the needs of the timber industry. One need was to investigate the dynamic field performance of timber bridges in relation to vehicular loading. The AASHTO load and resistance factor design specifications recommend a dynamic load allowance 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 obtain dynamic performance data, 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 performance data including deflection and acceleration, as well as to assess the overall condition state of the bridges. The nine bridges tested were found to have fundamental frequencies between 5 Hz and 11 Hz as well as a dynamic load allowance of 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.
