InTrans / Aug 14, 2018

Live-Load Distribution on Glue-Laminated Timber Girder Bridges

Increased use of timber bridges in the U.S. transportation
system has required additional research to improve the current design methodology of these bridges. For this reason, the U.S. Forest Service, Forest Products Laboratory
(FPL), and the Federal Highway Administration have supported several research programs to attain the objective listed above. This report is a result of a study sponsored by the FPL, with the objective of determining how highway truckloads are distributed to girders of a glued-laminated timber bridge. The American Association of State Highway and Transportation Official (AASHTO) load and resistance factor design (LRFD) Bridge Design Specification provides live-load distribution provisions for glued-laminated girder timber bridges that were used in previous AASHTO Specifications.
The AASHTO live-load distribution provisions were reviewed in this report.
Field-test results were used to review the current AASHTO
LRFD glued-laminated timber girder bridge-design specifications and to validate analytical results obtained by finite-element analyses. With the validated analytical models, parametric studies were performed to determine the worst-case live-load distribution factors that can be used to calculate the design moment and shear for glued-laminated timber girders. Simplified live-load distribution equations that can be used to determine these distribution factors were developed and are provided in this report. These equations take into account how load is distributed to the bridge girders,
considering the effects of span length, girder spacing, and clear width of the bridge.