Load Rating of Wood Bridges Using LRFD and ASD

Abstract

Most timber bridges are currently designed in accordance with the American Association of State Highway and Transportation Officials (AASHTO) Standard Specifications for Highway Bridges, 17th Edition (SSHB), which follows an allowable stress design (ASD) approach. However, the Federal Highway Administration (FHWA) will be requiring all highway bridges designed after October 1, 2007, to follow the AASHTO LRFD (Load & Resistance Factor Design) Bridge Design Specifications, 3rd Edition . As the transition from ASD to LRFD nears, engineers need to be aware of the differences. Information will be presented about more recent changes to the design codes and differences between AASHTO's Manual for Condition Evaluation and Load Resistance Factor Rating (LRFR) of Highway Bridges , which uses the LRFD method, and AASHTO's Manual for Condition Evaluation of Bridges, 2nd Edition (MCEB), which uses the ASD method. This paper will provide an overview of the pertinent issues related to wood bridge load ratings. Examples are included that illustrate the dramatic reduction in rated load capacity when sawn wood and glued-laminated timber stringers are load rated by the LRFD method compared to the ASD method.