Longitudinal Stringers with Transverse Deck Bridges
The longitudinal stringer with transverse deck bridges are common for 30-100 ft. spans; typically used for spans greater that 50 ft. The stringers are horizontally laminated and the decks are vertically-laminated panels.
Stringers are usually four to six feet on centers with 5 1/8" or 6 ¾" transverse deck thickness, dependent upon stringer spacings and bridge loadings.
Dry-use condition should be used for design of stringers and wet-use for deck panels. Both stringers and deck panels should be pressure preservatively treated per AITC 109 and AWPA C28.
The American Institute of Timber Construction (AITC) recommends a live load deflection criteria of L/300 for timber bridges. AASHTO recommends L/500; other criteria are stated to be acceptable. Considering traffic volume, speed limits, relative deflections of components in the bridge structure, and field experience, a deflection limit of L/300 is generally sufficient for timber bridges.
Cookson Bridge - horizontal girder and vertical flanges Standish Ave Bridge over Bear River, MI
Bridges Page 3