Wood Decks

Article 9.9 of the AASHTO LRFD (7th Edition, 2014) addresses design requirements of wood decks and wood deck systems. The use of wood deck systems on steel girders is typically limited to structures in special environments (forests, bike trails, etc.), or structures with severe weight restrictions. The United States Department of Agriculture, Forest Service, has developed Standard Plans for Timber Bridge Superstructures (20) that design engineers can reference for the various types of wood decks that can be used with steel girder bridges.

To determine the load distribution and force effects for structures with wood decks, Article 9.9.3.1 allows the bridge designer to use one of the following methods: Approximate methods specified in Article 4.6.2.1, orthotropic plate theory, or equivalent grid modeling. The live load flexural moment and shear for beams with transverse wood decks can be determined using the approximate methods provided in Article 4.6.2.2.2a for interior beams, and Article 4.6.2.2.3b for exterior beams. The distribution factors for interior beams are based on girder spacing and type of wood deck, and the distribution factors for exterior beams are to be determined using the lever rule.

If the girder spacing is less than 36 inches, or 6 times the nominal depth of the deck or the deck system including the girders, the deck should be modeled as an orthotropic plate or equivalent grillage. For wood decks with closely spaced supporting components, the assumption of infinitely rigid supports upon which the approximate methods are based, is not valid. Therefore, two dimensional methods such as orthotropic plate theory or equivalent grid models are recommended to obtain force effects with reasonable accuracy.