A reliability based design procedure for wood pallets

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Virginia Polytechnic Institute and State University


Pallets are widely used to efficiently store and handle goods and are often subjected to bending and impact loads. The consequences of structural failure of a loaded pallet can include loss of goods, increased labor and equipment costs, and possible severe or fatal injury to humans. - The pallet industry, which annually consumes nearly 20% of all lumber manufactured in the United States, recognized a need for a rational design methodology, based upon engineering principles, to ensure consistent safety and economy in pallets of any geometry. To satisfy this need a cooperative research project between Virginia Tech, the U. S. Forest Service, and the National Wooden Pallet and Container Association was established. The objective of the project was to develop methods to design pallets for strength, stiffness, and durability. A primary expected benefit of the design methodology is to allow comparison of different pallet designs on a performance basis, without the need for extensive physical testing. This dissertation presents the results of this cooperative research project.

The developed methodology was computerized (Pallet Design System (PDS)) and is intended to allow pallet manufactures to obtain estimates of the maximum safe load capacity or the member dimensions required to resist known loads. Additionally, the program produces estimates of the durability and cost-per-use for pallets in specific service environments. PDS is limited in scope to pallets with up to four stringers and a maximum of 15 deckboards. Five different load types and four support modes can be analyzed. These include uniformly distributed and concentrated loads, and racked, stacked, and sling support modes. The techniques for estimating the strength and stiffness are based on matrix structural analysis and classical beam theory. The deckboard-stringer joints are modeled as spring elements, the stiffness of which are based upon characteristics of the fastener. Most fasteners commonly used in pallet construction (i.e. threaded nails or staples) can be analyzed. A probabilistic design technique based on mean value methods was applied in PDS to ensure safety in the resulting designs. The safety index was calibrated to pallet designs associated with warehouse load data. The physical properties of the material are estimated using either a modified clear-wood property approach (ASTM D-245 method), or in·graded testing of pallet lumber. The durability estimates are based upon studies of field data and economic analysis.