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Va. Tech Studies Pallets Assembled With Structural Composite Lumber
Study focus is performance of pallet joints, best fasteners to use with structural composite lumber

By Staff
Date Posted: 6/2/2004

            The Virginia Tech pallet and container research laboratory recently completed a study that examined the use of structural composite lumber (SCL) in pallets. The study focused on performance of pallet joints as well as the most appropriate type of fasteners to use with structural composite lumber.

            Joints of pallets assembled with structural composite lumber performed similarly or better than Southern yellow pine, often exceeding it in performance, according to a draft report based on the study.

            Structural composite lumber includes all structural members manufactured using wood veneers or board strands with the grain directions of all plies oriented parallel to the length of the member. They include Parallel Strand Lumber (PSL) or Parallam™, Laminated Veneer Lumber (LVL) or Microllam™ and Laminated Strand Lumber (LSL) or Timberstrand™. Plies or strands of wood are bonded together with exterior adhesives.

            Parallel laminated lumber out-performs conventional lumber when either face or edge-loaded. Structural composite lumber is also highly predictable and uniform.

            There has been no previous research concerning joint performance of pallets made with structural composite lumber.

            The objectives of the Virginia Tech study were:

n      Measure the relative performance of pallet joints fabricated with low-grade SCL stringers or blocks and compare performance of SCL with solid Southern Yellow Pine.

n      Determine the effect of certain fastener characteristics on the performance of pallet joints fabricated with low-grade SCL stringers or blocks.

n      Identify optimum fastener design for use in fabricating pallet joints made of SCL stringers and blocks.

n      Determine the applicability of the Pallet Design System predicted withdrawal resistance (FWR) models of pallet joints for joints assembled with SCL blocks or stringers.

            The research tested the hypothesis that SCL can be used as an alternative wood pallet raw material. With correct fasteners, SCL and SCL with plywood decking provide similar performance joint stability to that of more common pallet materials.

            Pallets and components were subjected to a number of tests for shear resistance, lead board impact, stringer impact, rack bending and more.

            One series of tests measured fastener withdrawal resistance. The tests were conducted with four types of nails, all 2-1/2 inches long – three with helical threads ranging from 11 gauge to 12.5 gauge and an 11-gauge annular thread nail. The 11-guage annular thread nail was significantly more resistant than the others. All the SCL materials except Timberstrand showed greater withdrawal resistance than pine. Parallam exhibited the greatest withdrawal resistance with each type of nail.

            In tests of connection shear resistance, there was no significant difference between pallets made of Microllam and pine. Pallets made with Timberstrand performed slightly lower than the others. Different fasteners made only a marginal difference in connection shear resistance. Pallets made of SCL were relatively unaffected by fastener selection in these tests, but fastener selection significantly impacted performance of connections assembled from pine.

            In drop tests, pallets made of SCL were more rigid than those made from pine. There was no significant difference among pallets made from SCL.

            In tests to measure stringer resistance to impact from forklift tines, Timberstrand performed the best of the SCL components and pine; the other types of SCL and pine performed similarly.

            In bending tests of racked pallets, pallets made of Microllam and Parallam were significantly stronger and stiffer than pallets constructed of pine and Timberstrand stringers.

            The researchers concluded that pallet joints made from SCL performed similarly or better than pine, often exceeding its performance. Also, the predicted fastener withdrawal resistance model in the Pallet Design System software program may be used to design pallet joints assembled with SCL.

            The researchers also found that fastening joints of SCL can damage the material, causing splits. Smaller nails may be effective in reducing damage, they suggested, adding that further research should be conducted.

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