Lateral Bearing Strength of Douglas-Fir Lumber-Plywood Joints Treated with Fire Retardants

More than 4,700 lumber-plywood joints were fabricated from different combinations of untreated and fire-retardant treated (FRT) Douglas-fir dimension lumber and 3/8-inch plywood. Five types of nails and 3 staples (6d or equivalent) and 3 fire-retardant treatments were included. Joints (specimens) were exposed in cold, standard, and warm humid conditions then tested in lateral bearing after 2 and 7 years. Final failure in the majority of the tests was fastener heads pulling through the plywood. Considering all factors, greatest strength values were obtained from joints made with 6d hot-dipped galvanized nails. The 6d monel nail was next best overall, and was best in warm, humid conditions. When exposed in cold and standard conditions, about equal results were obtained from joints treated with NON-COM, Pyresote, and borax-boric acid fire retardants. In warm, humid conditions, however, specimens treated with Pyresote were lowest in strength. The recommendation of not using FRT wood exposed in warm humid conditions for prolonged periods should be followed, or considerable judgment should be exercised regarding types of fasteners to be used. Although some joints did not lose strength after 2 years’ exposure in warm humid conditions, nearly all lost strength after 7 years; in many joints the fasteners corroded away, resulting in zero strength. For each of the eight fasteners, there were many joints fabricated from different combinations of treated and untreated lumber and plywood that gave as good results as untreated control specimens. In general, however, greatest values were obtained from untreated controls; those joints with untreated plywood were next best; those with both lumber and plywood treated were lowest in strength.

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