Some Problems in the Design and Performance of Laminated Wood Trusses

For this study the Forest Products Laboratory of Canada designed a laminated arch for use inside a building 47 feet 6 inches wide, 21 feet 6 inches high at the peak, and with a wall height of 14 feet 3 inches. The arch was constructed of Canadian white spruce; laminations were 7/8 inch and of random lengths, glued with a casein glue. It was erected outdoors and coated with lead paint. The arch was put, under a total load of 48,600 pounds and remained this way for several months; deflection readings were taken along with records of climatic conditions. The load was then reduced to 27,800 pounds and deflection readings continued. Conclusions were that temperature was a main factor in deflection of the arch. The design allowed a load of 700 psi but under these conditions the stress brought the material to the plastic range. Therefore the arch could have been strengthened by deepening the section or by using thinner laminations.

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