Species Response to Prestress in Curved Laminated Wood Beams

Based on tests of small, clear curved beams, this paper presents curvature-stress factors for several species with respect to the ratio of individual lamination thickness to radius of curvature. Individual species in differ in their response to prestressing involved in forming a curvature and, in addition, the effects of prestressing are different in beams that are loaded on the convex or concave surface. Douglas-fir and two South American hardwoods were used to test the ratio of lamination thickness to radius of curvature (t/R). Four additional tropical American hardwoods were tested at the single t/R ratio of 1/100. For the hardwoods employed in this study, curvature-stress factors were found to be related to wood characteristics readily determinable from conventional static-bending data. The Wilson equation currently used in design does not take these differences into account and gives a rather conservative curvature-stress factor of 0.80. Curvature-stress factors for Douglas-fir ranged from 1.10 to 0.95 over the range in t/R from 1/180 to 1/80 for convex-loaded beams and from 1.00 to 0.83 over the same range in t/R in the case of concave loading.

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