Curvature-Stress Factor in Laminated Wood Beams

Pseudotsuga menziesii and Tabebuia pentaphylla were used to study th e influence of prestressing laminated curved members and to determine curvature-stress factors compared with straight nonprestressed laminated beams. Individual species act differently under prestressing so no conclusions can be made prior to results. As shown in this study, Douglas-fir was not prestressed beyond its proportional limit until formed to a radius less than about 100 times individual lamination thickness, whereas roble blanco only formed a radius less than about 80 times individual lamination thickness. All tests were run only on the convex face, not the concave face. Thickness of laminations to radius of curvature (t/R) ratio were up to 1/80 without breakage or serious reduction in strength. This portrayed that the Wilson equation now used is much too restrictive for values of strength as disclosed by these tests. At a maximum curvature of (t/R = 1/80) the curvature-stress factor of Douglas-fir was 0.87 — in other words, 87 percent of the strength of a comparable straight beam was found.

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