Proposed: New Stress Values for the Coast White Woods

In 1956 heavy infestations of wooly aphis on Weyerhaeuser Co. tree farms focused attention on the Coast white woods, which are western hemlock (Tsuga heterophylla), Pacific silver fir (Abies amabilis), grand fir (Abies grandis), and noble fir (Abies procera). Among problems worked on was one of sales outlets for the timber that would yield a satisfactory return on investment consistent with performance properties of the wood. It was determined that the bulk of the Coast white woods were in Washington and Oregon with lesser quantities in California, Idaho, and Montana. Volume-wise this amounted to 189 billion bd.ft. of timber or 200 billion bd.ft. of lumber. This represents 2.5 billion bd.ft. per year cut on a sustained yield basis over 80 year rotation or a perpetual supply of 250,000 homes per year. If higher stress levels could be adopted less lumber per house would be required and the resource would provide 300,000 homes per year. Liaison with the U.S. Forest Products Laboratory at Madison, Wisconsin, on the subject resulted in a series of studies, the results of which indicated that Wood Handbook values were low. Average specific gravity for hemlock was found to be 0.40 compared to Wood Handbook value of 0.38. The specific gravity of the firs was 0.36 as compared to a Wood Handbook value of 0.35. In addition to the higher specific gravity value indicated by these studies, two other considerations deserve attention. First, a rafter, joist or stud never fails in place, partly because it is overdesigned and partly because loading is transferred to adjacent members. Secondly, dry strength values have been set arbitrarily at 10 percent above green values when actual tests of clear specimens have shown 25 percent higher values. Steps are being taken to obtain more information. A U.S. Forest Service-industry study of species densities is one such move. Washington State University has a program to develop non-destructive test methods. U. S. FPL, DFPA, Potlatch Forest, Inc., Weyerhaeuser Co., and others have programs with similar objectives.

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