EMSR Lumber: Will It Make Good Roof Trusses?

January 1, 1965, marked the beginning of the third year that Electro-Mechanical Stress Rated (EMSR) lumber has been in the marketplace. The predominant uses have been in various types of roof trusses and trussed rafters. The EMSR grading system is much more capable of separating lumber into graduated strength and stiffness classes than the older visual stress grading methods. For example, 1500f Industrial grade western hemlock has traditionally been regarded as having a stiffness rating of 1,540,000 psi, and the same grade of Douglas-fir and southern pine has been rated at a stiffness rating of 1,760,000 psi. Material of comparable stiffness by the EMSR system has higher strength ratings. Tests of full-scale trussed rafters made from EMSR lumber of two grades and three species compared very favorably with the visual stress grade of western hemlock. Allowable stresses for EMSR lumber are given as well as the performance of trussed rafters on a 27-1/2 foot span with a 5 in 12 pitch under dead load of 25 psi.

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