2.1. Basic Strength and Elastic Properties of Wood
2.10 DESIGN VALUES, TABLES 2-6 AND 2-7.
Strength properties of various species for use in calculating the strength of aircraft elements are presented in tables 2-6 and 2-7. Their applicability to the purpose is considered to have been substantiated by experience.
The values in table 2-7 are based on a moisture content of 20% and should be used for design of structural parts of aircraft to be used under tropical conditions where high relative humidity, approximately 90% or over, is prevalent for long perlods of time, or more or less continuously.
2.100. Supplemental notes.,
2.1000. Compression perpendicular to grain.
Wood does not exhibit a definite ultimate strength in compression perpendicular to the grain, particularly when the load is applied over only part of the surface, as it is by fittings. Beyond the proportional limit, the load continues to increase slowly until the deformation becomes several times as great as at the proportional limit and the crushing is so severe as to damage the wood seriously in other properties. A "probability" factor was applied to average values of stress at proportional limit to take account of variability, and the result was increased by 50 percent to get design values comparable to those for bending, compression parallel to grain, and shear as shown in tables 2-6 and 2-7.
2.1001. Compression parallel to grain.
Available data, indicate that the proportional limit for hardwoods is about 75 percent and for softwoods about 80 percent of the maximum crushing strength. Accordingly, deslgn values for fiber stress at proportional limit were obtained by multiplying maximum crushing strength values by a factor of 0.75 for hardwoods and 0.80 for softwoods, and for a difference in the factors for the "rate and duration of load."
2.11. NOTES ON THE USE 0F VALUES IN TABLES 2-6 AND 2-7.
2.110. Relation of design values in tables 2-6 and 2-7 to slope of grain.
The values given in tables 2-6 and 2-7 for grain slopes as steep as the following:
(a) For compression parallel to grain: 1 in 12.
(b) For bending and for tension parallel to grain: 1 in 15.
When material is used in which the steepest grain slope is steeper than the above limits, the design values of tables 2-6 and 2-7 must be reduced according to the percentages in table 2-8.