Timber properties include the timber's strength and grade. Timber remains an important material because it is readily available in much of the world. Also, it is relatively easy to work with when using common tools. As an organic material, the strength properties of timber are influenced by factors such as the species of the tree, the direction of a load, and the size and grade of the lumber. These factors are considered in Tables C-1 through C-7. Other factors that could influence the strength of timber are not covered in this manual.
C-1. Table C-1 lists recommended civilian-design stresses based on the species, the size, and the grade of the timber members. Use this table for all semipermanent and permanent designs. Use it anytime a significant safety factor is wanted or when conditions allow the accomplishment of the mission using these conservative stress values.
C-2. Table C-2 lists allowable loads for expedient and temporary bridges based on timber sizes. Field experience has validated these higher stress values for temporary bridges that are not expected to have a heavy traffic load. Bridges designed or classified using these values should be watched closely for any signs of reduced carrying capacity. These bridges will have much higher maintenance requirements than those designed or classified with more conservative stress values. The data in Table C-2 is based on the assumption that the beam and stringer members are of select structural grade. Do not use this table if there are any doubts about the grade of timber.
C-4. The allowable stresses depend on the direction of loading of the timber member. Figure C-1 shows the radial, tangential, and longitudinal directions for the cross section of a log. The radial direction proceeds from what was the center of the tree out to the edge. The tangential direction is parallel (or tangent) to the growth rings of the tree at any particular point. The longitudinal direction represents the direction of growth for the tree. For example, the longitudinal direction of a tree trunk is up and down. The longitudinal direction is said to be parallel to the grain. The radial and the tangential directions are perpendicular to the grain.
C-5. Another factor that influences the allowable stress is the grade of lumber. Grading of a particular species is done according to the rules of the agency responsible for inspecting commercial timber for that species. Select a structural grade timber that generally has no knots and very few other imperfections, which can reduce strength. Grade No. 1 has no knots, but has slightly more imperfections than select structural grades. Higher grade numbers allow a greater number of knots and imperfections. The higher the grade number the lower the timber quality.
C-6. The grades listed in Table C-1 are commercial grades common in the US. Timber members obtained through the AFCS should match one of these specifications. If using native timber, try to locate similar civilian specifications or use the values in Chapter 3 for timbers of an unknown species. Another alternative is to compare a foreign species to a similar species listed in Table C-1. Such a comparison requires extensive experience to ensure that any crucial differences are not overlooked.
Figure C-1. Log Cross Section
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