construction, it is advisable to determine which species and grades are 

 available locally before design values are selected. 



(2) Classification of Structural Lumber . Because the effects of 

 knots, slope of grain, checks, and shakes on the strength of lumber vary 

 with the loading to which the piece is subjected, structural lumber is often 

 classified according to its size and use. The three major classifications 

 are as follows: 



(a) Dimension lumber- -pieces of rectangular or square cross 

 section, 2 to 4 inches thick and 2 inches or more wide (nominal 

 dimensions) graded primarily for strength in bending edgewise or 

 flatwise but also used where tensile or compressive strength is 

 important; dimension lumber may be further classified as joists 

 and planks, for material 5 inches or more in nominal width, and 

 as light framing or structural light framing for material 2 to 



4 inches wide; 



(b) beams and stringers- -pieces of rectangular cross section, 



5 to 8 inches (nominal dimensions) and larger, graded for strength 

 in bending when loaded on the narrow face; and 



(c) posts and timbers—pieces of square or nearly square cross 

 section, at least 5 by 5 inches (nominal dimensions) and graded 

 primarily for use as posts or columns but adapted to 

 miscellaneous uses in which bending strength is not especially 

 important . 



(3) Characteristics Affecting Strength . Aside from the natural 

 properties of the species, the major characteristics affecting the strength 

 of a piece of lumber are the sizes of knots or holes and their locations, 

 the sizes of checks or shakes and splits and their locations, the amount of 

 wane or absence of wood, slope of grain, degree of density or rings per 

 inch, and the condition of seasoning. All these characteristics are taken 

 into consideration in the stress grading of a piece of lumber. These 

 conditions are illustrated in Figure 60. 



(4) ASTM Standards . There are two ASTM standards which serve as 

 principal references in the assignment of working stresses of lumber. One 

 standard is ASTM D2555, "Methods for Establishing Clear Wood Strength 

 Values," which sets forth procedures for establishing strength values for 

 clear wood of different species in the unseasoned condition and unadjusted 

 for end use. Such procedures may be applied to a single species or to a 

 group of species where growth and marketing conditions justify such grouping. 

 The other standard is ASTM D245, "Methods for Establishing Structural 

 Grades for Visually Graded Lumber," which sets forth reduction factors to be 

 applied to the clear wood values and provides procedures for determining 

 strength ratios, based on knots and other characteristics, which, when 

 applied to the adjusted clear wood values, results in working stresses for 

 the various commercial grades of any species. This standard also provides 

 adjustments for degree of density and for condition of seasoning. 



(5) Lumber Grading Rules. Lumber grading rules. are, in effect, 

 specifications of quality. In the rules the maximum knots, slope of grain 



239 



