and other strength reducing characteristics are described in sufficient 

 detail that the procedures of ASTM D245 can be applied and working stresses 

 can be assigned to the specified quality. It is common practice to give 

 each grade a commercial designation such as No. 1 for best, No. 2 for next 

 best. This means that the purchaser orders the commercial grade which 

 qualifies for the values used in design. 



(6) Machine Graded Lumber . While most structural lumber has 

 design values assigned on the basis of visual grading to meet a minimum 

 quality specification, there is a growing trend toward the nondestructive 

 testing of lumber by machine. In this method a piece of lumber is passed 

 flatwise through a series of loading rollers and the stiffness, or modulus 

 of elasticity, is automatically recorded. Through correlation with pre- 

 viously established test data, bending strength and other strength properties 

 are assigned to each piece tested. At present, machine grading is supple- 

 mented by visual grading particularly in the assignment of horizontal or 

 longitudinal shear values. 



(7) National Design Specification . The principal reference for 

 working stresses for commercial grades of structural lumber is the National 

 Design Specification for Wood Construction, available from the National 

 Forest Products Association, Washington, D.C. The design value information 

 in this specification is taken from the published rules written by the 

 American Lumber Standards Committee (ALSC) and other grading rules writing 

 agencies. When these values are used, each piece of lumber is required to 

 be identified by the grade mark of a lumber grading or inspection agency 

 recognized as being competent. 



The National Design Specification provides for design of single member 

 uses of lumber and other structural timbers, and also for repetitive member 

 uses of lumber where load sharing is known to exist between repetitive 

 framing members which are spaced not more than 0.6 meter (24 inches), are 

 not less than 3 in number and are joined by floor, roof, or other load- 

 distributing elements adequate to support the design load. For repetitive 

 member uses, the design values in bending are higher than those for single 

 member uses, as provided in the National Design Specification. 



4. Selection of Timber Piles . 



a. Round Timber Piles . Recommendations for the use of timber piles in 

 foundations may be found in the American Wood Preservers Institute (1967). 

 The ASTM D25-73, Standard Specifications for Round Timber Piles, classifies 

 round timber piles according to the manner in which their load -carrying 

 capacity is developed. There are two classes: 



(1) Friction Piles . Friction piles are used when pile capacity is 

 determined by the friction developed in contact with the surrounding soil, 

 along with the compressive strength of the timber piles used. Table 29 

 from ASTM Standard D25-73, lists size requirements for friction piles. 



(2) End-Bearing Piles . End-bearing piles are used when pile 

 capacity is determined primarily by the end-bearing capacity of the soil at 

 the pile tip, along with the compressive strength of the timber piles used. 



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