The anisotropic nature of wood may be confusing to the designer during 

 his first experience with its use, but as he gets to know the material he 

 finds that engineering design with wood can be interesting as well as 

 productive in the way of lower construction costs. The discussion which 

 follows provides a brief description of the various mechanical properties of 

 structural wood as they affect engineering design. 



(1) Tension Parallel to Grain . A force generating tension parallel 

 to grain, as shown in Figure 55, creates a tendency to elongate the wood 

 fibers and to cause them to slip by each other. Resistance to tension 

 applied strictly parallel to grain is the highest strength property of 

 wood. This resistance, however, is substantially reduced when the force is 

 applied at an angle to the grain or when the cross section of the piece is 

 reduced by knots or holes. 







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t 







f 

















Figure 55. Tension parallel to grain. 



(.2) Tension Perpendicular to Grain . A force generating tension 

 perpendicular to grain tends to separate the wood fibers along the grain. 

 This is the direction in which wood has the least strength, and because it 

 is not good practice to apply loading to induce tension across grain, 

 design values are not provided for this strength property, except for 

 special applications. 



(3) Compression Parallel to Grain . A force generating compression 

 parallel to grain, as shown in Figure 56, creates a tendency to compress the 

 wood fibers in the lengthwise position. As with tension, resistance to 

 compression parallel to grain is affected by the angle of load to grain and 

 by the presence of knots or holes. 



(4) Compression Perpendicular to Grain . A force applied perpen- 

 dicular to grain, such as the bearing under the ends of a beam as shown in 

 Figure 57, tends to compress the wood at its surface. While the wood becomes 

 more dense as it is compressed, this action causes slight displacement of 

 the supported member. Thus, limits are placed on loading in bearing perpen- 

 dicular to grain. 



(5) Shear Parallel to Grain . A force applied in the manner 

 illustrated in Figure 58 causes one section of the piece to shear or slide 

 along the other section in a direction parallel to grain. In a loaded beam 

 where the induced stress on the one side is compression and on the other 

 side is tension, as illustrated in Figure 58, shearing stress is created 



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