(6) Shear Perpendicular to Grain . Shear perpendicular to grain is 

 not a design factor in solid wood because effective control is applied 

 through limits on design stresses in shear parallel to grain and compression 

 or bearing perpendicular to grain. 



(7) Fiber Stress in Bending . A force or set of forces applied 

 perpendicular to a beam, as shown in Figure 59, creates compression in the 

 fibers on the side to which the force is applied and it also creates tension 

 in the fibers on the opposite side. Thus, there is a tendency to compress 

 the fibers on the compression side and to elongate the fibers on the tension 

 side. As the stress is distributed from the extreme fibers or outside faces 

 toward the center or neutral axis of the piece it is reduced in intensity. 

 Thus, deviations in slope of grain and the presence of knots or holes in 

 these outside faces tend to reduce the resistance in the extreme fibers and 

 the bending strength of the beam. 



1 



i * . * 





COMPRESSION 



i 



I 





Atension 



Figure 59. Fiber stress in bending, 



(8] Proportional Limit, Static Bending . The proportional limit 

 occurs at the point where the induced strain or deformation ceases to be 

 proportional to the stress or applied load, as determined by the standard 

 test method. Stress at proportional limit is computed by the standard 

 method. All conventional methods of structural design for wood are within 

 the proportional or elastic limit. 



(9) Modulus of Rupture, Static Bending . The modulus of rupture is 

 computed from the ultimate load or the point at which the piece breaks under 

 the standard bending test method. Loading by test beyond the proportional 

 limit shows an increasing rate of deformation, without a specific yield 

 point, until ultimate load is reached. 



(10} Modulus of Elasticity, Static Bending . The modulus of elas- 

 ticity is a measure of stiffness and is computed on the basis of the load 

 and deformation within the proportional limit. 



b. Design Values For Structural Lumber . 



(1} General . Design values are assigned to lumber in a scientific 

 manner to provide material of predictable strength properties to meet the 

 requirements of engineering design. Because of the varying nature of the 

 different species of trees, there is a wide range of stress values from 

 which the designer can make his selection. However, to avoid delay during 



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