SAFE STRENGTH OF MATERIALS 



In bending, there is a combination of all three of these 

 forces. As an illustration of this take a green twig, cut it off 

 square and bend it near the cut end. It results in the crush- 

 ing of the fibers on the inner part of the curve, the failure by 

 tension of the fibers on the outer part of the curve, and often 

 that the portion of the stick between the bend and the end is 

 split, indicating that the endwise compression on the inner 

 side of the curve and the tension on the outer part of the curve 

 have been so great, pulling in opposite directions, that the 

 material of the twig shears through the heart. 



In this connection it is interesting to notice the impor- 

 tance of the depth of a beam. For instance, an increase in 

 the depth of a beam increases its stiffness more rapidly than 

 the amount of material is increased. In fact, the ability to 

 support a load increases as the square of the proportional in- 

 crease in the depth of the material. That is, a 2x8 will sup- 

 port 4 times as much as a 2x4, but contains only twice as 

 much material. 



Safe Strength of Materials. The following table will 

 show the safe strength of several materials: 



Table III. Safe strength of materials. 



A No. 3 bright Bessemer steel wire will safely stand 1000 pounds in 

 tension. 



