MATERIALS AND THE STRENGTH OF MATERIALS 49 



When a beam is loaded to its limit, bending moment = 

 section modulus X modulus of rupture. 



This is a general equation which applies to all beams. 



82. Factor of safety. In the design of machinery it is 

 customary to make the parts several times as strong as 

 would be needed to carry normal loads. The number 

 of times a piece is made stronger than necessary simply 

 to carry the load is called the factor of safety, and in 

 farm machine design it varies from 3 to 12. 



For a more complete discussion of this subject see any 

 work on mechanics of materials. 



AVERAGE STRENGTH OF MATERIAL PER SQUARE INCH 



Values for the strength of timber were obtained from 

 U. S. Forestry Circular No. 15. If the load or stress be 

 continued for a long time the ultimate strength of timber 

 will be only about one-half the above and for this reason 

 much lower values are often given in architects' hand- 

 books. 



For a more complete table see any engineers' hand- 

 book.* 



*"Architects' and Builders' Pocket-Book." By 

 "Materials of Construction." By J. B. Johnson. 



F. E. Kidder. 



