CHAPTER IX 

 SAFE STRENGTH OF MATERIALS 



The Factor of Safety. In the use of any material it is of 

 course important to know how much force or pressure it will 

 resist. In testing materials the actual weight necessary to 

 strain, bend, break, or crush them is accurately determined. 

 It is evident that the largest load which a given material will 

 carry cannot safely be placed upon it. The fractional part 

 of the greatest strength which it is considered advisable to 

 use, determines what is generally spoken of as the " factor 

 of safety." For instance, a steel rod one inch square, which 

 of course has a cross-sectional area of one square inch, would 

 probably support, without breaking, any load up to 60,000 

 pounds, providing the load was suspended directly by it. 

 But in case of silo building there should not be a force of more 

 than 15,000 pounds placed upon it. This is called the safe 

 strength of this material for this purpose, and is approxi- 

 mately J4 of the ultimate or greatest strength. It is gener- 

 ally spoken of as the use of a factor of safety of 4. If human 

 life were more continuously depending upon this material, 

 as is the case in residences, office buildings, or theatres, the 

 factor of safety used would probably be 5 or 6. Thus the 

 safe strength of this material would then be 10,000 to 12,000 

 pounds per square inch. In masonry materials for silos the 

 factor taken is not less than 8. 



Just what portion of its ultimate strength can be safely 

 utilized, depends considerably upon the material and the seri- 

 ousness of results in case a failure should occur. As already 



