STRESSES AND STRAINS 125 



subjected to fatigue, produced by constant stress due to the 

 load it may have to sustain. 



The preceding reasons are sufficient to require the factors 

 now adopted by conservative constructors in all engineering 

 work. These factors are 3 to 5 for structural steel, and 

 6 to 10 for cast iron. 



Stone and brick are very unreliable and a high factor of 

 safety should therefore be used. Usually, the factors 

 employed are 10 for compressive stresses, 15 for tensile 

 stresses, and from 10 to 20 for bending stresses. Some 

 engineers, however, do not use such high factors. Stone- 

 work or brickwork is even more unreliable than stone or 

 brick themselves, but the same factors are often employed. 

 Usually, however, the strengths of stonework and brickwork, 

 instead of being given as ultimate strength, are arranged in 

 tables to give the safe, or allowable, stress per square inch or 

 per square foot. This value embodies its own factor of safety. 



Concrete is another material that sometimes requires a 

 high factor of safety. In reinforced-concrete work a factor 

 varying from 4 to 6 is usually employed, although some 

 engineers use a higher one. 



Ordinances in various cities throughout the country 

 govern the allowable load that building materials should 

 carry. 



A structure withstanding shocks or containing rapidly 

 vibrating machinery is more liable to fail than one in which 

 the loads are quiet, even if the latter has the greater loads. 

 Therefore, in designing a structure to withstand vibrating 

 loads or shocks, the designer, to make the structure safe, 

 uses a larger factor of safety. Usually, this factor averages 

 from two to three times the ordinary factors, except those 

 for timber, which need not as a rule be increased. 



The average ultimate strength of various metals and 

 woods are given in the accompanying tables. 



