26 FACTOR OF SAFETY. Art. 24 



The ultimate strength of structural steel may be said to be 

 included between the limits of 50000 and 70000 ibs. per sq. in., 

 the allowed variation for any particular grade being from 8000 

 to 10000 Ibs. per sq. in. The ultimate strength of steel may vary 

 from about 48000 Ibs. per sq. in. in the softest kind up to nearly 

 400000 Ibs. per sq. in. in hard drawn wire. 



24. Factor of Safety. To obtain working unit stresses for 

 use in designing structures, it was and is still, to a large extent, 

 the practice of engineers to divide the ultimate strength by a 

 factor, varying from 3 to 10, called the factor of safety. Thus, 

 if the ultimate strength of the steel used be 60000 Ibs. per sq. in., 

 and the factor of safety be taken as 5, the working stress will be 

 12000 ibs. per sq. in. 



The choice of a factor of safety involves many considerations 

 such as the impossibility of knowing accurately the amount and 

 effect of all loads and stresses, undiscoverable and unavoidable 

 defects in material and workmanship, and deterioration due to 

 corrosion and icear. 



The distribution of the load and its possible future increase 

 are nearly always more or less uncertain. Then too a structure, 

 or certain parts, may be liable to receive accidental shocks as well 

 as those which are incidental to varying loads; these produce 

 impact stresses which are especially uncertain (5). 



Even when the loads are definitely known, theory is fre- 

 quently not adequate to the determination of the resulting 

 stresses except upon certain assumptions. Unavoidable eccentric 

 stresses are seldom calculated, but they have an influence upon 

 the choice of a factor of safety. 



The factor of safety is always taken less for wind load 

 stresses and for combinations of stresses which can only occur at 

 rare intervals, than for other stresses. 



The factor of safety is always taken greater for materials 

 which vary greatly in quality than for those which are uniform 

 and may be depended upon to be nearly as determined by tests. 

 The qualities of stone, timber and cast iron are much less uni- 

 form than those of steel and wrought iron. 



There will be imperfections in workmanship as well as in 

 material which no amount of inspection and testing can elimi- 

 nate. Working stresses must be based upon available material 

 ami workmanship. 



