Art, 24. 



FACTOR OF SAFETY. 



27 



A steel structure over a railway will suffer much more from 

 corrosion than one over a highway or one entirely enclosed. 

 Neglect of repairs may result in injury to parts not subject to 

 wear ordinarily. 



A temporary structure is given a smaller factor of safety 

 than a permanent one for numerous reasons. 



The damage that would result from the failure of one 

 structure or part might be much greater than from the failure 

 of another. 



It is plain that the choice of a factor of safety depends upon 

 the judgment of the engineer, and an inexperienced designer 

 should be guided by precedent, which usually means by some 

 standard specification. Specifications do not give factors of 

 safety at all but working stresses directly, and those may vary so 

 much that it is scarcely proper to speak of a factor of safety. 

 Working stresses may be based upon the elastic limit instead of 

 on the ultimate strength; this is a rational basis if the material 

 has a yield point or definite elastic limit. 



Specifications treat stresses in different ways, resulting in 

 widely different working unit stresses, as illustrated below for a 

 tension member of a truss for a railway bridge, designed in ac- 

 cordance with three different specifications. The discrepancy 

 in results is usually much larger. 



In the last case the working unit is applied to an equivalent 

 static stress, while in the other cases an allowance is made for 

 impact in the working stress, the allowance in the second case 



