32 



FATIGUE OF IRON AND STEEL. 



Art. 28 



working stresses for bridges upon the range of stress in order to 

 take account of "fatigue," but this has been largely abandoned. 

 According to this practice, working stresses were determined 

 from a formula commonly called the Launhardt Formula, which 

 for stresses of one kind was of the form 



, . /., , minimum stress \ 



working stress=a (l + maximum gtreg j 



which usually also provided for impact and such other things as 

 a factor of safety makes provision for, "a" being a constant 

 depending upon the judgment of the engineer writing the speci- 

 fication. Since working stresses are always taken well within 

 the elastic limit, "fatigue" need not be considered in designing 

 structures; moreover, the influence of periods of rest between 

 loadings must be considerable 1 . Some engineers who used the 

 "Launhardt Formula" neglected "fatigue," but used this for- 

 mula as an impact formula. There have not been enough experi- 

 ments like those mentioned in Art. 5 to determine the proper 

 allowance to make for impact stresses in bridges. 



It is generally agreed that if sufficient impact stress is in- 

 cluded with dead and, live load stresses, a working stress equal to 

 about half the elastic limit affords ample security when good 

 steel is used. 



*For an extensive discussion of this whole matter, see the Transac- 

 tions of the American Society of Civil Engineers, Vol. 41, p. 166. 



