110 



STKEHGTH OF MATERIALS 



90. Cooper's modification of Johnson's straight-line formula. In 

 his standard bridge specifications, Theodore Cooper has adopted John- 

 son's straight-line formulas, modifying them by the introduction of 

 a factor of safety. Thus, for medium steel, Cooper specifies that the 

 following formulas shall be used in calculating the safe load. 

 For chords 



?. = 8,000 - 30 - for live load stresses, 



F 



t 



P I 



- = 16,000 - 60 - for dead load stresses. 



f t 



