290 TESTING OF STRUTS. 



where / is, as before, the elastic limit stress in compres- 

 sion of the material, and a is a constant whose value is 



~ 



E 



This value of a is dependent on the fact that the curve is 

 tangent to that of Euler. Johnson* has found that this 

 agrees very well with the plotted results of tests. Here 

 f ' = 36,0001bs. per square inch and 



(36,000) 2 

 ~ 4 . T 3 . 30,000,000 



Professor Johnson gives the following as reliable 

 average values : 

 For wrought-iron columns, pin ends, 



(where ^ is not greater than 170) 



? = 34,000 - 0-67 (I 



For wrought-iron columns, flat ends, 



(where ._ is not greater than 210) 

 fc 



p 



= 34,000 - 0-43 ( l \ 



For mild-steel columns, pin ends, 



(where ^ is not greater than 150) 



-A = 42 . - ' 97 (ff 

 For mild-steel columns, flat ends, 



(where -^ is not greater than 190) 



f = 42,000 - 0-62 (1)' 



For cast-iron columns, round ends, 



(where = is not greater than 70) 

 fc 



= 60,000 - V ( 



For cast-iron columns, flat ends, 



(where -=_ is not greater than 120) 



fc 



= 60,000 - 



* Johnson's " Materials of Construction," p. 362. 



