CAMBKIA STEEL. 



413 



UNIT STRESSES FOR STRUCTURAL TIMBER. 



(Expressed in Pounds per Square Inch.) 



be used without increasing the live load stresses for impact. 



L = length in inches. D = least side or diameter in inches. 



The relation between the strength of the lowest 10 per cent, group of tests 

 and the average strength for each series, the relation between the elastic limit 

 and the ultimate strength, as well as the fact that the live load stresses are 

 not to be increased for impact, are all to be taken into account in determining 

 the general relation between the safe stress and the average ultimate stress; 

 it being always remembered that it is more rational to relate the safe unit 

 stress to the elastic limit of the material than to its ultimate strength. 



As large columns not over 15 diameters in length may not develop more 

 than 70 per cent, of the strength of short blocks, the column formulas are 

 arranged to give approximately these relative values at the given limit of 

 length when L, the length of the column in inches, equals 15 times its least 

 diameter D, also expressed in inches. 



It is expected that these unit stresses will be revised at intervals of a few 

 years, whenever new results of timber tests are published, or when the expe- 

 rience of bridge engineers who have adapted them shall indicate that revision 

 is desirable. 



