CAMBKIA STEEL. 407 



The following statements are made in Bulletin No. 12, U. S. 

 Department of Agriculture, Division of Forestry: 



RECOMMENDED PRACTICE. 



"Since the strength of timber varies very greatly with the 

 moisture contents (see Bulletin 8 of the Forestry Division), the 

 economical designing of such structures will necessitate their being 

 separated into groups according to the maximum moisture 

 contents in use. 



MOISTURE CLASSIFICATION. 



"Class A (moisture contents, 18 per cent.) Structures freely 

 exposed to the weather, such as railway trestles, uncovered 

 bridges, etc. 



"Class B (moisture contents, 15 per cent.) Structures under 

 roof but without side shelter, freely exposed to outside air, but 

 protected from rain, such as roof trusses of open shops and sheds, 

 covered bridges over streams, etc. 



"Class C (moisture contents, 12 per cent.) Structures in 

 buildings unheated, but more or less protected from outside air, 

 such as roof trusses of barns, enclosed shops and sheds, etc. 



"Class D (moisture contents, 10 per cent.) Structures in 

 buildings at all times protected from the outside air, heated in the 

 winter, such as roof trusses in houses, halls, churches, etc. 



"For long-leaf pine add to all the values given in the tables, 

 except those for moduli of elasticity, tension and shearing, for 

 Class B, 15 per cent. ; for Class C, 40 per cent. ; and for Class D, 

 55 per cent. For the other species add to these values, for Class 

 B, 8 per cent. ; for Class C, 18 per cent., and for Class D, 25 per 

 cent." 



Based upon the above classification of structures, the two 

 following tables have been figured to facilitate calculations of 

 allowable loads for wooden beams and columns. 



Proportion of the Values given in the "Tables of Safe 

 Loads for Wooden Beams," Pages 416 to 421 inclusive, 

 to be used in order to obtain the Safe Loads for the 

 various classes of structures referred to above. 



