12 



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. 



( 'lax.s C (moisture contents, 12 per cent). — Structures in buildings 

 unheated, but more or less protected from outside air, such as roof 

 trusses or barns, inclosed 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. 



The following tables of safe loads have all been made out for Class A, 

 with the intention of making them applicable to bridge-trestle construc- 

 tion. To make these applicable to the other classes make the following 

 modifications: 



For longleaf 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. 



For the modulus of elasticity add only one-half of the above percent- 

 ages. For tension and shearing use the tabular values— whatever the 

 percentage of moisture. 



For longleaf and shortleaf pine these modifications are quite correct, 

 the percentage of increase of strength of the former being about twice as 

 great as that for the latter between the green and diy condition. This 

 percentage of increase is not so well known for the other species, but 

 tests that have been made indicate a percentage of increase at least as 

 large as for shortleaf pine. Until further tests have been made, there- 

 fore, the modifications given above may safely be used. 



Note. — The reductions for moisture as given above in the case of longleaf pine 

 appear somewhat at variance with results obtained since. In the case of other spe- 

 cies they rest on assumption, for which experimental data are still largely lacking; 

 it will, therefore, be proper to use the same with caution. 



The moisture condition at 18 per cent is one difficult to obtain under natural con- 

 ditions; it would, therefore, have been more desirable if the author had started from 

 the green condition, which is fixed. The following values for green condition are 

 here added for the four Southern pines, on which alone the Forestry Division has 

 reliable data: 



Transverse strength or modulus of rupture, 



green 



Compression endwise, green 



Relative strength as a mean of transverse 

 and eompression: 



Green 



Half dry 



Yard dry 



Room drv 



Average 

 change. 



100 

 122 

 146 

 182 



R. E. Fernow. 



