TIMBER PHYSICS SOUTHERN PINE. 



353 



Influence of moisture on strength. 



Average of all \ahd tests. 



Bending strength . 



Ci ushmg endwise . . 



Per cent 

 of moist- 



Cu- 

 ban. 



Long- 

 leat. 



Lob- 

 lolly. 



ure.a 



b, 430 



7,660 



7, 370 



[ 33 



J 20 



10, 050 



8, 900 



8, 650 



1 15 



11,950 



10, 900 



10, 100 



I 10 



15,300 



14, 000 



12, 400 



[ 33 



5,000 



4, 450 



4,170 



J 20 



6,600 



5, 450 



5,350 



\ 15 



7,850 



6,850 



6, 500 



1 io 



9, 200 



9,200 



8, 650 



Short- 

 leaf. 



6, 900 

 8,170 

 9, 230 

 11, 000 

 4,160 

 5, 100 

 5,900 

 7,000 



Relath e values. 



Bending strength . 



Ciushing endwise .., 



Mean of both bending and 

 crushing strength 



Per cent 

 of moist- 

 ure. a 



33 

 20 

 15 

 10 

 33 

 20 

 15 

 10 

 33 

 20 

 15 

 10 



Cu- 

 ban. 



100 

 llo 

 142 

 181 

 100 

 132 

 157 

 184 

 100 

 125 

 149 

 182 



Long- 

 leaf. 



100 

 116 

 142 

 182 

 100 

 122 

 154 

 206 

 100 

 119 

 148 

 194 



206 

 100 

 122 



187 



Lob- 



Short- 



lolly . 



leaf. 



100 



100 



117 



118 



138 



134 



168 



160 



100 



100 



128 



122 



156 



142 



168 

 100 



138 

 164 



Aver 

 age. 



100 

 117 

 139 

 173 

 100 

 126 

 152 

 191 

 100 

 122 

 146 

 182 



a 33 per cent green, 20 per cent half dry, 15 per cent yard dry, 10 per cent room dry. 



S 6 7 8 9 



Fig. 93.— Variation of compression strength with moisture. 



It will be observed that the strength increases by about 50 per cent in ordinary good yard seasoning, and that 

 it can be increased by about 30 per cent more by complete seasoning in kiln or house. 



Large timbers require several years before even the yard-season condition is attained, but 2-inch and lighter 

 material is generally not used with more than 15 per cent moisture. 



WEIGHT AND MOISTURE. 



So far the weight of only the kiln-dry wood has been considered. In fresh as well as all yard and air-dried 

 material there is contained a variable amount of water. The amount of water contained in fresh wood of these 

 pines forms more than half the weight of the fresh sapwood, and about one-fifth to one-fourth of the heartwood ; in 

 yard-dry wood it falls to about 12 to 18 per cent, while in wood kept in well-ventilated and especially in heated 

 rooms it is about 5 to 10 per cent, varying with size of piece, part of tree, species, temperature, and humidity of air. 

 Heated to 150° F. (6o J C.) the wood loses all but about 1| to 2 per cent of its moisture, and if the temperature is 

 raised to 175° F. there remains less than 1 per cent, the wood dried at 212° F. being assumed to be (though it is not 

 really) perfectly dry. Of course large pieces are in practice never left long enough exposed to become truly kiln-dry, 

 though in factories this state is often approached. 



As long as the water in the wood amounts to about 30 per cent or more of the dry weight of the wood there is 

 no shrinkage 1 (the water coming from the cell lumen) and the density or specific gravity changes simply in direct 



] In ordinary lumber and all large size material the exterior parts commonly dry so much sooner than the bulk 

 of the stick that checking often occurs, though the moisture per cent of the whole stick is still far above 30. 

 H. Doc. 181 23 



