The Wood for the Job 



617 



THE STRENGTH PROPERTIES of WOod 



that most concern the woodworker in- 

 clude bending strength, compression 

 strength, stiffness, and toughness. 



Bending strength is a measure of the 

 load-carrying capacity of the members 

 that are ordinarily used in a horizontal 

 position and rest on supports. 



High bending strength is required in 

 barn rafters, girders, stringers, wagon 

 tongues, and scaffold platforms. If the 

 only available wood is low in bending 

 strength compared with better-suited 

 species, the deficiency can be overcome 

 by increasing the size of the member 

 used. An increase of 10 percent in the 

 height of a beam increases its bending 

 strength by 21 percent. Both the vol- 

 ume and bending strength of a beam, 

 however, increase in direct proportion 

 as the width is increased. Woods high 

 in bending strength for farm and home 

 building include ash, beech, yellow 

 birch, cherry, Douglas-fir, rock elm, 

 hickory, the western larch, locust, hard 

 maple, oak, southern yellow pine, and 

 walnut. 



Compression strength of wood is the 

 measure of its ability to resist a load 

 applied in such a direction that it tends 

 to crush the member, as in a post or 

 column. Good compression strength is 

 essential for members used to support 

 houses, garages, barns, storage bins, 

 and the like, because they hold up a 

 load. It is not important in such items 

 as fence posts. 



Low compression strength can be 

 compensated for in some instances by 

 the use of proportionately larger mem- 

 bers. In the construction of small 

 buildings, then, the size requirements 

 of posts where the length is less than 1 1 

 times the smallest dimension are de- 

 termined by bearing area, stiffness, and 

 stability rather than by actual com- 

 pression strength. Because these re- 

 quirements necessitate the use of posts 

 large enough to carry greater actual 

 compressive loads than are ever placed 

 upon them, no particular considera- 

 tion need be given to the compression 

 strength endwise in selecting a wood 

 for small houses. Where exceptionally 



heavy loads are involved, as in sup- 

 ports for bins or root cellars, the com- 

 pression strength of the members 

 should be considered. If the length is 

 greater than 11 times the smallest di- 

 mension, the stiffness of the member 

 becomes the controlling factor, and the 

 compression strength can be disre- 

 garded. Of the woods used in farm 

 and home building, those high in com- 

 pression strength include white ash, 

 eastern redcedar, cherry, Douglas-fir, 

 hickory, western larch, locust, hard 

 maple, southern yellow pine, redwood, 

 and walnut. 



Stiffness is a measure of the resist- 

 ance to bending or deflection under a 

 load. It assumes importance in floor 

 joists of houses and in studding, where 

 it is more important than the actual 

 breaking strength. Lack of stiffness in 

 these members will result in plaster 

 cracks in ceilings and vibration of 

 floors. Stiffness is important also in 

 shelving, ladder rails, beams, ax han- 

 dles, and long, slender columns. Con- 

 struction practices can compensate for 

 the lack of stiffness, on the one hand, 

 or nullify the advantages of using wood 

 with high stiffness on the other. In- 

 creasing the size of a member will in- 

 crease its stiffness, but the use of wood 

 that is not fully dry at the time of in- 

 stallation will result in a loss in stiffness 

 of the structure as a whole, because the 

 wood, as it dries, may shrink or split, 

 so that the fastenings, bracing, and 

 bridging will not hold so well. Woods 

 high in comparative stiffness that are 

 used in farm and home building in- 

 clude white ash, beech, yellow birch, 

 cherry, Douglas-fir, rock elm, western 

 hemlock, hickory, western larch, locust, 

 hard maple, oak, southern yellow pine, 

 the Sitka spruce, and walnut. Defects, 

 such as knots, checks, and shakes have 

 little effect upon stiffness. In light 

 building construction, therefore, mate- 

 rial of the sound, though knotty, 

 grades may be used to good advantage 

 for joists and studs because stiffness is 

 more important than breaking strength 

 in those items. 



Toughness is a measure of the ca- 



