44 



TIMBER. 



Tested in compression endwise (fig. 29), the fibers act as so many 



hollow columns firmly grown together, and when the load becomes too 

 great the piece fails in the manner illustrated in fig. 

 31. This failure is a very complex phenomenon ; in 

 wood like pine the fibers of the plain in which failure 

 occurs become separated into small bodies; they tear 

 apart aud cease to behave as one solid body but act 

 as a large number of very small independent pieces. 

 Like the strands of a rope these small bodies offer 

 but little resistance to compression; they bend over, 

 and the piece "buckles." 



It is evident that a vertical position and a regular 

 arrangement of the fibers increase the resistance 



and that therefore the medullary 



rays and oblique position of fibers 



in crossgrained and knotty timber 



tend to reduce the strength in com- 

 pression. 



From the following table of strength 



in tension and compression it will be 



seen that these two are not always 



proportional, the stiffer conifers excelling in the latter, the tougher 



hard woods in the former: 



Fig. 29.— Compression 

 endwise. 



Fig. 30.— Longitudinal shearing. 



Ratio of strength in tension and compression, showing the difference between rigid conifers 



and lough hard woods. 



Hickory 



Elm 



Larch 



Lougleaf pine 



i: 



Ratio : 



Tensile 



strength. 



compressive 

 strength. 



3.7 

 3.8 

 2.3 

 2.2 



A stick 1 square inch 



in cross section. 

 Weight required to— 



Pull apart. 



Pounds. 

 32, 000 

 29, 000 

 19, 400 

 17, 300 



Crush 

 endwise. 



Pounds. 

 8,500 

 7,500 

 8,600 

 7,400 



Strength in compression of common American tvoods in well-seasoned select pieces. 



[Approximate weight per square inch of cross section requisite to crush a piece of wood endwise.] 



Pounds. 



(1) Black locust, yellow and cherry birch, hard maple, best hickory, longleaf 



and Cuban pines, and tamarack 9, 000+ 



(2) Common hickory, oak, birch, soft maple, walnut, good elm, best ash, 



shortleaf and loblolly pines, western hemlock, and Douglas fir 7, 000+ 



(3) Ash, sycamore, beech, inferior oak, Pacific white cedar, canoe cedar, Law- 



son's cypress, common red cedar, cypress, Norway and superior 

 spruces, and fir 6, 000+ 



(4) Tulip, basswood, butternut, chestnut, good poplar, white and other com- 



mon soft pines, hemlock, spruce, and fir 5, 000+ 



(5) Soft poplar, white cedar, and some western soft pines, and firs 4, 000+ 



