TENSION. 



43 



Strength in cross-breaking of well-seasoned, select jrieces. 



(1) Robinia (locust), hard maple, hickory, oak, birch, best 



ash and elm, longleaf, shortleaf, and Cuban pines, 

 tamarack 



(2) Soft maple, cherry, ash, elm, walnut, inferior oak, and 



birch, best poplar, Norway, loblolly and pitch pines, 

 black and white spruce, hemlock and good cedar 



(3) Tulip, basawood, sycamore, butternut, poplars, white 



and other soft pines, firs, and cedars 



Strength of 

 the extreme 

 fiber 

 SWl 



*—% bd 2 



per square 



inch-. 



Pounds. 



13,000 



10,000 

 6,500 



Approximate weight 

 which breaks a stick — 



1 by 1 inch 



and 12 inches 



long. 



Pounds. 



720 



550 

 350 



2 by 2 inches 



and 10 feet 



long. 



Pounds. 



570 



440 

 280 



'^^^^^mmm^mm 



a 



TENSION AND COMPRESSION. 



When a piece of wood is pulled lengthwise, in the manner shown in 

 fig. 2G, part of the fibers are torn asunder or broken, but many are 

 merely pulled or shredded out from between their 

 neighbors. Since failure in tension thus involves 

 lateral adhesion as well as strength of fibers, it is 

 affected not only by the nature and dimensions of the 

 fibers but also by their arrangement. Owing to their 

 transverse position the medullary rays (a large part of 

 all woods) offer but one-tenth to one-twentieth as much 

 resistance as the main body of fibers and moreover 

 weaken the timber by disturbing the straight course of 

 the fibers and the regularity of the entire structure. 



The resistance is also 

 much affected by the 

 position of the grain. 

 The perfectly cross- 

 grained piece a (fig. 27) 

 sustains but about one-tenth to one- 

 twentieth of the load which is sup- 

 ported by the straight-grained piece c, 

 and it is evident that the piece b, 

 which represents the ordinary case of crossgrain, is likewise weakened 

 by the oblique position of the grain. 

 This explains the detrimental i nTu- 

 ence of a knot on the underside of a 

 board, as in fig. 28. Since the lower 

 side of the board, in bending, is 

 stretched, the upper side being com- 

 pressed, the fibers Of the lower Side FlG - 28— Effect of knots and their position. 



are subjected to tension and the wood of the knot, like the piece of 

 crossgrained wood, offers but little resistance. Commonly the defect 

 is greatly increased by a season check in the knot itself, so that the 

 knot affects the strength of the board like a saw cut of equal depth. 



Fig. 26.— Specimen 

 in tension test. 



Fig. 27.— Straight and cross grained wood. 



