78 PRACTICAL STRUCTURAL DESIGN 



Wood is composed of actual horizontal fibers, instead of the 

 imaginary fibers, or horizontal planes, considered in analyzing 

 shear in beams of homogeneous material. If wood were equally 

 strong in all directions, a shearing failure in this material would 

 also be indicated by the appearance of diagonal cracks. 



Shear in Wooden Beams 



In wooden beams the dangerous shear acts along the neutral 

 plane and the beam may split, thus by shearing action being con- 

 verted into two shallow beams, which will then break by bending, 

 for the upper half must carry the whole load and the lower half 

 carries the whole load when the upper half is destroyed. The 

 strength in shear of wooden beams should be tested by the follow- 

 ing formula. If the distributed load found by this formula is 

 smaller than that found by the bending formula, increase the size 



of the beam. 



, TT 4/ifrs 

 W = 57 



o 



in which W = the load the beam will carry without failing in shear. 

 b = breadth in inches, 

 h = height in inches, 

 s = shearing stress per square inch, usually one-tenth 



the maximum fiber stress in bending. 

 The above formula is derived as follows: 



W 

 TF V^ _2_ W JL_3TF 



= 2 a ~ jhb " |/i6 " 2 X 2hb ~ 4hb 



and, therefore W = 5 



o 



Modulus of Rupture 



The modulus of rupture is a measure which represents a com- 

 bination of all the forces that tend to break a beam ; i.e., the com- 

 bined action of tension compression, shear, and crippling. It was 

 formerly used in beam design to obtain the breaking load, which 

 was divided by some factor of safety to determine the safe load. 

 To-day it is used only for materials in which it is difficult to sepa- 

 rate the different stresses, as, for example, clay, stone, and plain 

 concrete. The moment of resistance, using an allowable safe 



