510 The Philippine Journal of Science 1924 
In large beams the weight should be taken into account in 
calculating the fiber stress. Three-fourths of the weight is 
added to the load for this reason. 
GLOSSARY ” 
Elastic limit—The elastic limit is that point where the 
distortion ceases to be in proportion to the load. A timber 
stressed beyond the elastic limit will not resume its original 
form immediately upon the removal of the load. 
Elasticity.—Elasticity is the property of changing form with 
the application of force and recovering at once upon release 
from the force. In any elastic material the amount of com- 
pression or deformation is proportional to the force applied. 
Fiber stress at elastic limit—Fiber stress at elastic limit is 
the stress obtained in a timber by loading it to its elastic limit. 
It is the greatest stress the timber will take under a given 
loading and immediately return to its former position. 
Mechanical properties.—Mechanical properties are the prop- 
erties of wood which enable it to resist deformations, loads, 
shocks, or forces. Thus the ability to resist shearing forces is 
a mechanical property of timber. 
Modulus of elasticity.—Modulus of elasticity is the ratio of 
stress per unit area to corresponding strain per unit length, the 
distortion or strain being within the elastic limit. It is the 
measure of the stiffness or rigidity of a substance. 
Modulus of rupture—Modulus of rupture is the computed 
fiber stress in the outermost fibers of a beam at the maximum 
load and is a measure of the ability of a beam to support a 
slowly applied load for a very short time. The formula by 
which modulus of rupture is computed is the same as that for 
fiber stress at elastic limit, the maximum load being substituted 
for the elastic limit load. It is a definite quantity, and the 
personal factor does not enter to any great extent into obtain- 
ing it. It is consequently not so subject to error as the fiber 
stress at elastic limit, and for that reason is used more than 
any other value to represent the strength of wood. Modulus 
of rupture should always be considered in calculating the 
strength of beams to be used as stringers, floor joists, etc. 
* Mechanical Properties of Woods Grown in the United States, Bull. 
U. S. Dept. Agr. 556 (1917) 20-22. 
