14 BULLETIN 556,, U. S. DEPARTMENT OF AGRICULTURE. 
tively free from defects, can be expected to have a modulus of 
rupture about three-fourths as large as that of small clear pieces cut 
from it. 
The modulus of rupture of small clear individual pieces will occa- 
sionally vary more than 40 per cent above or below the average 
modulus of rupture. Pieces giving very low values are almost 
invariably lacking in density, while very strong pieces are excep- 
tionally dense. 
Figures on the variation of modulus of rupture are given in 
Table 3. 
Safe working stresses for carefully selected structural timbers, 
with all exceptionally light pieces excluded, subjected to bending 
in dry interior construction and where only small deflections are 
allowable are about one-fifth the modulus of rupture values given 
in the table for green material. (Table 1.) In some interior con- 
struction where beams may be allowed to sag somewhat without 
damage, the working stresses may be slightly increased. But for 
timbers used in bridges or other structures exposed to moisture, the 
working stress should be slightly lower. However, beams can not 
be correctly designed on the basis of outer fiber stress in bending 
alone. Strength in longitudinal shear must also be taken into 
account. (See p. 17 for allowable shearing stress.) 
MODULUS OF ELASTICITY. 
The modulus of elasticity is a measure of the stiffness or rigidity 
of a material. In the case of a beam modulus of elasticity is a 
measure of its resistance to deflection. The formula (see p. 24) 
connecting modulus of elasticity, load, and deflection shows that 
the deflection under a given load varies inversely as the modulus 
of elasticity; that is, a beam with a high modulus deflects but little. 
Modulus of elasticity is of value in computing the deflections of 
joists, beams, stringers, etc., and in computing safe loads for columns. 
The values given are derived from the static bending test, but are 
applicable to both beams and columns. 
In building construction the means by which the various members 
are held in place, inequalities in workmanship on the various parts, 
differences in the quality of the timber in all parts of the structure, 
and shrinkage due to the adjustment of the moisture content of 
the various members to that of their surroundings give rise to 
unequalized local stresses, often very large. When these stresses 
become equalized through the gradual readjustment of the mem- 
bers, deflections greater than those calculated from the average 
moduli of elasticity will be found. For this reason it is good prac- 
tice in the design of structures to use values for moduli of elasticity 
about one-half those given in Table 1. 
