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. 



