MECHANICAL PROPERTIES OF WOODS GROWN IN UNITED STATES. 13 



The values derived from this test are applicable to beams of any 

 size by the use of the formulas given on page 24, except for the de- 

 fects that occur in the larger sizes. 



In all cases it is best to use the results from tests of green material 

 in determining allowable working stresses in large timbers, since 

 defects are usually introduced in drying large timbers with the result 

 that often there is no increase of strength. However, timbers which 

 are always dry may be allowed a slightly higher stress than those 

 exposed to the weather or subject to moisture. 



FIBER STRESS AT ELASTIC LIMIT. 



Fiber stress at elastic limit (see definition, p. 21) is very important 

 in determining the proper working stresses for a beam. A beam 

 loaded to its elastic limit in static bending for a short time will 

 recover its form immediately upon removal of the load. If the 

 same load is allowed to remain, complete failure will ultimately 

 result. Consequently, the necessity of keeping working stresses 

 below the elastic limit is apparent. It is recommended, however, 

 that working stresses be calculated not from the elastic limit, but 

 from the modulus of rupture, and for the following reasons: There is 

 a personal element in determining the elastic limit; slight inaccuracies 

 in measurements of deflections often cause considerable error in 

 elastic limit values; defects in structural timbers may be such that, 

 in testing, certain portions are stressed to or beyond the elastic 

 limit without discovery; and there is an element of safety in the 

 differential of strength between the elastic limit and modulus of 

 rupture values. 



MODULUS OP 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. The assumptions on which 

 this formula are based hold only up to the elastic limit, hence modu- 

 lus of rupture is not a true fiber stress. It is, however, a universally 

 accepted term, and the values are quite comparable for various 

 species and sizes of timber. It is a definite quantity, and the per- 

 sonal factor does not enter to any great extent into obtaining 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. A green structural timber, if compara- 



