[bovey] presidential ADDRESS 23 



Since the kiln-drying diminishes the tensile and sheai'ing strengths 

 and greatly increases the compressive strength, an absolutely dry beam, 

 under a transverse load, will fail either by a tensile fracture or by longi- 

 tudinal shear. Failure on the compression side rarely takes place ; onl}-, 

 in fact, when the compressive strength of the wood is very low in com- 

 parison with the tensile, and when the pi'oportion of hard and soft fibre 

 is uniformly distributed over the upper and lower halves of the section, 

 may initial failure by crippling be expected. 



The question of how far in practice it is desirable to eliminate the 

 moisture depends on obtaining the best balance between the effect on the 

 tensile, shearing and compressive strengths. It should be remembered 

 that drying causes seasoned cracks to become more pronounced, and as 

 frequently the^e extend to a great depth and run the whole length of 

 the beam, it often happens that only a slight layer is left to hold the 

 beam together. Thus a beam, with such defects, though otherwise sound 

 and clear, offers very little resistance to longitudinal ohear, and the beam, 

 in fact, should be regarded as made up of two or more superposed beams. 

 Beams should always be placed so that the relative strengths in tension 

 and compression may be exerted to the best advantage. 



The elimination of the moisture naturally makes the beam stiffer 

 and gives it a higher coefficient of elasticity. The deflection is directly 

 proportional to the load so long as the latter is less than about 60 percent, 

 of the ultimate strength. 



NV hen the saturation is greater than that at the normal condition of 

 the timber, that is, the condition corresponding to the saturation of the 

 atmosphere, it will be generally found that the greater amount of 

 moisture is detrimental to the transverse strength because the strength on 

 the comjjression side of the stick is being rapidly reduced almost to the 

 point of crippling and this weakening is not proportionately balanced by 

 the increase of strength on the tension side. The deflection under a given 

 load is greater and more continuous, and failure results from a less amount 

 of load than would be the case were the timber in its normal condition. 

 Timber of average good quality, under such conditions, would fail natur- 

 ally by compression until the surface of resistance under compression 

 balances and so overcomes the tensile strength of the remainder of the 

 section, after which failure may occur under tension. 



A precisely similar beam containing a normal amount of moisture 

 corresponding to the saturation of the atmosphere, will give much higher 

 results, the coefficients of transverse elasticity and ultimate strength 

 being both much greater. This increase in sti-ength is due to the greater 

 resistance to failure on the compression side produced by the drying. 



in order to determine the effect of the moisture upon timbers of large 

 scantling, a Douglas Fir beam 16 feet 4 inches X 15 inches X 6 inches, was 

 placed on supports 15 feet 6 inches apart and loaded with 1,000 lbs. at 



