TESTS OP WESTERN YELLOW PINE CAR SILLS, ETC. 11 



RATE OF GROWTH. 



The effect of the rate of growth on strength was studied in small, 

 clear pieces, for there are so many factors which influence the 

 strength of material in structural sizes that it is impossible to draw 

 any conclusions regarding the effect of rate of growth from the 

 results of tests on such timbers. 



Figure 2 shows the relation of the strength values obtained from 

 the various tests to the number of rings per inch. The small number 

 at each circle on the curve indicates the number of tests used to 

 obtain that average. It will be observed that the maximum strength 

 values are reached in pieces having a rate of growth approximately 

 20 rings to the inch. 



The modulus of rupture varies considerably with the rate of 

 growth, while the remaining factors are influenced to a less degree. 



The dry weight increases with the number of rings per inch until 

 it reaches its maximum at 26 rings per inch and remains about con- 

 stant thereafter. 



DRY WEIGHT. 



Without question, the dry weight of wood, all other things being 

 equal, is the best criterion of its strength. But in order to obtain 

 reliable and definite results it was deemed advisable to study the 

 effect of this factor in small, clear pieces only, as in the case of the 

 rate of growth. Figure 3, in which the results of the tests are given, 

 shows that the various strength factors increase with the dry weight 

 of the wood, the greatest increase being in the modulus of rupture. 



SEASONING. 



The results of moisture determinations made on the car sills and 

 joists tested are indicated in figure 4. The green car sills show an 

 average moisture content for the entire section of 39.2 per cent; the 

 green joists, an average moisture content of 35.7 per cent. In the 

 case of both car sills and joists the outer portion of the green beams 

 showed a smaller moisture content than the intermediate portions. 

 This variation was doubtless caused by slight surface seasoning 

 before testing. The air-seasoned car sills showed an average moisture 

 content for the whole section of 12.6 per cent and the joists 13.3 per 

 cent. The moisture content within the sections varied but slightly, 

 increasing from the minimum in the outer portion to the maximum in 

 the inner portion. The material tested was all air seasoned under 

 cover, the car sills for about 16 months and the joists for 11 months. 



EFFECT OF SEASONING UPON STRENGTH. 



Seasoning, as a rule, increases the strength of wood. Although 

 a lowering of the moisture content is accompanied by an increase 



