6 BULLETIN 497, U. S. DEPARTMENT OP AGRICULTURE. 



RELATION BETWEEN STRENGTH VALUES. 



The chart (fig. 1) showing the relation between strength values 

 was made by plotting along one vertical line the various strength 

 values for each green car sill and the corresponding air-seasoned sill. 

 The order of succession of the pieces was obtained by first plotting 

 the values for the modulus of rupture of the green sills, beginning 

 with the maximum and descending in order. The other values for 

 the same beams were then plotted on the same vertical lines. The 

 chart shows that, with one exception, the strengths of the air-dry 

 beams are in the same order as those of the green beams to which 

 they are matched. However, the air-seasoned material shows a 

 greater range in values than the green material. This is due, in 

 part at least, to the variable moisture content of the air-dry sills. 

 It will be observed that the pairs of pieces having the greater 

 modulus of rupture in the green condition showed the larger in- 

 crease in strength in air-seasoning. The other strength properties 

 of the air-seasoned material are somewhat more erratic than those 

 of the green. This also is partly due to variable moisture content. 



Table 11 is a summary of the averages of all the preceding tables. 

 It gives, in condensed form, the corresponding values of green and 

 air-seasoned material of each size tested, and also the percentage of 

 increase of those values in seasoning. 



RELATION BETWEEN PHYSICAL CHARACTERISTICS AND 

 MECHANICAL PROPERTIES. 



DEFECTS. 



The natural defects found in timber have an important influence 

 on its strength values. In order to secure a more specific knowledge 

 of this influence the timber was graded before being tested. 



Table 6.— Results of 'bending tests of small clear sticks taken from western 

 yellow pine joists, 2-inch by 2-inch by 30-inch beams, 28-inch span. 



Seasoning. 



Number 



of 



tests. 



Rings 

 per 

 inch. 



Moisture. 



Weight per cubic 

 foot. 



Fiber 

 stress at 

 elastic 

 limit per 

 square 

 inch. 



Modulus 



of 

 rupture 



per 

 square 

 inch. 



Modulus 



of 

 elasticity 



As 

 tested. 



Oven 

 dry. 



per 

 square 

 inch. 



Green: 



Average 



91 

 1 



1 



58 

 1 

 1 



16.5 



41.0 



5.5 



17.1 

 27.0 



8.0 



Per cent. 

 33.4 

 133.7 

 19.0 



11.7 



• 13. 5 



10.6 



Pounds. 

 35.8 

 58.3 

 26.6 



30.9 

 38.3 



24.8 



Pounds. 

 26.9 

 40.8 

 21.0 



27.6 

 34.2 

 22.1 



Pounds. 

 3,530 

 6,080 

 1,910 



6,622 

 10, 270 

 2,890 



Pounds. 

 5,560 

 8,530 

 3,630 



11,045 

 15,960 

 6,670 



Thou- 

 sands of 

 pounds. 

 1,149 



Maximum 



Minimum 



Air seasoned: 



1.765 

 '583 



1,611 



Maximum 



Minimum 



2,236 

 964 





