TIMBER PHYSICS BEAMS AtfD COLUMNS. 



359 



SIZE OF TEST MATERIAL. 



The long-standing idea of engineers and other consumers to have wood tested more nearly in 

 the sizes used in ordinary practice led to the adoption of test sizes, generally varying from 3 by 3 

 inches to 4 by 4 inches. Besides this, special inquiries with different kinds of timber into the 

 relation of large and small tests were instituted to ascertain the correctness of the general dogma 

 which claimed that tests on small pieces could not be utilized, since such pieces for their very size 

 gave higher values of strength. This investigation involved full-size columns as well as beams, 

 and was continued throughout the entire period of the timber-physics work. It led to a number of 

 the most interesting and highly valuable results, as will appear from the following statements: 



Selected tests of columns and compression pieces from the same i7'ees compared. 









Hatio 



Small pieces 



Large 











Number 

 of tree. 



Length. 



I 



d 



(average of 

 whole tree). 



columns. 



Eelative value. 



Deflec- 

 tion. 



Failure. 







(a) 



(&) 



(a) 



(b) 













Pounds per 



Pounds per 













Feet 





sq.inch. 



sq. inch 







Jneh. 



i 



239 



12 



U 



6,700 



6 J 00 



100 



91 



0.7 



Sheared* 



240 



12 



14 



7,000 



0,9G0 



100 



99 



0.1 



Compression. 



241 



12 



15 



6, 900 



6,500 



100 



94 



0.7 



Bo. 



:*09 



12 



12 



6,800 



6,500 



100 



96 



4 



Bo. 



312 



32 



10 



6,100 



6,300 



100 



103 



0.4 



Bo. 



In these columns (nearly one-tenth of all longleaf pine columns tested) the strength was so nearly the same as that 

 of the short pieces that it appears as if flexure had hut little to do with the failure, the small differences being amply 

 accounted for hy a larger number of defects in the columns. Should this prove true in general for wooden columns 

 as ordinarily designed, the problem would become simply a study of the influence of defects and of proper inspection. 



The nature of the failures would also point in this direction: 



Of 86 columns 32 failed normally, i. e., in simple compression; 22 were crushed near the end; 14 failed at knots, 

 and 19 by shearing, the rupture usually beginning at or near the ends ; a small knot proved sufficient to cause a large 

 column, 20 times as long as its diameter, to fail at 14 inches from the end. 



The deflection in the average for all columns (12 to 20 feet long) was only about 1 inch for the maximum 

 load, when, to be sure, destruction had progressed for some time; at the elastic limit the deflection was only about 

 one-half as much. These results would seem to warrant the statement that for pine columns at least, in which the 

 ratio of height to least diameter does not exceed 1 in 20, none of the accepted column formulae are applicable, the 

 nature of the failure being mostly in simple compression, and depending more on specific defects than on the design 

 of the column. 



STRENGTH OF LARGE BEAMS AND COLUMNS. 



Owing to the fact that much wood testing has been done on small, select, and perfectly seasoned pieces, usually 

 from butt logs, the values thus obtained seemed to differ very markedly from the results on large timbers usually 

 very imperfectly seasoned, and it was claimed that tests on small sizes always furnished too high values, just as if 

 the differences were due to sizes alone. 



While, to be sure, a small piece may be so selected that defects are excluded, the grain straight and in the 

 most favorable position with regard to the load, the assumption of the difference in strength of small pieces from 

 that of large-sized sticks has never been made good experimentally. 



Since it appears desirable to compare the results from large beams and columns not only with the average 

 data obtained from the general test series on small 4 by 4 material, but also with the average strength of small pieces 

 cut from the same beams and columns, a special inquiry into the legitimacy of such a comparison was made. This 

 study involved over 100 separate tests, and proved the very important fact* that uninjured parts of broken beams 

 and columns do not suffer in the test. The large-sized beams varied from 4 by 4 to 8 by 16 inches. 



Tests of large and small oeams — Bending strength. 



Number or tests involved 



Longleaf 



Loblolly 



Shortleaf 



Small beams, 



general 

 test series. 



1,986 



Lbs. per sq. in. 



11, 300 



10, 000 



9,300 



Large beams. 



Total. 



127 



Lbs. per sq. in. 



11, 500 



10,800 



9,200 



Beams from 



which 



small beams 



were cut. 



57 



Small beams 



cut from 

 large beams. 



236 



Lbs. per sq. in. I Lbs. per sq. in 



9,800 



10, 300 



8,700 



10, 100 



10, 000 



8,700 



From the preceding table it would appear that large timbers, when symmetrically cut (i. e., with the center of 

 the log as center of the beam), develop as beams practically the same strength as the average of the small pieces that 

 may be cut from them, and sometimes even higher values ; tho explanation being that cut in this manner the extreme 

 fibers which are tested in a beam come to lie in that part of the tree which, as a rule, contains the strongest timber. 



