TIMBER PHYSICS BKAMS AXD COLUMNS. 



3f)9 



SIZE OF TEST MATERIAL. 



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

 the .sixes used in ordinary practice led to the adoption of test si/.cs, generally varying from .'{ by .". 

 inches to 4 by 4 inches. I'.esides this, special inquiries with ditl'erent 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 

 ;; -ive. 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 tents of column* and compresiion pieces from the same trees compared. 



In these columns (nearly one tenth of all longleaf pine columns tested) the strength was so nearlytliesamensth.it 

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

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

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



The nature of t.hn failures would also point in this direction: 



Of 8(1 columns UJ failed normally, i. e., in simple compression ; 22 were crushed near the end; 14 failed at knots, 

 and 111 by shearing, the rupture usually beginning at or near the ends; a small knot proved sufficient to cause a large 

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



Hie (letle 'tion in the average, for all columns (I'J 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 tlie 

 ratio of height to least diameter does not exceed 1 in 20, none of the accepted column formula! 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 OK I.AROK BEAMS AND COLUMNS. 



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

 from Imtt 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 sixes always furnished too high values, just as if 

 the differences were due to si/.cs 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 largo-si/.ed 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 scries 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 beams Bending strength. 



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

 the lug 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. 



