20 



HARDWOOD RECORD 



January 25, 1917 



done by C. P. Sharpies. He handled 12,961 samples in connection 

 with the tests. The less important woods were dismissed with a 

 few, perhaps only one or two tests each; but important woods were 

 tried out more carefully, in some instances from twenty to forty 

 sticks being put through the machine. 



These tests proved what was already pretty well known, that 

 wood varies greatly in strength. That holds true quite naturally 

 between species of different kinds, as between white pine and ma- 

 hogany; but it holds also between sticks of the same species, and 

 between different parts of the same tree._ The thirtj-eight pieces 

 of white oak tested by Sargent showed a differnce of 180 per cent 

 between the weakest and strongest. The variations between sam- 

 ples of other woods are no less remarkable. The final figures pub- 

 lished to show the strength of woods are only averages made up of 

 the weakest, the strongest, and all between, of each kind of wood. 

 Tests have been made by many persons to determine how strong 

 wood is. Sargent 's tables include more American woods than those 

 by any other authority. Some engineers have questioned their cor- 

 rectness, and in some instances companies or societies have made 

 tests for their own use, of the few kinds of timber in which tlicy 

 are specially interested. 



There is no such thing as figures showing absolutely the strength 

 of any wood. Figures true in one ease are not true in another. 

 There is nothing better than averages to depend on. The builder 

 of a steel bridge or house frame knows almost to a pound the load , 

 tlie steel will carry; but sueli precise knowledge can not be had 

 when wood is used. And yet the remarkable fact stands out that 

 there are fewer collapses in wooden structures than in those of otlier 

 materials. That fact is known in Europe as well as in America. 

 It is not because wood is stronger than the other building materials; 

 some are much stronger thau wood. It is because architects, 

 knowing the wide variations in the strength of woods, always 

 figure within the margin of safety. They make allowances in 

 order to be sure the timbers will carry the loads allotted to them; 

 and the result of long experience, and in many countries, has dem- 

 onstrated that the safest structures have been those made of wood. 

 Baterden, the English timber engineer, writing on this subject, said: 

 "It is because of this great variation in tests that in the calcu- 

 lation of stresses for timber structures it has been usual to allow a 

 much larger margin of safety than in the case of steel structures, 

 and probably this is why we have had a singular immunity from 

 failures in timber structures, the margin of safety allowed being 

 riiueh greater than was really necessary." 



Tlie need of uniform and reliable data on the strength and otiicr 

 physical properties of the commercial woods of this country is 

 keenly felt by builders and other wood users. What is wanted is a 

 set of figures that are as accurate as circumstances will allow, and 

 which the ordinary user can understand if he will do a little think- 

 ing on the subject. Sargent's tables, already referred to, are a 

 monument of industry and research, but most of Sargent's figures 

 '-e intelligible to engineers only; and the correctness of the fig- 

 ures has been questioned in many instances. For several years the 

 United States Forest Service has been compiling wholly new sets of 

 figures showing the characters and qualities of wood. There is 

 ample reason to believe that these figures will be complete and ac- 

 curate; the only cause for impatience being the long delay in 

 making them public. However, it is a gigantic task to collect and 

 test samples of wood from all parts of the United States, and there 

 must be some delay in ordor to attain completeness and accuracy. 

 Such figures, when tliej' come, will fill a long-felt want. 



Eatio of Weight to Strength 

 The weight of any wood per cubic foot is as well known as is its 

 modulus of rupture per square inch. If a ratio exists it should not 

 be difficult to show it. White oak weighs 46.35 pounds, and its 

 lireaking strength is 12,516 pounds. It thus appears that for every 

 pound weight per cubic foot, this wood has a breaking strength of 

 270 pounds. The ratio of weight to strength for all otlier commer- 

 cial woods may be calculated in the same way, by dividing tlie 

 modulus of rupture figures by weight figures. This has been done 

 for a number of woods in the following talilc, and it is clcnrlv 



broiight out that a wood 's weight 

 terion of its strength. The valu 

 Sargent's tables: 



is not in all cases a reliable cri- 

 es below were calculated from 



Species 



Strength 



in pounds 



per one 



pound weight 



Common cottonwood 443 



I^ongleaf pine 392 



Sweet birch 380 



Beech 374 



Basswood 371 



Arborvita; 364 



California big tree 359 



Yellow poplar 349 



Wliite ash 843 



Sitka spruce 336 



Shellbark hickory 325 



Black spruce 314 



Mahogany 309 



Strength 

 in pounds 

 per one 

 Species pound weight 



Black walnut 309 



Black locust 309 



White elm 295 



White oak.. ., 270 



Sugar maple 270 



Persimmon 250 



Osage orange 238 



Mangrove 233 



Black willow 214 



Red gum 208 



Lignum viata; 155 



Ironwood (Cyrilla raceniifliprn I 104 



Use Influenced by Strength 

 The relation of weight and strength is not always considered by 

 users of wood. Strength is thought of in practice oftener than 

 weight. That holds particularly true when large timbers are used; 

 but in many situations where small pieces are employed, both 

 weight and strength are given consideration. A few pounds saved 

 in making a vehicle is economy if strength is not sacrificed. There 

 is not much choice between maple, hickory and ash for wagon axles, 

 so far as strength goes; but an axle of black willow or red gum 

 would need to be fifty per cent heavier, and of course, much larger, 

 to have the same strength as hickory. Nearly all woods used in 

 vehicle construction are considered from the standpoint of both 

 weight and strength. Even the boards of which farm wagon bodies 

 are made are of very strong woods, compared with their weight. 

 Cottonwood and yellow poplar rank high in that respect. Weight 

 for woiglit they are stronger than oak. 



Automobiles that use wooden frames demand material that is 

 strong, tough, stiff, and of comparatively light weight. That is 

 a difficult combination of qualities to be possessed by a single wood, 

 yet a few have them. Ash meets the requirements well; but it is 

 not the only wood possessing great strength in proportion to 

 weight. Others that might compete more or less successfully with 

 it are yellow birch, sugar maple, hickory, and black 'locust. 



The builders of skiffs and other light boats want light, strong 

 woods. Not a superfluous pound is tolerated. One of the best, 

 when extreme lightness is wanted, is arborvitae. Pound for pound, 

 it is stronger than hickory. Canoe makers have always liked it. 

 Before the discovery of America the Indians were using this wood 

 for frames of their bark canoes, because it made the lightest frames 

 I)ossiblc, yet strong enough to stand punisliment in rapids and whirl- 

 pools of rivers and in rougii land portages where the canoes were 

 carried. What the red men knew they had learned by experience. 

 Though the makers of light boats and light vehicles scrutinize 

 their woods to make sure that not an unnecessary pound gets in, 

 yet the manufacturer of aeroplanes is much more exacting. This 

 machine is a late invention, but the builders have ransacked the 

 whole w^orld for woods of lightest weight, with greatest strength, 

 for if the boatmaker rejects superfluous pounds, the airshij) manu- 

 facturer reduces weight to the last ounce. IJe must, however, 

 take into account stiffness as well as strength, and that shortens 

 the list of woods from which to choose. 



Frames of aeroplanes are made largely of Sitka spruce of the 

 Pacific Coast and of red spruce of West Virginia. No wood has 

 been found superior to these for frames; but the propellers are of 

 mahogany, walnut, and spruce, generally of built-up stock, glued 

 together. Formerly propellers were solid spruce, but since the 

 beginning of the European war, walnut and mahogany have become 

 tlie principal propeller woods. They are very strong for their 

 weight, and they possess the additional advantage that if struck 

 by bullets the wood will not split and splinter. Tlie bullets simply 

 cut small holes in passing through and the propeller is not wrecked 

 by splitting, as might result if spruce were used. The air pressure 

 on a propeller blade revolving 600 times a minute and moving 

 forward SO miles an hour may exceed 100 pound per square loot. 

 Wood for Packing Boxe.s and Crates 

 Makers of packing boxes and crates in the United States con- 



