January 25, 1917 



HARDWOOD RECORD 



19 



any of these are present in a stick, its_atrengtb-4s-4iarbiB— tlT'lre ^leaving more room for the guns and gunners, thereby increasing 

 below normal, and in testing timbers, such defects are supposed to 

 be largely absent. 



Cross Breaking 



Wood resists forces applied in different ways and from different 

 directions. So great a load may be placed on top of a post that 

 the piece crushes or careens and snaps. This is common in mines 

 where pit props are crushed by the weight of the mine roof. Ex- 

 perienced miners can determine when props are about to be crushed 

 by the load upon them. A small piece of thick plank called a 

 "cap" rests on top of the prop, and the weight of the mine roof 

 is on the cap. When the load is growing too heavy, the mine boss 

 notices that the top of the prop is being forced into and through 

 the wooden cap. When that happens it is accepted as a danger 

 signal that the overloaded timbers are about to collapse. 



The mine prop is not the only piece of timber that must be 

 strong when the end 

 of the piece receives 

 the shock or the 

 load. Posts of build- 

 ings must sustain 

 similar stress, but 

 still more striking 

 instances are met 

 . with in wooden rail- 

 way cars where the 

 beams and sills re- 

 c e i V e tremendous 

 bumps and thrusts. 

 If they cannot sus- 

 tain the pressure 

 and shock, the car 

 crushes. Many 

 examples have dem- 

 onstrated, however, 

 that wooden cars, 



with their heavy oak or yellow pine frames, show enormous 

 strength, and many times they pass through wrecks without serious 

 damage, while steel cars next to them in the train were crushed 

 beyond repair. 



Often the severest strain on ship timbers is end pressure on the 

 beams, and when violent shocks and stress are to be met, the 

 wooden ship still holds a high place, in competition with steel ves- 

 sels. Explorers whose ships must ram their way through polar ice, 

 go in wooden vessels because trials of the most desperate sort have 

 shown wooden ships more dependable than steel when beset by 

 floes and icebergs. 



During the war of 1812 the tremendous superiority of American 

 war vessels over those of the enemy was attributed, by some Euro- 

 pean writers, to the use of locust posts and braces in American 

 ships. So strong was this timber that pieces of small sizes sufficed, 



ASH SILL PLIES OF FRANKLIN CAR 

 The Strength anil Stiffness of This Wood Peculiar ly Fits It for the High Class Service Which It Is 



Required to Give 



the rapidity and accuracy of the fire which speedily turned the 

 scale of victory. 



The strength of timber is not usually measured by weight or 

 shocks upon the end, as in the mine props, ships and cars above, 

 but by the load which a beam will sustaiii when horizontal with 

 the ends on supports. The process of measuring stress of that 

 kind is sometimes called "cross breaking." It is what the ordi- 

 nary person has in mind when he discusses the strength of wood, 

 but other tests may be made by twisting or cross-shearing. 



When engineers speak of cross breaking they use the term "mod- 

 ulus of rupture," which means, in a general way, the measure of 

 the load required to break a beam of stated size, and with end 

 supports a specific distance apart. Sargent gives the modulus of 

 rupture (cross-breaking strength) of white oak at 12,516 pounds 

 per square inch, and longleaf pine at 16,100 pounds. 



These figures have 

 a particular and 

 exact meaning to 

 engineers, but to 

 the layman they 

 mean little unless 

 explained, and the 

 explanation is pret- 

 ty technical. But a 

 simple translation 

 of the technical lan- 

 guage may be made 

 into plain English 

 and anybody can 

 understand it. 



As a means of 

 illustration, take a 

 piece of wood more 

 than a foot long and 

 2% inches square. 

 Place this little beam on supports exactly one foot apart, and hang 

 weights on the middle of the beam until it breaks. The weight 

 that breaks it corresponds nearly enough to what engineers call 

 modulus of rupture. A white oak stick of that size and placed in 

 that manner, will break when a weight of 12,516 pounds is hung 

 from its center — and that means that the modulus of rupture of 

 white oak is 12,516 pounds. When stated in that way the figures 

 have a meaning. When it is said that the modulus of rupture of 

 longleaf pine is 16,100, it means that a weight that heavy would 

 be just sufficient to break a longleaf pine stick of the dimensions 

 given above. 



Tests of Timber Strength 



Machines of various kinds have been used in determining the 

 strength of wood, and have been in use many years. Large num- 

 bers of beams and sticks have been broken to find out how strong 

 they were. As a part of the census of 1880, tests were made on 

 405 kinds of wood by Charles S. Sargent. The actual work was 



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A SLAM MAY BURST A BOX 



It Is Important That Manufacturers of Boxes and Crates Should Select 



Woods Strong Enough to Carry the Load to Its Destination 



/ 



A PAIR OF AEROPLANE PROPELLERS 



This Is Perhaps the Severest Test of Wood That Man Has Ever Been 



Called Upon to Make In the Whole History of the Human Race 



