+<i TECHNICAL PKOPERTIKS OF WOOD. 



case of cask-staves, and other curved pieces. Thoroughly dried 

 steamed wood, and also injected wood, has lost all tiexibility 

 and is very brittle. 



Section YIII. — Strength of Timber. 

 1. General Aeeonnt. 



By the strength of timber is meant the resistance it ofters to 

 the separation of the fibres of which it is composed. Separa- 

 tion may be due to tensile, crushing, twisting, shearing, and 

 bending actions. 



The ultimate strength of timber is measured by the force in 

 lbs. per square inch of section which must be exerted in order to 

 break it. 



In countries using the metric system the unit is a kilogram 

 per square centimeter, and since the atmospheric pressure on 

 one square centimeter is very nearly one kilogram, the breaking 

 force is frequently expressed in atmospheric pressures (at.). 



[The elastic strength, or load which may be borne without 

 causing permanent deformation, is about one half the ultimate 

 strength ; the working load to which it would be wise to subject 

 wood being only about one tenth of the ultimate strength. This 

 large factor of safety is advisable in order to provide for the 

 following contingencies : — 



Possibility of unsoundness in the timber ; 



Provision against deterioration ; 



Allowance for extraneous forces not taken into account in the 



calculation ; 

 To give a capability of resisting the extra transient effect due 

 to the abrupt application of a load. 

 The effect of the sudden application of a load in causing strains, 

 such as would approximately be due to a fast moving train 

 running on a bridge, is twice that due to the same load ap]»lied 

 gradually. 



A load repeatedly removed and reapplied is called a live load, 

 as distinguished from the so-called dead load due to the weight 

 of the structure itself, which is applied once for all. The 

 liability of the sudden application of a live load requires that 

 the factor of safety should be twice that for a dead load, and if 

 there is a possibility of the load being applied by impact, or 

 reversed in direction, the factor should be greater. — Tr.] 



