used as a factor of reduction for applying- to the census report values 

 for the species upon which the Forestry Division has so far made no 



tests. 



The information for these species is very meager, but the values given 

 are considered safe, though probably not as economical as they might 

 be if more extensive tests had been made. 



As will be seen from an inspection of Table IV, the factor of safety 

 is not a constant quantity, but ranges from 2 to 5. 



In general, any composite structure should be of equal strength in 

 all of its parts. This does not mean that they should all have the same 

 factor of safety. 



This factor is a function of three things: 



(1) The importance of the piece in the structure. 



(2) The amount of ignorance as to the strength of the material. 



(3) The amount of ignorance as to the amount of the imposed load. 



In many cases the failure of one piece will not endanger the struc- 

 ture as a whole. In other cases, even if the whole structure is wrecked, 

 no serious calamity results. In these cases a small factor of safety may 



be used. 



The values given for the modulus of elasticity for all the species except 

 redwood and cedar will give, for the average condition,* a deflection 

 equal to about one two-hundredths of the span, which has been assumed 

 as the maximum allowable. This is about equal to a factor of safety of 

 5 on the total deflection at rupture. The exceptions to this are the 

 two species above mentioned, but these are not used for beams. 



The crushing strength across the grain in Table IV is based upon a 

 crushing of 3 per cent of the cross-sectional height of the piece. This 

 point may be compared to the elastic limit f in the cross-breaking tests. 

 While absolute failure does not occur at this point, yet it is a point 

 beyond which it is unsafe to go. The point of absolute failure in this 

 test is, more or less, an imaginary point, and the above percentage of 

 crushing has been selected as an arbitrary representative thereof. As 

 failure does not occur here, however, a factor of safety of 3 is deemed 

 necessary and sufficient for this kind of a load. 



* See page 13. t See Bulletin 8, page 23. 



