11 



cases it will be too large, so that it is thought to be fairly representa- 

 tive thereof. It is also assumed that the stringers are butting, each 

 having but 6 inches bearing at each end on the cap. 



A factor of safety of 3 (see discussion of Table IV) is necessary and 

 sufficient for the proportioning of areas subject to load across the 

 grain. 



From an inspection of Table III, under the head of "Stringers" — 

 the latter being correctly designed to resist the bending moment — 

 we see that stringers of none of these species would have sufficient 

 bearing area upon a 12 by 12 inch cap. Colorado pine comes nearest, 

 with a factor of safety of 2.55. The resistance of the cap under the 

 stringer depends upon the kind of stringer used. For example, with 

 a white-pine cap and Douglas-fir stringers, the cap has a factor of 

 safety of 1.54. The last column but one in the table gives the factor 

 of safety for the bearing value of cap on the post. 



There are only four species, i. e., longleaf and shortleaf pine, cedar, 

 and oak, that have sufficient strength in crushing across the grain to 

 enable the cap to resist the thrust of the post. Oak is the only species 

 that has sufficient strength to enable the cap to resist the pressure of 

 all kinds of stringers. 



There has been no such parsimon}^ exercised in the design of the 

 posts, however. The factors of safety here range from 15.8 to 24.4. 



Now, although the stringers in cross breaking have a factor of safety 

 of 5, and the posts have a factor of safety of 20, the structure as a 

 whole has a factor of safety of only 2, approximate^.* Since these 

 values are intended to represent only the average condition in prac- 

 tice, they are very unlikely to represent any actual condition, being 

 functions of such variable quantities as the proof load, length and 

 height of span, height of stringer, etc. 



They serve to show, however, the wholh r inefficient allowance that 

 has been made in the bearing area, and it is probable that in nine out 

 of ten cases no attempt has been made to proportion this surface 

 according to conditions in hand. 



RECOMMENDED PRACTICE. 



Since the strength of timber varies very greatl} 7 with the moisture 

 contents (see Bulletin 8 of the Forestry Division), the economical 

 designing of such structures will necessitate their being separated 

 into groups according to the maximum moisture contents in use. 



MOISTURE CLASSIFICATION. 



Class A (moisture contents, 18 percent). — Structures freely exposed 

 to the weather, such as railway trestles, uncovered bridges, etc. 



*This is based upon the assumption that it is dangerous to strain a cap or stringer 

 in crushing across the grain more than 3 per cent of the height. (8ee page 14. ) While 

 this amount of crushing is not a failure in the sense of collapse of the structure, yet 

 unless the piece is soon taken out and replaced, the structure as a whole will be in 

 danger. 



