10 



latter values to be correct, we find that where a factor of safety of say 

 5 to 12 was supposed to exist, this quantity has a range of from 0.6 to 

 25. The former value occurs for the crushing strength across the grain 

 by which the bearing area on the cap of a trestle bent should be pro- 

 portioned. Less seems to be known about this quality of timber than 

 any of the others, though very little is known of any of them. Two 

 companies reported equal values for the crushing strength endwise and 

 across the grain. 



Most of the safe loads assumed by these companies for this latter 

 factor would, if actually employed, be dangerously unsafe. Fortu- 

 nately, perhaps, trestles have not been designed according to these or 

 any other safe loads, but according to standard sizes which have 

 proved themselves capable of doing the required duty — at least for 

 a time. But even these furnish an altogether insufficient factor of 

 safety in many cases, while in others it becomes five times as large 

 as necessaiw. No values whatever were given for the modulus of 

 elasticity. 



Table III. — Showing the great range in value of the factor of safety for the different por- 

 tions of timber trestles, according to the practice now prevailing. 



[Span=14feet; length post=12 feet; posts, caps, and sills, all 12 inches square. Li>ad=100-ton con- 

 solidation engine of the P. R. R. Maximum moment =98,000 foot-pounds on one rail; maximum 

 shear=36,000 pounds on one rail; maximum bent load=91,800 pounds on both rails.] 



Species. 



Longleaf pine . . 

 Shortleaf pine.. 



White pine 



Norway pine ... 

 Colorado pine . 



Douglas fir 



Redwood 



Cedar 



Cypress 



White oak 



Stringers, all 

 lOincheshigh. 



Factor of 



safety. 



In 

 cross 



break 

 ing. 



For 

 bear- 

 ing on 

 cap. 



1.92 

 2.30 

 2.32 

 1.82 

 2.55 

 1.70 



Caps. 



Factor of safety for bearing value under 

 stringej-s of — 



Long- 

 leaf 

 pine. 



1.66 



1.92 

 1.92 

 1.32 

 1.28 

 1. 01 

 1.50 

 1.03 

 •-'.21 

 1.07 

 3.58 



Short- 

 leaf 

 pine. 



2.28 

 2. 28 

 1.50 

 1.52 

 1 . 92 

 1.78 

 1.23 

 2. 07 

 1.28 

 4.20 



White 

 pine. 



3. 38 

 3.38 

 2.32 

 2.25 

 2.83 

 2.03 

 1.81 

 3. 91 

 1.89 

 0.30 



Nor- 

 way 

 pine. 



2.74 

 2.71 

 1.87 

 1 . 82 

 2. 30 

 2.13 

 1.40 

 :;. is 

 1.53 

 5.10 



Colo- 

 rado 

 pine. 



3.04 

 3.04 

 2.08 

 2. 02 



2. 55 

 2.37 

 1.03 



3. 54 

 1.70 

 5.00 



Doug- 

 las fir. 



2.25 



2. 25 

 1.54 

 1.50 

 1.89 

 1.75 

 1.21 

 2. 02 

 1.20 

 1. 20 



Cy- 

 press. 



2.97 

 2.97 

 2. 01 



1 . 98 

 2.49 



2. 32 

 1.59 

 3.40 

 1.00 

 5. 5 1 



Factor 



of 

 safety 



for 



bearing 



value 



under 



post. 



Factor 



of 

 safety 



for 

 posts. 



24.4 

 20. 5 

 17.1 

 is. 4 



15.3 

 21.4 

 15.8 

 17.1 

 10.4 

 19.5 



This table gives, for a set of conditions representative of the present 

 practice, the factors of safety obtaining for the posts and for the bear- 

 ing value of stringers on caps and caps on posts, using the safe loads 

 given in Table IV and a uniform height of stringer of 16 inches. 

 That is to say, the load assumed and the dimensions of caps and posts 

 and height of stringer are supposed to represent the average condi- 

 tions in practice. The necessary width of stringer, however, has been 

 determined by using the safe loads recommended in Table IV. While 

 in some cases this resulting width, from which the bearing area is com- 

 puted, will be too small to represent the average practice, in other 



