28 



Even if the stringer crushed all this amount, it would still be able to 

 crush as much more before being in danger. This would not happen, 

 however. The corbels may be beveled toward the ends an eighth of an 

 inch if desired, but it is scarcely necessary. 



In the last design, by using four stringers 7i by 14i inches each, with 

 a 12-inch cap, the corbels are avoided, sufficient bearing area being 

 obtained without them. 



POSTS AND CAPS. 



The posts in these last two designs are much lighter than those in 

 the first, but, before weathering begins, have a factor of safety of about 

 9. Takiner off one-half inch from the cross-sectional dimension to allow 

 for weathering, after being in service for some years they still have a 

 factor of safety of 7£. The dimensions were obtained as follows: 



Assuming four posts in the bent, we have - ^r — (22,950 pounds), to be 



carried by each post, or 11.48 tons. These posts will be about 11 feet 8 



inches long and have a ratio of ^ = about 20, probably. From an 



inspection of Table IV we find that for this ratio a 7 by 7 inch longleaf- 



pine post will suffice. 



With an oak cap, the safe bearing value of which is 400 pounds per 



square inch, we will require an area of cap and sill at the end of each 



22 950 

 post of AL = 57.4 square inches. Adding to this the mortise area, 



we have 72.4 square inches which will require a post a trifle over 8.5 

 inches square. Then adding a half inch to each dimension to allow 

 for weathering, we have a 9 by 9 inch post, requiring caps and sills of 

 the same size. But with posts of this size w T e may use cypress or any 

 of the weak, cheap, but durable timbers. 



The designs in figs. 3 and 4, though capable of carrying twice as 

 much load as that shown in fig. 2, show a saving of $5 per span, equal 

 to 36 cents per linear foot of track, and 28 per cent less timber. 



Assuming that this would be representative of one-half the total 

 mileage of timber trestle bridges, i. e., 1,000 miles, we have a total 

 saving every nine years of $1,900,000, which is equal to an annual 

 expenditure of $211,000. This capitalized at 4 per cent gives a capital 

 of $5,275,000. These 1,000 miles of trestle use annually about 

 1^0,000,000 feet, B. M., of valuable timber, 35,000,000 feet of which 

 might readily be saved. 



The tables of cost accompanying these designs upon which the above 

 figures have been based are, of course, subject to great modification, 

 depending upon the location, condition of the market, etc. It is 

 thought, however, that they give a fair representation of the average 

 conditions. 



