In this manner is formed the following table, giving the scantling of sticks for railroad stringer bridges, of twenty feet span and under.
| Span. | Breadth. | Depth. | |
|---|---|---|---|
| 5 | 12 | 12 | |
| 10 | 12 | 13 | |
| 12 | 12 | 15 | |
| 15 | 12 | 18 | |
| 18 | 12 | 20 | |
| 20 | 12 | 21 | inches. |
The first scantlings exceed the requirement of the rule, but are none too large to resist the shocks to which such sticks are exposed.
Cross-ties of plank, 2 or 3 by 6 or 8 inches, and plank braces underneath, (as shown in the fig. at the end of chapter VIII.,) should be bolted to the main timbers; the same bolt passing through the tie beam and plank. The longitudinal pieces should be firmly notched and bolted to the wall-plates, and these latter either built in or scribed on to the masonry.
Fig. 61.
172. For a span of from 20 to 50 feet, we may use the combination shown in fig. 61. The piece A B, must be so strong as not to yield between A and D, or D and B. The pieces C E must be stiff enough to resist the load coming upon them which is as follows. A locomotive engine of the heaviest class will not exceed fifty tons weight, each pair of driving wheels will support ten tons, and on each side five tons, 2240 × 5 = 11,200 lbs.; or to allow for shocks and extra strains, 15,000 lbs. Each brace, then, must support seven thousand five hundred pounds, which for compression simply would require only seven and one half square inches of sectional area; but the brace being inclined, the strain is increased as follows:—
A E to E C as 7,500 to X.
And A E being ten feet, and A D fifteen feet, E C becomes eighteen feet, whence
10 to 18 as 7,500 to 13,500 lbs.;