Fig. 62 C.
| Span. | Rise. | A B. | C E. | Rod b. | |
|---|---|---|---|---|---|
| 20 | 8 | 12 × 12 | (5 × | 8)—2 | 1¼ inches. |
| 25 | 10 | 12 × 15 | (5 × | 9)—2 | 1⅜ inches. |
| 30 | 12 | 12 × 18 | (5 × | 10)—2 | 1½ inches. |
| 35 | 13 | 12 × 20 | (5 × | 10)—2 | 1⅝ inches. |
| 40 | 14 | 14 × 21 | (5 × | 12)—2 | 1⅝ inches. |
| 45 | 15 | 14 × 22 | (6 × | 12)—2 | 1¾ inches. |
| 50 | 16 | 14 × 24 | (6 × | 12)—2 | 1¾ inches. |
The braces, (column 4,) being in pairs and blocked together. In spans exceeding twenty-five feet, the braces d f, and the rods f g, should never be omitted. The size of the rod g f, is found by considering A, d, f, as a small bridge.
175. In all light bridges, like the one under consideration, all parts should be fastened by bolts, to prevent springing by reaction. A bridge with but little inertia, or dead weight, tends to jump up when the engine has passed over it. Fastening takes the place of weight in a large span.
As soon as the rise admits, the points C, on each side of the bridge, should be connected to resist lateral motion. When the height is not enough for this, the same points may be joined to a floor beam extended out beyond the truss.
Though the dimensions are given for this plan up to fifty feet span, it is very seldom advisable to go beyond twenty-five or thirty feet; as frames consisting of a few long timbers are not so rigid, and free from vibration, as those made of a greater number of short pieces.
176. In extending this system one hundred or two hundred feet, we see at once that the pieces A c, B c, would become very long and would need to be made large and heavy. We should always so proportion any beam in a bridge that it is at once able to resist all of the several strains to which it may be exposed, without being unnecessarily large.
As to compression, the above system might be extended to almost any amount; but the braces would yield by flexure.
Instead of producing the braces A c, A′ c′, fig. 64, to their intersection, we stop at c and c′, insert c c′; to prevent the approach of these points, suspend the points B and B′ from c and c′, and commence again with the braces B D, B′ D; and so on as far as necessary.