| Span. | Rise. | Chord. | Posts. | Braces. | Lattice. |
|---|---|---|---|---|---|
| 50 | 8 | 2—8 × 10 | 1—8 × 10 | 2—6 × 7 | 2 × 6 |
| 60 | 9 | 2—8 × 10 | 1—8 × 10 | 2—6 × 7 | 2 × 6 |
| 75 | 10 | 3—8 × 10 | 2—8 × 10 | 3—6 × 8 | 2 × 6 |
Fig. 100. 50 to 75 feet.
| Span. | Rise. | Chords. | Posts. | Braces. | Centre braces. | Rods. | |
|---|---|---|---|---|---|---|---|
| No. 1. | No. 2. | ||||||
| 50 | 8 | 2—8 × 10 | 1—8 × 10 | 2—6 × 7 | 5 × 5 | 1½ | 1 |
| 60 | 9 | 2—8 × 10 | 1—8 × 10 | 2—6 × 7 | 5 × 5 | 1⅝ | 1⅛ |
| 75 | 10 | 3—8 × 10 | 2—8 × 10 | 2—6 × 8 | 5 × 6 | 1¾ | 1¼ |
212. In dimensioning small bridges, like the above, in estimating the maximum load, more regard must be given to the weight of momentary loads than (as in large bridges) the weight per lineal foot, as the weight of the bridge itself, when under fifty or sixty feet span, is inconsiderable. The greatest load that can come upon a single post or rod, is that from the driving wheels of a locomotive. If the whole engine weighs forty tons, there will be ten tons on each pair of drivers, or five tons or 11,200 lbs. on each wheel; which, being applied over a length of ten feet only, may be considered as at a single point, and all parts must be able to bear such load. In large spans, where the weight is great, if the truss is strong enough to support the bridge and load, it will safely resist the effects of a sudden application of passing trains.
Note.—On the Static and Dynamic deflection of Bridges. Considerable variance of opinion exists as to the relative deflection of bridges, produced by stationary and by moving loads. Neither experiment nor theory has exactly settled the point.
Experiments upon the Elwell bridge, (Epsom and Croydon Railway, England).
| Velocity in feet per second. | Deflection in inches. |
|---|---|
| 0 | 0.215 |
| 25 | 0.215 |
| 31 | 0.230 |
| 32 | 0.225 |
| 54 | 0.245 |
| 75 | 0.235 |
The bridge being a cast-iron girder of forty-eight feet span, load thirty-nine tons.
Experiments on the Godstone bridge, (S. E. R. R. England).