The thickness of web is usually a little greater at the bottom than at the top, and varies from 1/14 to 1/24 of the depth of the girder. The bottom rib is usually made from six to eight times as wide as it is thick, and the top rib from three to six times as wide as thick, so that, in the example above given, we could have as dimensions for the parts
Top Flange, 4¼ X 1½ inches nearly.
Bottom Flange, 6 X 2½ inches nearly.
Web, 1½ inches thick.
The simplest bridge, consisting of a single stick, to span openings of 20 feet and under, is calculated according to the formula
Example.—The depth of a beam, of 12 feet span and 12 feet wide, to support a load of 22400 lbs. will be
= 15 in. nearly.
The following Table was calculated by the above rule—and the dimensions altered according to the actual practice of the writer.
| Span. | Breadth. | Depth. |
|---|---|---|
| 4 | 10 | 12 |
| 6 | 10 | 12 |
| 8 | 12 | 12 |
| 10 | 12 | 13 |
| 12 | 12 | 15 |
| 16 | 12 | 18 |
| 18 | 12 | 20 |
| 20 | 12 | 22 |
These dimensions will give ample strength and stiffness. Fig. 1, Plate I. gives an illustration of this kind of bridge—in which a, a, are the bolsters or wall plates, shown in section, to which the bridge beams are notched and bolted. Fig. 1, A, Plate I, shows the method of diagonally bracing these beams by planks, dimensions of which in general use are 6 to 8 by 2 to 3 inches. The track should rest on ties, about 6 inches by 8 or 10 inches—the same bolt confining the ends of the ties and diagonal braces when practicable. These ties should be notched on the string pieces 2 or 3 inches—without cutting the stringers. Below is a table giving general dimensions, in inches, of the several parts of a bridge of this description.