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.

Span. Bolsters. Stringers. Ties. Braces. Diameter of Bolts.

4 12 x 12 10 x 12 6 x 8 2 x 8 1 inch.
10 12 x 12 12 x 13 6 x 8 2 x 8 1 "
16 14 x 14 12 x 18 6 x 8 2 x 8 1 "
20 14 x 14 12 x 22 6 x 8 2 x 8 1 "


Each bolt must have a washer under the head, and also under the nut.