Fig. 72.
As a bridge composed of a curved rib when the span is large yields at D, C, and E, fig. 71, when the load is applied in the middle, the strength must of course be increased by increasing the depth of the rib; and not to make this too heavy, a framed or built beam should be used as in fig. 72. Here it must be remembered that the two ribs must be so framed as to resist both tension and compression; for when a load is placed at D, the lower rib will be extended at d, and compressed at c′, and e; while the upper one will be compressed at D, and extended at C and E.
OF THE ROAD-WAY.
196. The flooring of any system is about the same; consisting of transverse floor beams, placed either on the top or bottom chords, (according as the road-way is more or less elevated above the water-way,) which support longitudinal timbers, upon which are placed cross-ties. In some cases, two curves of diagonal plank have been placed across the floor beams, spiked at right angles to each other, by which the bridge is considerably stiffened laterally.
General dimensions for the floor may be thus:—
| Transverse timbers, 3 feet from centre to centre, | 8 × | 14 |
| Track strings, notched 2 inches to floor beams, | 12 × | 14 |
| Cross-ties placed one foot apart, (clear,) | 3 × | 6 |
Fig. 73.