Handbook of Railroad Construction; For the use of American engineers. / Containing the necessary rules, tables, and formulæ for the location, construction, equipment, and management of railroads, as built in the United States.

by George L. Vose

208. In crossing rivers or bays open to navigation, it is required from any companies building a bridge, to leave a free passage for shipping. This is done by making that part of the bridge over the channel movable; (a draw).

Fig. 86.

Draws may lift up, (being counterbalanced,) may slide back upon the fixed part of the bridge, or may turn on a pivot. Fig. 86 shows a draw much used at present, and answering every purpose. Each half of the movable part must be calculated as a small bridge. The rods c c c support the overhanging part of the draw while open. The whole revolves upon a centre pin and a set of rollers.

CENTRES.

209. Centres are temporary wooden frames, used in the construction of stone arches. Their duty is to hold the masonry, while it is unable to support itself.

For arches from five to fifteen feet span, a centre made of boards or planks, fig. 87, is all that is necessary. For longer spans, when the ground beneath the arch can be used, the form, fig. 88, answers well. When there is no support but the abutments or piers, something similar to fig. 89 must be adopted. This is the plan adopted by George Rennie, chief engineer at the Waterloo bridge over the Thames at London.

Fig. 87.