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.
Centres are strained in a different manner as the arch progresses; first at the haunches, and last at the crown. Excess of weight at any point causes a settling at such, and a rise takes place at some other place. By loading the arch temporarily, such motions are checked.
These frames are placed vertically upon the pieces F F, which being connected with the braces D D by the folding wedges _c c_, admit of adjustment of the height of the centre. The distance between the ribbed frames depends upon the form of the arch, and the span, or upon the weight to be supported; varying from one to four feet. The centres are covered with a course of narrow plank, placed parallel with the axis of the arch, upon which the voussoirs rest.
210. The method of putting a bridge upon the masonry is shown in figs. 90, and 91; the former when the road-way is upon the upper, and the latter when upon the lower chord.
211. In figs. 92 to 100, are given several plans for spans from five to seventy-five feet. Fig. 92, shows the simple beam braced beneath with diagonal plank; the bolts passing through the ties, stringers, and braces. The stringers are bolted to the wall plates, and when the bridge is upon a curve notched also, by cutting the bolster. Fig. 92 A shows the plan. This form answers for openings from five to twenty feet. From fifteen to thirty feet, we may use figs. 94, 95, 96, and 97. From twenty-five, to fifty and sixty feet, figs. 93, 97, and 98. And from fifty to seventy-five feet, figs. 99 and 100.
The following tables give reliable dimensions for bridges upon the above plans.
Span. Bolsters. Stringers. Ties. Braces. Bolts. 5 12 × 12 12 × 12 6 × 10 2 × 10 1 inch 10 12 × 12 12 × 13 6 × 10 2 × 10 1 inch 15 14 × 14 12 × 18 6 × 10 2 × 10 1 inch 20 14 × 14 12 × 24 6 × 10 2 × 10 1 inch