PRACTICE OF BRIDGE BUILDING.
WITH regard to the practice of bridge building, the chief With regard to the practice of bridge building, the chief objects may be arranged as follows : I. The situation is generally determined by local cir cumstances, in a town by streets, and in the country by the roads adjacent. If the bridge is of a great size, or the foundations difficult, it is frequently advisable to choose the most favourable situation for the bridge, even at the expence of changing the approaches. It is abso lutely necessary that the access be commodious, but where practicable, from the approaches being in a curve, the general outlines of a bridge are seen to most advantage. The situation should be chosen where the river runs in a straight course for a considerable way above it, and where the channel rather contracts at some little distance below. The direction of the bridge should form a right angle with that of the river above it. Rock should, if possible, be obtained for the founda tions of the abutments and piers ; next to rock, hard clay, or clay mixed with gravel, or otherwise firm gravel ; but loose gravel, mud, or quicksands, are to be avoided as much as possible.
2. When the situation has been determined, a map or plan should be made of the channel of the river and adjacent banks, also of the streets or roads which are to be connected with each end of the bridge. There should likewise be a section, sheaving the breadth of the bed of the river, and.the form of the banks on each side. Upon this section should be marked the lines of high and low water, to be determined by the marks of the greatest floods, and the best information which can be procured from the oldest inhabitants, and most observ ing people in the neighbourhood. The consistence of the bed should also be ascertained, by boring with pro perly constructed augers, especially in the scite of the abutments, piers, and wing-walls. There should like wise be a longitudinal section, shewiug the declivity of the bed of the river, for at least 200 yards above and below the proposed situation.
When these steps have been taken, and the nature of the materials ascertained, the engineer has obtained sufficient data to enable him to make a suitable design. We shall at present confine ourselves to bridges con structed with stone.
leading objects in forming a design are, 1st, The passage for the water under the bridge. 2d, The making a perfect roadway over it : And, 3d, The decorations.
1. The number and dimensions of the arches must depend on the breadth of the river, the nature of the foundations, the height of the banks, and the quality of the materials to be used. If the foundations are good, the banks high, the stone hard, and to be procured of proper dimensions, large arches will be found advisa ble ; if the reverse is the case, the size of the arches must be diminished, and their number increased ; if more arches than one are required, their number should be odd, in order that there may be an arch in the middle of the river : the middle arch should be the largest, and those on each side should diminish, so that their spring ing remaining the same, their tops may form a declivity of about 1 in 24. When piers are placed in a river, the breadth between the abutments should be made greater than the natural breadth of the channel at that place, by about double the thickness of all the piers. Where the bed of the river is soft or loose, the increas ed width must be considerably more. In proportioning the thickness of the piers to the span of the arches, great latitude has been taken, as will appear from the following statement From this statement. it will be perceived, that the proportion has been varied from nearly one half to one ninth part of the span. As this important object has been discussed at length, when treating oldie principles, it is only necessary here to observe, that the thickness of the piers, besides the span and shape of the arch, is affected by its rise, and in no inconsiderable degree by their own altitude, and the quality of the stone of which they arc constructed ; it is desirable, in order to lessen the obstruction to the water-way, to make the piers as thin as prudence will admit. The shape of the piers and arches should also be calculated, to form as per fect a water-way as possible ; and on this account, all recesses and side projections, within reach of the water, should be avoided, and the connections of the abut ments with the banks should be formed on the same principles.