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BRIDGE. 



Designs for 

 bridges. 



Practice, be commodious, but where practicable, from the ap- 

 - ~ -' preaches 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 an- 

 gle with that of the river above it. Rock should, if 

 possible, be obtained for the foundations of the abut- 

 ments 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 shewing 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 observing people in the neighbourhood. 

 The consistence of the bed should also be ascertained, 

 by boring with properly constructed augers, especi- 

 ally in the scite of the abutments, piers, and wing 

 walls. There should likewise be a longitudinal sec- 

 tion, shewing the declivity of the bed of the river, for 

 at least 200 yards above and below the proposed si- 

 tuation. 



When these steps have been taken, and the nature 

 of the materials ascertained, the engineer has obtain- 

 ed sufficient data to enable him to make a suitable de- 

 sign. We shall at present confine ourselves to bridges 

 constructed with stone. 



The 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 pro- 

 cured of proper dimensions, large arches will be found 

 advisable ; if the reverse is the case, the size of the 

 arches must be diminished, and their number increa- 

 sed ; if more arches than one are required, their num- 

 ber 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 aide should dimi- 

 nish, so that their springing 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 dou- 

 ble the thickness of all the piers. Where the bed of 

 the river is soft or loose, the increased width must be 

 considerably more. In proportioning the thickness of 

 thepierstothespan of the arches, great latitude has been 

 taken, as will appear from the following statement : 



Pratt 





Passag 

 for IM 



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 of the 

 principles, it is osly 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 inconsi- 

 derable degree by their own altitude, and the quality 

 of the stone of which they are constructed ; it it de- 

 sirable, in order to lessen the obstruction to the water- 

 way, to make the piers as thin as prudence will ad- 

 mit. The shape of the piers and arches should also 

 be calculated, to form as perfect a water-way as pos- 



2 



sible"; and on this account, all recesses and side pro- 

 jections, within reach of the water, should be avoided, 

 and the connections of the abutments with the banks 

 should be formed on the same principles. 



2. If the bridge consists only of one small arch, 

 and there is little intercourse, the road-way may be 

 narrow and without footpaths ; if the bridge is long, 

 and there is much intercourse, the breadth must be 

 increased, and have proper footpaths. Since wheel 

 carriages have been in general use, few bridges, how- 

 ever small or remote from towns, have been made less 

 than fifteen feet in breadth over the parapets ; they 

 are more generally made from 18 to SO feet, in or 



