R HI D G E. 





applied in any wr.y which pi iiit, lias, of 



icil tO .111 i 



utsoii ncc, .in :ip- 



: increase of expence, the bebt cpa!:lird of the 

 competitor:-, of whom we have frequently l%.mnvu up- 

 ward>u: twenty for one project. 



The superstructures of wooden bridges have been 

 Constructed in .: , hut their abutments 



have usually been made of stone. In all, therefore, 

 winch !X);;udsthe touiidatioiuand masonry of tlie abut- 

 ments, ;s is precisely similar to wliat has been 

 described for stone bridges, only the weight and u-.-i- 

 zontal thrust of wooden bridges being ITHIL!I 

 quire a smaller proportion of mason; y. \Vhei >; the 

 streams are narrow, it is only necessary to lay beams 

 across, reaching between the abutments at the distance 

 from each other of fgurto five feet from middle to mid- 

 dle, and to have braces reaching from some: parr, ot the 

 f the abutments to the lower side ot each beam. 

 Besides these supports, the outer beams are usually 

 d up by means oi the side railing. Across the 

 beams, planking of three or four inches in thickness 

 is laid to receive the gravel. A plank is also fixed 

 along the inside of each railing, to support the side of 

 the roadway. The railing is secured by braces fixed 

 on the outside, or by strong iron knees on the inside. 



When the stream is wider than to be reached across 

 b,y one length of beams, the most usual way has been 

 to drive rows of piles at each length of beams, in lines 

 parallel with the current of the river, at about four or 

 five fc-ct distant from each other, (middle to middle,) 

 and iilso fixing braces up and down the river to sup- 

 port each pier, or row of piles. These piles are 

 driven and fixed in the same manner as described for 

 the foundations of stone piers. They have usually 

 diagonal braces crossing each side of i',\ r -.w of piles 

 in the form of an X and which art boil -d together at 

 each pile. Tenons are formed on th-- top of each 

 pile, upon which a cap is morticed ; and in order to 

 shorten the bearings, pieces of timber are laid imme- 

 diately under each beam, extending five to ten feet 

 cm each side of the cap. The rest of the structure is 

 completed in the same manner as ha? been described 

 ior one bay of beams. A very perfect model of this 

 oort of bridge was constructed by Mr Peter Nichol- 

 son, upon the river Clyde at Glasgow. It is only a 

 foot bridge, but the principle is excellent. See Plate 

 CII. 



Instead of driving piles for supports, frames are 

 sometimes constructed on dry land, which are after- 

 wards sunk in their proper .situations, after the bot- 

 tom has been made level to receive them. These up- 

 right frames Jiave grated frames attached to their 

 base, and which extend on each side of the upright 

 frame?. Through the spaces formed by this excess 

 of breadth, short piles are driven to keep the grating 

 and frames secure. These upright frames have their 

 sides covered with planking ; and in order to add to 

 their stability, the lower part is tilled with gravel or 

 small stones. Upon the edges of the frames which 

 face the stream, triangular pieces of cast iron are fix- 

 rd, to prevent ice or other matter floating down .the 



TAUT II. VL. IV. 



river from injuring tin- 



somptinv 



distil 



has 1 



on the ,S 







,but notwithstai :< n 



taken, liable to injt 



', of ;' ' 



from ban'; to l>:i:i!-. , v. . intermediate sup- 



ports. This IMS i it way* 



with grent ingenuity. When the bank* risr co. 

 dcrably above the level of the river, it is usual to con- 

 struct the supporting frames chiefly below the level 

 of the roadway. 



I . Merely by means of two principal rafters, who y; 



r ends stand on the abutments, and their pn 

 meet below the middle of the beam j or sometimei 

 the upper points pass the outer beams, and meet be- 

 low tlie top of tlr- Mde railing. 



'i. By the principal rafters rcMchiucf from the abut- 

 ments to an intermediate part of the horizontal beam, 

 and having a stretching piece between their t 

 There are instances of this being repeated in the sanv- 

 bridge below the level of the beams, and also again 

 in the side railing of the roadway. 



3. By constructing a series of short frames, and 

 placing them vertically in the mariner of stone roif- 

 .fi/irs, and upon these raising the structure of the 

 roadway and railing. This mode was pointed oat by 

 Palladio, as suitable for wooden bridges of great ex- 

 tent. Gautier acknowledges that he had adopted Pal- 

 ladio's idea, in forming a design of a wooden bridge 

 for the Seine ; and in England, the cast iron bridges 

 at Wearmouth, in the county of Durham, and at 

 Boston, in Linclnshire, are also formed on this prin- 

 ciple. 



4'. A very ingenious mode has been practised by 

 James Burn of Haddington, in a wooden bridge of 

 109 fret 3 inches span, and 13 feet 4 inches rise, over 

 the river Don, abrnit 7 miles from the city of Aber- 

 deen, on the road to Banff. Mr Burn takes a series 

 of frames in the form of an arch, but each set of 

 frames is laid horizontally across the soffit. The 

 bridge is 18 feet in width; and there arc two row* 

 of frames, or as it were two ribs placed four inches 

 distant from each other. Upon these arch frames, 

 a vertical fiaming is constructed to support the bear- 

 ers, the joists, plankin;;, and gravel of the road- 

 way. Sec Plate LXXXV1II. Thisbridge was erected 

 in 1803. Mr Burn built another wooden bridge upon ' 

 the same principles over the same river, at Granholm, 

 within four miles of Aberdeen, to open a communi- 

 cation with the extensive manufactory of James Had- 

 den, Esq. and Co. This bridge consists of two 

 arches, each 71 feet (i inches span, and 10 feet 6 

 inches rise : it has only 10 feet (i inches in width of 

 soffit, and consists of one row of frames. Here there 

 is, of course, a pier in the middle of the river. 



The same artist has likewise built, on the am? 

 principles, a wooden bridge of three arches ov:-r the 

 river South Esk, in the park of William Maule, Esq. 

 at Brechin in Angus-shire. The middle arch is 5S 

 feet span, and 10 feet rise ; the bridge 1 is 15 feet in 

 t v 





Pf.> 



ll! 



