540 



BRIDGE. 



Practice. 



fur* 

 XCIL 



had been embanked, so that the flood- waters passed 

 ' off more hastily, and in a greater body than for- 

 merly ; in order, therefore, to remove all obstacles 

 out of the way of future floods, and on account of 

 being within two miles of the best founderies in the 

 world, he recommended a cast-iron arch of 130 

 feet span. (See Plate XCII.) The magistrates 

 of the county agreed to this, and the Coalbrookdale 

 Company became contractors, both for the iron work 

 of the arch, and the masonry of the abutments. Mr 

 Telford, we understand, had some trouble in making 

 that Company depart from their former mode of 

 construction ; but he at last prevailed in keeping the 

 roadway low, and adopting the suspending principle, 

 by means of a rib on each side of the bridge, which 

 sprung from a lower base than the bearing ribs, and 

 rose above them to the top of the railing : thus the 

 bearing ribs were supported by the lower parts of 

 those before-mentioned, and were suspended by their 

 upper parts. The bearing ribs have a curve of 17 in 

 130, or nearly one-eighth of their span. The sus- 

 pending ribs rise 34 feet, or about one- fourth of their 

 span. There are cast iron braces, and also horizon- 

 tal ties. There are 46 covering plates, each 18 feet 

 in length, and one inch in thickness. They have 

 llaunches four inches in depth, and are screwed toge- 

 ther at each joint ; so that, by taking the curvature 

 of the bearing ribs, and being firmly secured at the 

 abutments, instead of a load, they compose a strong 

 arch. There being only one rib in the middle of 

 18 feet breadth of bridge, on each covering plate, a 

 cross rib or flaunch, four inches in depth, is cast at an 

 equal distance between the bearing ribs. The sus- 

 pending ribs are each 18 inches in depth, and 2f 

 inches in thickness, exclusive of a moulding. The 

 bearing ribs are 15 inches in depth, and 2i inches in 

 thickness, and e?,ch of the ribs are cast in three 

 pieces only, of about 50 feet each ; the braces are 

 .5 by 3 inches. The principal king posts are 10^ by 

 4-J inches. The springing plates are each 3 feet 

 bsead, and 3 inches thick, with openings to save me- 

 tal. The uprights against the abutments are 4J 

 .uclics square. The strongest uprigh'ts in the rail- 

 ing are '3 inches square, and those between them 1 

 inch. They are placed 6 inches apart, between 

 widdle and middle. The height of the railing above 

 the surface of the roadway, is 4 feet 9 inches. In 

 each spandrel there are three circular arches formed 

 with hard burned bricks, which preserve most of the 

 space open, but they are concealed by iron plates, 

 one inch in thicknesr, which form the outside facings. 

 On the eastern side of the river, although the banks 

 are not so very high or steep, the quality of the 

 ground being similar to that of the other iron bridge, 

 particular care was bestowed upon the abutmen-ts : 

 the space for them was excavated down to the rock, 

 which lay considerably under the bed of the river, 

 and the masonry was sunk into the solid part of tike 

 rock. It was built up chit-fly of square masonry, 

 and the rest of rubble, laid very close in regular 

 courses, and having the back part formed in the 

 shape of a wedge, pointing to ta.-; bank. The wing 

 walla were curved horizontally and vertically. At 

 the height of 10 u-et above t:i-_- low water, there is 

 a hauling- path on eaeh side of the river. This 



6 



Mic'l 



bridge, which was completed in 1796, has never Practi 

 shewn any appearance of failure i;i any of its parts; 

 nothing can be more perfect than the irn work ; 

 it is fitted as correctly as a piece of good carpentry. 

 It has been objected to this structure, that by con- 

 necting ribs of different lengths and curvature, they 

 are exposed to different degrees of expansions and 

 contractions. This appears just in theory ; and that 

 no discernible effect has hitherto been produced, is 

 probably from the difference being small ; but this 

 point will be discussed under the article IRON'. Ano- 

 ther objection is, an apparent heaviness in the span- 

 drels, from concealing the circular arches with iron 

 plates. For appearance, these spaces had certainly 

 better not been concealed, but they are not liable to 

 the objections made in the former iron bridge, be- 

 cause the space around them is all closely filled up, 

 and the roadway being formed with materials similar 

 to this filling up matter, distributes the pressure very 

 regularly. Upon the whole, considering the strength 

 acquired by placing the covering-plates with their 

 deep flanches, in the form of an arch, we doubt 

 whether a greater degree of strength can be had by 

 any other distribution of the same quantity of cast- 

 iron, viz. 173^ tons : it appears to us s that the up- 

 right standards, braces, and king posts, might be 

 made of smaller dimensions. 



We have been informed, that each of these two 

 first iron bridges, including abutments and roadways, 

 cost about L. 6000. 



The third iron bridge was constructed over the At Sund 

 river Wear, near Sunderland, in the county of Dur- land - 

 ham. Its projector was Rowland Burdon, Esq. a 

 gentleman of considerable landed property in that 

 county, and who, for some time, represented it in 

 parliament. The iron work was cast at the foun- 

 deries of Messrs Walkers of Rotherham, and erect- 

 ed under the inspection of Mr Thomas Wilson. The 

 confidence in the use of iron, for arches of great ex- 

 tent, was by this time established. The span of the 

 second arch, we have seen, is 30 feet more thar. that 

 of the first ; and, in this third instance, the span is 

 106 feet beyond that of the second, although its rise 

 is only the same as that of the suspending ribs at 

 Buildwas. The arch at Sunderland springs 60 feet 

 above the level of the surface of low water ; the 

 span is 236 feet ; the rise, or versed sine, is 34 feet ; 

 the width of the roadway 32 feet ; and there are six p , ._ 

 ribs. See Plates XCI. and XCIII. xCll 



In this arch, the mode of construction is very XCIII. 

 different from either of the former. Instead of work- 

 ing with pieces of iron from about 50 to 70 feet in 

 length, each rib is here composed of 125 smalt 

 frames, each about two feet in the length or curve 

 of tire rib, and five deep in the direction of the ra- 

 dius. In. each frame there are three pieces of four 

 inches square, which run in the direction of the 

 curve of the arch ; and these are connected 'n the 

 direction of the radius by two other pieces, four by 

 three inches. In each side cf the larger pieces, is a 

 groove thrc'.- inches broad by ^ of an inch ni depth ; 

 and opposite each cross piece there is a hole in the 

 middle of the groove. When th.- abutments were 

 brought up, and a scaffolding constructed across the 

 river between diem, six of these frames were placed 



