IRON BRIDGE, a species of bridge constructed, as its name implies, of iron, which was first applied to this pur pose in England, towards the close of the 18th century. The merit of having employed this material in bridge building, has been generally claimed for the English, but more accurate writers state that it really belongs to the Chinese. Be this as it may, it is certain that in no country in the world have been erected so many magnificent and stu pendous structures as the iron bridges in the United Kingdom.
Under the article BRIDGE, (11. 58, vol. i.) we gave some account of the " rise, progress, and present state" of bridge building, reserving to each respective head the more detailed information peculiarly belonging to it. Following that course, we now proceed to describe the several forms of bridges built of iron, as cast-iron arched bridges, cast-iron girder bridges-, cast-iron compound girder bridges trussed with malleable iron bars, tubular bridges of various kinds, &-c.
The first iron bridge erected in this country was over the Severn, a little below Colebrook Dale, where that river is narrow and rapid. The abutments, which are of stone, are brought to about 10 feet above the surthee of common low •ater, where they have each a platfbrm of squared freestone for ten feet breadth, which serves for a hauling way, and a base fir the arch to spring; ITpon this plattlmn, cast iron plates, four inches in thickness, are laid, and formed with sockets to receive the ribs. These plates, in order to save metal, have considerable openings in them. The prin cipal, or inner ribs, which are five in number, and which form the arch, are 0 inches by fii. The second row behind them, and which are cut off at the top by the horizontal bearing-pieces, are 1G. by 6 : the third row are 6 by 6 inches : the upright standards behind the ribs are 15 inches by 61 inches, but they have an open space in the breadth of 51; the back standards are 9 inches by 61, with projec tions for the braces: the diagonals. and horizontal ties, are 6 inches by 4, and the cast-iron tie bolts are 2+ inches diameter. The covering plates, which are 26 feet in length, reaching quite across the bridge, are one inch in thickness. The great ribs are each cast in two pieces, meeting at the keys, which, as the arch is circular, 100 feet 6 inches span, and 43 feet rise, are about 70 feet in length. There are circular rings of cast iron introduced into the spandrels, and there is a cast-iron railing along each side of the roadway of the bridge : the weight of the whole of the iron work is 3781 tuns. Behind the iron work, at each extremity of the
arch, the abutments are carried up perpendicularly of rubble masonry, faced with squared stone, and the wing-walls are also of the same materials.
The iron work was east and put together in a very mas terly manner, under the direction of Abraham Derby. of Colebrook Dale ; and the whole was completed in the yeqr 1779. The design was original and very bold, and was, as tar as concerned the iron work, well executed ; but being a first attempt. and placed in a situation where mare skill than that of the mere iron masterwas required. several defects became apparent when the bridge was completed. The banks of the Severn are here remarkably high and steep, and consist of coal measures, over the points of which vast masses of alluvial earth slide down, being impelled by springs in the upper parts of the banks, and by the rapid stream of the river, which WI:SO! VI'S and washes away the skirts below: the masonry of the abutments and wing-walls not being con structed to withstand this operation, was torn a+sunder, and forced out of perpendicular, more particularly on the western side, where the abutment was forced forward about 3 or 4 inches, and by contracting the span, of course heaved up the iron work of the arch. This was remedied under the direction of that able mason Mr. John Simpson, or Shrews bury, as far as the of the case would adinit of. by rumw rug the ground, and placing piers and counter-arches upon the natural ground behind it. I lad the abutments been at first sunk down into the natural undisturbed measures, and constructed of dimensions and form capable if' resisting the ground behind ; and had the iron work, instead of being formed in ribs nearly semicircular, been made Hat segments, pressing against the upper carts of the abutments; the whole edifice would have been much more perfect, and a great proportion of the weight of metal saved. have already stated that one row of the principal ribs formed the arch; the two rows behind are carried concentric with the inner row, until intersected by the roadway, which passes imme diately at the level of the top of the inner ribs. This has a mutilated appearance; the circular rings of the spandrels being less perfect than if the pressure had been upon straight lilies; for a circle is not well calculated for resistance, unless subjected to an equal pressure all round.