IRON BRIDGES.
In the practice of bridge building, there yet remains to be described a mode not only unknown to the an cients, but unquestionably a late invention of British ar tists. We are not perfectly certain with whom the pro posal first originated, whether k was solely with the Coalbrookdale Company, or whether the late Mr John `Wilkinson had some share of the merit; certain it is, he was very active in promoting the first iron bridge.
A thorough discussion of this subject would involve an investigation of the principles of working with iron as a material ; but for this we must refer to the article IRON; yet as our readers would naturally be disappoint ed, if, under the head of Bridge Building, no notice was taken of the modes hitherto practised in this important change of bridge operations; we shall therefore, not withstanding the length to which we have unavoidably been led to extend the present article, give a short ac count of the different modes practised in the principal iron bridges which have hitherto been constructed.
Thu first, as has already been observed, was that erected upon the Severn, a little below Coalbrookdale, where that river is narrow and rapid. See Plate XCI. The abutments are of stone ; they are brought up to about 10 feet above the surface of common low water; here they have each a platform of squared freestone for ten feet breadth, wnicn serves for a hawing way, and a base for the arch to spring from. Upon this platform, cast iron plates, four inches in thickness, are laid, and lornied with sockets to receive the ribs. These plates, in order to save metal, have considerable openings in them. The principal, or inner ribs, %%ilia are five in number, and which form the arch, are 9 inches by 61. The 2d row behind them, and which are cut off at the top by the horizontal bearing pieces, are 61 by 6 inches; the 3d row are 6 by 6 inches; the upright standards be hind the ribs are 15 inches by 61 inches, but they nave an open space in the breadth of 51; the back standards are 9 inches by 61, with projections for the braces; the diagonals, and horizontal ties, are 6 inches by 4 inches, and the cast iron tie bolts are 21 diameter. The cover ing 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 45 feet rise, arc about 70 feet in length. There arc circular rings of cast iron introduced into the span drels, 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 3784- tons. 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 arc also of the same materials.
The iron work was cast and put together in a very masterly manner, under the direction of Abraham Der by, of Coalbrookdale ; and the whole was completed in the year 1777. The design was original and very bold, and was, as far as the iron work goes, well executed ; but being a first attempt, and placed in a situation where more skill than that of the mere iron master was requir ed, several radical defects are now apparent. 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 clown, being impelled by springs in the upper parts of the banks, and by the ra pid stream of the river, which dissolves and washes away the skirts below : The masonry of the abutments and wing walls, not being constructed to withstand this ope ration, has been torn asunder, and forced out of the per pendicular, more particularly on the western side, where the abutment has been forced forward about 3 or 4 in ches, and, by contracting the span, has of course heav ed up the iron work of the arch. This has been reme died under the direction of that able mason Mr John Simpson, of Shrewsbury, as far as the nature of the case will admit of, by removing the ground and placing piers and counter arches upon the natural ground behind it. Had the abutments been at first sunk down into the na tural undisturbed measures, and constructed of dimen sions and form capable of resisting the ground behind, and had the iron work, instead of being formed in ribs nearly semicircular, been made flu segments, pressing against the upper parts of the abutments, the whole edi fice would have been much more p •rfect, and a great proportion of the NYPi:_thI of metal saved. We have al ready stated, that one row of the principal ribs formed the arch ; the two rows behind are carried con'entric with the inner row, until intersected by the roadway, 'which passes immediately at the level of the top of the inner ribs. This has a mutilated appearance ; the cir cular rings of the spandrels are less perfect, than if the pressure had been upon straight lines ; for a circle is not well calculated for resistance, unless equally pressed all round.