of bridge had been erected by him usually on common roads, where economy was necessary, as the traffic was but small, and also in localities where they might be dispensed with hereafter at the fancy of the landowner. He thought in such cases the timber lattice bridge might be adopted with advantage. Mr. Mallet had erected for him a large lattice bridge in 1847 at Thomastown, over the River Nore, and he believed it was still standing, without objection having been made against it. He would therefore express his decided conviction, contrary to the opinion of those who had spoken of the destructibility of the American bridges, that if timber bridges, and especially lattice bridges, were properly constructed, as they had been in this country, they might be adopted in many cases with perfect safety, and with great advantage in point of economy. Captain Moorsom added that he had found one great advantage of the lattice bridge to consist in the facility for repairs, and that he should have no difficulty in repairing a lattice bridge with a single track, while the trains were passing, provided it was of the ordinary character. Mr. MALLET said the Paper did not give any information, as to the particular sort and quality of timber used in these structures in America. Practical men knew that there were three important elements which chiefly determined the durability of any timber bridge; the quality of the timber, the way in which it was put together, especially the extent to which the joints were water tight, and the climate to which it was exposed. He was rather surprised at the very short life assigned to American timber bridges by Mr. Colburn, whose facts were nevertheless, he did not doubt, worthy of full acceptance in point of accuracy. It must, however, be borne in mind, that the climate of North America tried timber very severely; in addition to which he suspected, that the timber used was probably unseasoned, and not in all cases of the best quality. Upon these points, and whether due regard had been paid to the exclusion of moisture lodging between the opposing surfaces, he should deem it very desirable to have further information. The large annual expense for repairs and the short durability mentioned, did not quite coincide with European experience of wooden bridges. In the east of Germany, for example, it was well known that there were many wooden bridges, whose durability had been so great, that they were now actually objects of antiquity. The original bridge of Schaffhausen had stood, he believed, for more than forty years, before it was burnt by the French. The Germans and the Swiss, however, employed excellent timber, and they well understood how to put it together. In the case of the River Nore Viaduct, on the Waterford and Kil 1 Vide Minutes of Proceedings Inst. C.E., vol. xi., pp. 426–434. kenny Railway, alluded to by Captain Moorsom, Mr. Mallet's firm had been the contractors for the erection of that structure; the timber used was of admirable quality, the wales being pitch pine and the lattice bars choice crown memel. That viaduct was still standing, and was, he believed, sound, after being in use for fifteen years. He had passed over it last year, and it appeared to stand well. He had heard, however, that since Captain Moorsom had ceased to have charge of the line, piles and struts had been put in, forming a centre pier, and dividing the original long span of 200 feet into two spans of 100 feet each nearly. This appeared to him to be perfectly unnecessary-though it was only another example of a case that too frequently occurred, in which advice given by a second Engineer subsequent to the completion of a work by his predecessor, was based upon an assertion of original insufficiency. February 24, 1863. JOHN HAWKSHAW, President, in the Chair. No. 1,088.-" On the Reconstruction of the Dinting and the Mottram Viaducts." By WILLIAM FAIRBAIRN, F.R.S., M. Inst. C.E. IN the early period of railways, the passage of rivers and deep ravines was frequently effected, to save expense, by timber viaducts and bridges, instead of by more substantial structures of stone, brick, or iron. This has been demonstrated, by experience, to be a mistaken economy; and although, with a limited capital, the Engineer has to adopt measures in accordance with his means, yet at the present time, considering the facilities afforded by wrought iron, there does not appear to exist any inducement for the introduction of a perishable material into structures intended to be of a permanent character. Previous to the application of wrought iron, in the tubular girder and other forms in which it is now employed, there might have been some reason for the use of timber, in order to reduce the first cost. But it is now generally acknowledged, that it is safer and more satisfactory to increase the estimates, than to incur the necessity of frequent repairs and renewals, the risk of accident, and the danger arising from the use of a material subject, like timber, to rapid decay. In the construction of works in which strength and durability are required, the Engineer is justified in availing himself of the material best adapted for ensuring permanency. For bridges and viaducts of great width and span, wrought iron is, probably, the most suitable. A wroughtiron girder, when properly proportioned, contains within itself the elements of lightness, with great powers of resistance to strain; and although, like all other materials, it is liable to oxidation and decay when exposed to the action of the atmosphere, it may nevertheless, with a little trouble and attention, be preserved for many years, in a state of perfect security. These considerations have more or less guided Engineers for the last fifteen years; and hence the immense increase in the use of iron for road and railway constructions. The experiments for determining the strength and form of the Britannia and Conway tubular bridges led to a new era in bridge building. They developed an entirely novel principle of construction, which has been most successful. The many advantages resulting from the application of a tenacious, but flexible, material like wrought iron, both in plates and bars, may be estimated by the numerous structures of this class in almost all parts of the world. From what has been already accomplished, it is probable, that, in bridge building alone, the use of this material will confer on Engineers the power of crossing rivers and deep ravines with spans up to 1,000 feet; and the time is, apparently, not far distant, when even greater spans may be possible, by the employment of a quality of steel possessing a much higher degree of strength and uniformity in its tensile character than iron, so as to enable nearly one-half the weight of the present constructions to be dispensed with. It may, therefore, be reasonably anticipated, that the improvements in progress in the manufacture of steel and iron will still farther extend the solidity of works, in every department of the useful and the industrial arts. 1 The Dinting and the Mottram viaducts, on the Sheffield and Manchester Railway, were erected in the year 1843-44, under the direction of the late Mr. Joseph Locke, (M.P., Past-President Inst. C.E.) In the former there were five arches of 125 feet span, and in the latter, one of 150 feet and two of 125 feet span, all constructed of timber ribs on the laminated principle. At the springing of the arch, the ribs were fitted into cast-iron shoes, let into the masonry of the piers and the abutments. There were four main ribs in each arch of the Dinting viaduct (Fig. 1), and three in each arch of the Mottram viaduct; and these ribs were stayed by diagonal and cross braces. The uprights and diagonals in the spandrils were also stayed, and were morticed into longitudinal beams supporting cross joists, on which the permanent way was laid, at a height of 100 feet to 120 feet above the level of the river. As is generally the case with similar structures, after a few years the ribs became twisted, and the entire structure of the Dinting viaduct was so insecure, that it had to be trussed diagonally with iron rods to retain it in line, and to prevent it from bulging over in a lateral direction. Within ten or twelve years after erection, the timber in both the viaducts was so much decayed as to endanger their security, and the Author was consulted as to the best means of reconstructing them in iron. The late Mr. Jee, then the Resident Engineer, undertook however, to render them secure, and after much trouble and expense this was considered to have been accomplished. But this did not last long, as, in 1858, they were again in such a state as to alarm the passengers and the people in the neighbourhood, who drove and travelled several miles to avoid crossing either bridge. Under these circumstances it was finally determined to renew them with iron, and the Author being again 1 Vide Minutes of Proceedings Inst. C. E., vol. v., p. 216, for "A Description of the Dinting Vale Viaduct," by the late Mr. A. S. Jee. Also, Ibid. p. 219, for a Paper, "On the Arched Timber Viaducts on the Newcastle and North Shields Railway," by Mr. B. Green. |