Discussion. The Design Generally of Iron Bridges of Very Large Span for Railway Traffic (Includes Supplementary Table of Iron Girder Bridges for Railways).

Abstract

Vice-President, said considering the long list of American girders of great span erected since 1870, exceeding 300 feet, and reaching 515 feet, he thought English engineers might look with envy upon their American brethren, in having such numerous opportunities for the exercise of their professional skill. In the Author's description of the method in which Mr. Linville had erected the bridge over the Ohio river, and of the manner in which Mr. Shaler Smith had erected the bridge over the Kentucky river, there were examples of facilities of resource in American engineers which both the older and younger members of the profession might follow with great advantage. I n large spans it was a matter of considerable importance t o study the details of the structure. There was undoubtedly a considerable difference in the use of what the Author called pin-connections as compared with rivet-connections. H e had, to a certain extent, looked into that subject ; and he had found that in suspension bridges there was a surplus of metal of about 10 per cent. arising from the use of pin-connections, while in riveted girders the waste amounted to 30 per cent., so that there was a large saving in the former method. It was to be observed, however, that a pin-structure, which was a joint, was liable, to motion, and he was not quite sure whether in point of durability it was as good as the other. The sixteen bridges mentioned in the Author's tabular statement might be arranged under four classes : (l) what he had termed quadrangular bridges with pin-connections ; (2) containing one bridge, Mr. Brunel's, at Saltash ; (3) lattice bridges ; and (4) tubular bridges. He had endeavoured to compare the efficiency and structural merit of those bridges by the system of ascertaining their limiting spans. It was a mode of treating the subject which he had adopted some years ago. It was first published by Professor Rankine, and it afforded great facility in ascertaining what the weight of a girder should be without the trouble of calculating all its several parts; and in like manner it afforded the means of computing how far one girder was superior to another. He meant by the limiting span the length to which a girder could be carried, increasing all its proportions in like manner, when its own weight would produce upon it the strains which were said to arise in the girder itself. Or it could be carried a little further, by ascertaining the length to which the girder might be carried so as to bring upon it those strains which English engineers were in the habit of putting upon girders-5 tons tension and 4 tons compression. With regard to th9 quadrangular girders mentioned in the list, the first of