DISCUSSION. AMERICAN IRON BRIDGES. (INCLUDES PLATE)
Minutes of the Proceedings of the Institution of Civil Engineers
AMERICAN IRON BRIDGES. 557 Mr. ZERAH COLBURN said, he had refrained from entering more into detail, to avoid making the Paper tedious, but he should be happy to reply to any questions. No doubt, in looking at the diagrams, it would be considered that American engineers had practised great economy in designing these bridges. The fact was, that if bridges could not be constructed of iron at a moderate cost, the Railway Companies in the States would not adopt them, but would continue to use timber
... tinue to use timber bridges, which could be built at from &5 to &7 per lineal foot. It was on the score of cost, alone, he believed that American engineers had adopted cast iron for all parts in compression ; so many square inches of section could be put into the tubular form for one-third the cost in cast iron that would be incurred with wrought-iron plates. I n the case of the Green River Bridge, the t,op chords and the vertical posts were of cast-iron pipes. Care was requisite in casting the pipes. Mr. Fink tested the iron for all the pipes, and holes were drilled in the side that lay uppermost in the sand, to ascertain that the cores had not Boat,ed, and thus that the metal was of uniform thickness. The fact of cast iron being used in the top chords was a sufficient explanation, why bridges were not made continuous over two, or more, spans in America. I n nearly all these bridges almost the whole of the iron was made to do work in carrying the load. They no doubt differed from what English engineers were accustomed to, and he feared the diagrams would present an extraordinary appearance to English eyes ; but they were faithful representations. Mr. Colburn then called attention to the drawing of a bridge with truss rods to every upright post. The span was 125 feet, and the depth was 23 feet from the centre of the top chord t,o the centre of the bottom chord ; yet all the iron posts, the truss rods, the diagonal tension rods, and the leaning end columns were not together equal to a quarter of an inch in thickness of iron, if spread out as a plate, over the whole side, and half of that thickness was cast iron ; still no portion of the iron was subjected to a greater working strain than 4 tons to the square inch. Under ordinary circumstances a working strain of 3 tons to the square inch was tile utmost that existed. H e contrasted with that design some plate girder bridges, erected three years ago, on the Boston and Worcester KaiIway. They were 87 feet span and 7 feet (5 inches deep. The plates were G feet 3 inches wide and 7 feet 6 inches high. A t every vertical joint there was a pair of butt straps, 8 inches wide, double riveted on each side of the web, and over these a pair of angle irons, 3 inches by 6 inches. Midway between the vertica.1 joints, there were two angle irons, B inches by 3 inches, the longer sides being turned over at right angles, to form a lrnce by which they were riveted to the top and bottom chords. Mr. Philbrick, the engineer, adoptcd that form of stiffcning the Downloaded by  on [12/09/16].