On Some Common Errors in Iron Bridge Design

Each pair of these columns supports an area of bridge decking 70 feet long and 32 feet wide. The proof of the practical success of these columns is in every way most conclusive, for not only is the bridge on. an important main road with heavy traffic, but it is also at the part of the Yarra where the hydraulic conditions are of the severest kind. During the great flood of July, 1891, when two iron bridges were washed away and hundreds of suburban dwellings inundated, the water stood at the level shown in Fig. 2. The gradient of the flood surface for 50 chains above the bridge was at the rate of over 5 feet per mile, the hydraulic radius about 30 feet, and floating timber and other wreckage abounded. Nevertheless these slender columns stood absolutely uninjured, and that, although the bracing between them is by no means as massive as, in my opinion, it should be. The second example is the bridge carrying the North-Eastern Railway over the Racecourse Road, Flemington, near Melbourne. The railway is double line and is traversed by a busy suburban traffic propelled by tank engines of 49 tons weight. The bridge is situated at the entrance of the Newmarket Station and is exposed to the constant action of the Westinghouse brake. There are two spans of 51 feet each (discontinuous), four main, girders to each span, and the central support consists of four columns each made of four 3| x 3| x | angles of mild steel, with single rivetted lacing. The foundations are of Victorian bluestone, a 3 inch cube of which crushes with 40 tons pressure, and are 2| feet square for each column. The compressive stress on the metal of the angles is 4 tons per square inch. The columns are 15 feet high from stone foundation to girder seat and are 18 inches square. Strange to relate a second railway, carrying a practically identical traffic crosses the same road at a short distance, and here the columns are of cast iron filled with cement, 2ft. 3in. diameter, 1 inch thick, and the girders 44 feet span. Judging from experiments made with the University testing machine it would take 300 tons to crush a column of the former bridge and 4000 tons to crush one of the latter, and yet the latter carries a smaller load than, the former.