On Some Common Errors in Iron Bridge Design

6 material there' should be no difficulty in proportioning the cylinders of future bridges. As for those in existence, nothing- can be done, but as they usually err on the side of excess thei’e is not much cause for alarm. 2. Excessive and disproportionate size of columns.—By the term column is meant that part of the structure extending from the foundation to the girder seat. Its size is often made equal to that of the foundation, but there is no necessity that this should be the case, for while the size of the foundation depends on. the resistance of the material upon which it rests, that of the column depends upon the material of which it is made and which sometimes offers a greater resistance per square inch than the foundation does per square foot. For the sake of lateral and frictional suppoi't, the cylinder is usually, and properly carried up the full size from the foundation to the surface of the ground. Above this, however, there is no reason why it should not be as economically designed as any compression element of the super- structure. In many of the older bridges the columns are of most unnecessary size, adding seriously to the cost of the structure, and impeding the flow of water in the case of river bridges in an undesirable manner. This is certainly the case with the older New South Wales railway bridges already referred to, and also with some in Victoria. As examples of what has been success- fully done in the way of reducing this part of the structure to reasonable and economical proportions, two structures may be cited. Jhe first is the Johnston Street Bridge, Collingwood, near Melbourne, shown in Fig. 2. This is an iron bridge built about 20 years since by C. Rowand, Esq., C.E., to replace a large timber arch that failed through decay. It consists of three spans of nearly 60 feet each, extending between the stone abutments of the old timbei’ arch, and having as intermediate supports wrought iron columns filled with concrete, which for slightness present a most extraordinary contrast to the usual practice at the time it was built. Their dimensions are as follow :— Height from top of cast iron cylinder to girder seat ... ... 45 feet Diameter ... ... ... 2 feet Thickness of metal ... ... |nch Dead load for each column ... ... 40 tons Live load for each column ... ... 50 tons