On Some Common Errors in Iron Bridge Design

43 cross girder. Properly such cross girders should carry no load, for if loaded their deflection will affect the verticality of the main girder. This, however, can hardly be insisted upon in practice. (£) The chords may be bent sideways by the pressure of wind upon the girder itself or upon objects supported by it. This pressure will be great in. the case of plate web girders and of that chord of an open girder that carries the deck. It is to be met by inserting a proper system of diagonals forming with the chords of the main, girder, a complete horizontal triangulation. It is certainly desirable that such a triangulation, made of stout T or angle section, be added to the older tubular girder bridges and others that do not possess it. The smaller the width as compared with the span, the more important this horizontal system is. There are some cases, however, in which it is not needed. The first is when the bridge is provided with a continuous metallic deck, as is now often the case. Such deck forms a most efficient horizontal web. The second is when the bridge is built on a skew such that the cross girders attached at or near the end of one main girder meet the other main girder at a fourth to a third of the span, from one end. Here the cross girders themselves form an efficient bracing. (<) The compressed chord may buckle or fail as a long column. To prevent this, it must be rigidly held at frequent intervals by some system of bars preventing any small initial lateral bending from increasing. In an ordinary discontinuous girder the top chord is compressed, and if the deck is on the top, the same bracing that resists the pressure of wind on the top chord, deck, and load, will meet this requirement. If, however, the deck is on the bottom, the top chords may, if the girder be deep enough, be braced together overhead, with a complet© triangulation, extending from end to end. This is by far tli© most satisfactoiy method, and is usually found in the larger American