On Some Common Errors in Iron Bridge Design

12 forms of open girders including the X girder, the N girder, the W or Warren girder, the Pratt & Whipple girders so deservedly popular in America, and of late years, frequently adopted by English engineers, and other forms too numerous to mention. Some of them are of equal depth throughout, the top and bottom surfaces being parallel, while others vary in. depth, having one or sometimes both surfaces curved. To discuss all these various forms in detail would require a very large volume indeed far beyond the dimensions of the present brochure. All that can. be here attempted is to clearly state the leading principles of strong and economic construction, and indicate how they have been transgressed in times past. These are as follow : («) In beams or girders having continuous plate webs the material should be concentrated as far as possible from the neutral axis so as to give a maximum moment of resistance, and only sufficient material be left in the connecting web to enable the whole mass to act as one beam. (<5 ) The web should be sufficiently stiffened or reinforced so as to enable it to bear transverse compression at points of support, or of heavy isolated load. (c) In open web girders the structure should consist of a continuous series of triangles connecting the points of application, of external force, the sides of which should be perfectly straight, and the angles common, and which should be so arranged that determinate equations of equilibrium may be obtained for every angular point. (^) The vai’ious bars composing the frame should be propor- tioned to the stress they undergo, should be efficiently jointed, and if in compression should be of such a section as not to evade their duty by lateral bending or wrinkling. (e) In all forms of girder no unnecessary material should be used in any part, and the proportions and arrange- ments of parts should be such as to give the greatest possible strength for a given amount of material and workmanship.