On Some Common Errors in Iron Bridge Design

25 corrugation of the thin metal. Figure 21 shows a full size sample of bicycle tube that has been crushed in the University testing machine, its original dimensions being shown by dotted lines—the metal is -035 thick and the ultimate load 4400 lbs. Unfortunately the circular tube is very unsuitable for the convenient attachment of other parts, and consequently has been generally rejected by the designers of framed structures, although the great Forth Bridge and the modern bicycle frame are instances to the contrary. A further reason for rejecting the circular tube is, in structures of ordinary size, the inaccessibility of the interior for purposes of inspection, clean- ing, painting, and repair. To describe and discuss all the sections that have used for .compression members for chord and web purposes would extend this paper far beyond permissible limits. It must therefore suffice to enunciate general conditions to be complied with, and point out instances of conspicuous transgression of these conditions. (a) A good compression section should have a large radius of gyration in every direction. ($) If the column is prevented from bending in one plane by the attachment of other parts, and is not so pre- vented, oi’ not sö effectively prevented from bending- in a plane at right angles to the first, it should, if its radius of gyration varies, be placed with its maximum radius of gyration in the second plane. (f) Thin unsupported edges should be avoided, or, if unavoid- able, should not be counted as part of the effective section, as they are very Hable to buckle. (d?) Flat surfaces should not be made too thin in proportion to their width. A proportion of 1 to 30 is quite small enough. If this be passed the central part of the flat face becomes of little value for resisting compression. (^) If two compression members be connected together with the object of preventing their bending in the plane of the connecting pieces, those pieces should be arranged so as to constitute an efficient web system, forming with the two compression pieces a complete girder designed for resisting bending.