On Some Common Errors in Iron Bridge Design

37 the Warren girder if the diagonals are not connected at the points of intersection. If they are it is increased about 50 per cent. At the root of a cantilever or over an. intermediate support in a continuous girder, the secondary stresses, both in chords and web members, attain a value nearly twice as great as those in a girder simply supported at the ends and designed to carry distributed moving loads, such as ordinary road and railway bridges are subjected to. This is due to the fact that both chords and web members are stressed most heavily at the same time, which is not the case at the centre of an ordinary discontinuous girder the maximum stress conies on the chords under full load, and on the web members under a load extending from one end to the centre. The foregoing results, while very far removed from the alarming statements of Bender, nevertheless err on the side of pessimism. The subjoined facts all indicate certain, sources of relief from secondary stress, which neither Ritter’s nor the author’s methods of investigation take account of. 1. The full stress of 5 or 6 tons per square inch is not main- tained throughout any structure. Many parts are, for convenience, to obviate the use of too many differ- ent sections of metal, or to facilitate jointing, made 10, 20, or even, sometimes 50 per cent, more massive than calculation requires. Hence the deformation of the structure is reduced below what it would be if more closely designed. This tends to reduce secondary stress. 2. In both Ritter’s and the author’s methods it is assumed that the chord sections are so much more massive than those of the web members as to completely over- power them, and compel them to accommodate their direction to that of the chords. This is by no means absolutely true. Assuming as a fair average that the web members are one-fourth as stiff as the chords, there will be a rotation of the joints, tending to reduce the secondary stresses in the web members by 25 per cent, and increase those in the chords. As the previously determined secondary stress in the