On Some Common Errors in Iron Bridge Design

36 To carry out this method, diagrams of one of the simplest of which Fig. 38 is a greatly reduced copy, have been made, and. from them the following results have been obtained :— The smallest secondary stress so far as he has gone, the writer has found in the simple Warren girder, consisting of one series of equilateral triangles. In this girder, assuming that the transverse dimensions of the chords and web members to be equal, and their sections symmet- rical about the neutral axis, the secondary stress in the web members is about four times that in the chords. • Taking a Warren girder of 8 panels as shown in Fig. 38, of 100 feet span, with all members 1 foot wide, stressed to 6 tons per square inch with ordinary provision for partial loading, and a modulus of elasticity of 26,000,000, the secondary stress in the chords was 8 per cent, of the primary, and in the web members 30 per cent, of the primary. Any reduction in the width of any bar (as measured in the plane of the frame) without altering its sectional area, reduces the secondary stress in the same propor- tion. Hence it is desirable to keep the web members of such girders as narrow as other considerations will permit. The usual trough or T section employed for chords is subject to only about half of the increase of stress, that a rectangular or other section symmetrical above and below experiences. This is due to the fact that the neutral axis lies so much closer to the side where the primary and secondary stresses are additive. Ihe N girder or that with alternate vertical and diagonal web members is subject to considerably more secondary stress than the Warren. of equal dimensions and width of bars, especially in. the- vertical members. It also shows an intensification of secondary stress at the midspan if made with an even number of panels. At each of these points the secondary stress is at least 50 per cent, greater than at the corresponding points of the Warren, girder. In the X girder, Fig. 23, the distribution of secondary stress is peculiarly complicated. That in the chords is variable, attaining maxima values at alternate panel points, and. vanishing, or nearly so, at the other alternate panel points, its maximum value being about double what it is on the corresponding Warren, girder. That in. the diagonals is about equal to what exists in