On Some Common Errors in Iron Bridge Design

38 web members was, other things being equal, about four times that in the chords, this means a distinct gain. 3. By virtue of resistance to secondary stress and entirely apart from primary stress, a certain small portion of the load is carried. 4. It is doubtful if ever the tightest rivetted joints are absolutely rigid, and any microscopic yielding or adaptation in them tends to reduce secondary stress. In order to provide for secondary as well as primary stress, with economy it is recommended, in new designs. (rt) That other things being equal or nearly so, preference be given to the Warren or equilateral type of girder, with a single system of triangulation. (/>) That chords of trough or T section be preferred to those that are symmetrical about their horizontal neutral axis. (<r) That all members be kept as narrow in the plane of the girder as other conditions permit. (</) That the unit stresses in the web system be made about 10 per cent, less than those adopted in the chords. (< ?) That girders of ample depth, say not less than of the span, be preferred to shallower ones. (/) That those portions of continuous girders immediately above non-terminal supports be made about 15 per cent, more massive than the corresponding parts of ordinary girders. This extra strength to taper away till it disappears at the point of contraflexure. Existing bridges should have their stresses determined by the method previously described, and where the secondary stress and .primary stress together exceed the usually permitted primary stress by more than 30 per cent., either the load should be reduced or the weak part strengthened. This strengthening must be made without increasing the breadth of the bar. Girders of the N type with broad web members, such as that shown in Fig. 17, where the diagonals have a width of one-eighth of their length and are very firmly rivetted, should be looked upon with the greatest suspicion and taken in hand first, also continuous girders of all types.