On Some Common Errors in Iron Bridge Design

17 2. To check the tendency of the web to buckle under the diagonal compression that pervades it, being most intense near supports and least midway or there- abouts between, them. To comply with the former condition we need a .maguire, mas- sive vertical pillar at the encl of every ordinary girder, and a .still more, massive one at the piers of continuous girders or those with overhanging ends. Strange to say, however, these verticals are often seen inserted when there is no need for them, and omitted where there is the maximum stress. A notable instance is to be seen in a small continuous three span bridge on the North-Eastern Railway about two miles from Melbourne. Should a very heavy concentrated load, as for example a large ■column in a building, be imposed at any particular point, this column should be continued as a vertical stiffener to the bottom of the girder. This case does not however often occur in bridge- work. These stiffeners are usually made of uniform section from top to bottom of the girder. This may be justified by convenience •of construction, but is not required for strength. As we pass from the bottom to the top of the girder at a point of support, or from the top to the bottom at a point of concentrated top load, the compression in the vertical gradually discharges itself into the web in the form of a shear, or its equivalent, a set of diagonal compressions and tensions, and thus dies away. Hence the vertical at a support should be of the full section required by the reaction of that support at the bottom and diminish to nothing at the top, and that at a concentrated top load of full section for the load at top diminishing to nothing at the bottom. In large girders this fact may be macle use of to save material. The other use of verticals in plate girders is to prevent the web from buckling oi’ being thrown into waves by the diagonal compression due to the shear. Rankine in his “ Civil Engineering” treats the web as a long ■column tending to buckle under the diagonal compression, and measures the length on an angle of 45 deg. between top and bottom chords, or between vertical stiffeners, whichever happens to be the smaller. He then applies the excessively high safety factor of six. There are two most serious errors in this treat- 2a