On Some Common Errors in Iron Bridge Design

which means a valuable gain in strength. There would be no difficulty and but little expense in altering these structures now, removing the cross pieces one or two at a time when the bridge was free from live load, and replacing them by square plates as in lig. 28. An even better but slightly more expensive arrange- ment would be to insert a complete triangulation similar to that shown for a different purpose in Fig. 8. lig. 29 represents a type of braced strut appearing in. the late Sir John Hawkshaw’s great railway bridge over the Thames at Charing Cross, Fig. 15, and copied thence extensively in Europe and in New South Wales. It requires but little consideration to detect the weakness of this arrangement. Suppose there is a tendency for the member to bend to one side, the rectangular panels will become rhomboidal, one diagonal being extended, and the other reduced in length. Now if these diagonals are straight they will oppose the maximum resistance to such distortion, but if bent or crooked, as in the Charing Cross Bridge, the extended one will tend to straighten, and the compressed one to become more crooked. Experiments have been macle at the University on different types of compression members, and have shown that great advantage ensues from replacing this unsatis- factory double system of crooked flat bars, by a single diagonal system of straight bars, as shown in Fig. 30. This improvement might easily be applied to actual structures, taking advantage of times when they are free from live load to remove the one and insert the other. Fig. 31 is a photograph of a number of experimental com- pression pieces tested at the University, arranged in order of merit. The best of these carried, in proportion to the metal it contained, rather more than twice the load that the worst did. These specimens were free to bend in their own plane, but braced at three points in a direction at .right angles to their own plane, and represented compression diagonals in closely latticed girders intersected and kept from bending in the plane of the main girder by the tension diagonals. The figures at the bottom indicate the ratio of the load carried, to the weight of the structure, and thus express the relative values of the various systems. The first and third of these models have angle bars and the second and fourth channel bars for their sides.