Some years ago, engineering thought tended toward staffcally determinate structures--toward three-hinged rather than toward hingeless arches.
Today, that trend is largely reversed. Study of indeterminate structures by the use of models has grown rapidly. In line with this advance and contributing to it, the model arches in the Department of Railroad Engineering in the Franklin Institute Museum, being built to full size and of structural materials, should play a helpful part.
Under one rail, the hingeless concrete arch spans fifty-five feet, and in line with it, the hingeless steel arch spans fifty-two feet. Under the other rail, the three-hinged concrete arch spans twenty-two feet, the one-hinged concrete arch spans twenty-six feet and the steel truss-girder spans fifty-eight feet.
The hinges of the three-hinged arch and of the one onehinged arch have their pins resting in pedestals which are built of steel slabs, of stepped-up sizes, welded together. This is one of the first times that such pedestals have been used.
The live load on the arches consists of a modern locomotive, Baldwin No. 60,000. Its weight, complete with fuel and water and including the tender, is 7Ol,OOO pounds. Its weight as at present, without fuel and water is 532,000 pounds. Instruments measure the deflections at various points as the load moves slowly along the arches. These measurements allow comparisons between the actual and the theoretical for a live load movement of sixteen feet.
The dimensions, the properties, and the influence lines for all of the arch ribs are given on diagrams in a large frame beside the arches. They allow ready computation of the theoretical deflections.