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There seems to be a notion prevalent among the uninitiated (engineers too often included) that there is some inherent virtue in cantilever bridges which renders them superior to ordinary structures, in what particulars, however, the said uninitiated are not often able to state, although they generally claim that it is in economy.
This notion is entirely erroneous; for cantilever bridges are always inferior in rigidity to bridges of simple truss spans, and, excepting for certain peculiar conditions, are also always more expensive. These exceptional conditions are but two, viz., deep gorges to be crossed by single spans, and the impracticability of using false work because of danger from washout.
If there be assumed a river crossing of very great length, in which the bed-rock is approximately horizontal and where the conditions affecting erection are not unusually dangerous, there is no possible layout for a cantilever bridge which will be as inexpensive as a structure consisting of simple truss spans of equal length, provided that the said length be the most economic one possible. That this fact is not generally known is proved by the occasional building of a cantilever bridge in a place where the conditions do not call for one. For instance, there was no good reason whatsoever for making the great Poughkeepsie Bridge a cantilever structure, because by using the same number of piers and making all the spans alike the cost of the substructure would not have been at all increased, but probably diminished, while the weight of metal in the super-structure and towers would have been lessened materially. It is true there may have been a little saving in cost of false work,
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