| ECONOMICS OF CANTILEVER AND SUSPENSION BRIDGES | 93 |
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drawn through less than three located points, and in many cases the number was four or more. The regularity of all the curves proves that there was no error of any magnitude in the figuring which located the points thereof. It does not mean, however, that the author's calculations contained no errors. Unfortunately, several mistakes crept into the work, but the plotting invariably pointed them out and led quickly to their satisfactory correction. In the diagrams the abscissae represent main-span lengths in feet, and the ordinates show the total costs of structures in dollars, the recorded units being millions. In figuring the weights of stiffening trusses for suspension bridges, the author made an important modification in one of the formulae given in Chapter XXVII of "Bridge Engineering."* Equation 15 thereof has been employed without change, when wind stresses are ignored; but the following formula for the weight per foot of both trusses has been added to cover the case where the effect of the wind load is considered:
The corresponding equation when the wind stresses are ignored is:
The greater of the two values of T given by these equations is, of course, the one to use in the estimate of total weight of metal. The division of total metal weight between carbon steel and nickel steel was made by the author's judgment, based upon the curves in his two before-mentioned papers and upon the assumptions of material distribution adopted when preparing the suspension-bridge computations. No error of any magnitude exists because of this assumed distribution, although, of course, the method employed is only approximately correct. Whenever a proper weight curve for cantilever structures was not available, the author fell back upon the general curves for weights of metal in trusses and laterals that record the various double-panel weights in cantilever arms and anchor arms as multiples of the corresponding double-panel weight of the suspended span, which general curves were first given on Plate X of "De Pontibus," and afterwards were reproduced in Fig. 25j of "Bridge Engineering." In establishing the general assumptions for the layouts of both cantilever and suspension bridges, with one exception they were made as favorable as possible for each type, that exception being that, for the sake of appearance, the anchor arms of each cantilever structure were made of the * This modification has lately been incorporated in the third thousand of that treatise, now on sale. |
