In determining the economic span-lengths, computations were made for the volumes of concrete in shafts, volumes of caissons, volumes of cribs, total lengths of piles below crib bases, and weights of metal in spans, but no notice was taken of the cost of flooring, as that is a constant for any type of bridge.

It might be well to mention that while the abscissae, of the diagrams give the span lengths measured from center to center of end pins, the costs of structure per lineal foot were computed by using the distance from center to center of piers.

In making each of these cost estimates there was assumed a structure of indefinitely great length and unvarying profile, so that the sum of the cost of the steel work in a span and the cost of a complete pier divided by the horizontal distance between adjacent-pier centers gives the comparing cost per lineal foot of structure, although, as before indicated, not the complete cost thereof.

The results of all calculations made were plotted on cross-section diagrams, but only a few thereof have been reproduced herein. However, the important deductions from all the estimates have been tabulated. The plotting was done with the utmost care, and due consideration was given to a proper determination of the economic span-length. As previously indicated, a number of arithmetical errors were located and corrected by reason of irregularities in the curves, thus making the latter truly reliable. In almost all cases, at least four points were plotted from computations, in order to locate the curves of cost for substructure and for the steelwork of superstructure; and a combination of these was used for locating a few intermediate points on the curve which gives the combined cost of substructure and steelwork. In a few instances, though, three points for the lower curves were found to be sufficient for a correct plotting of the upper curve.

Recording Diagrams and Table

On Figs. 18d to 18g, inclusive, are graphically recorded specimen diagrams of the results of the special calculations. Each diagram contains three curves, one for substructure, one for steelwork in superstructure, and the other for a combination of these two. The computed cost points therefor are marked on the three curves, respectively, by circles, squares, and diamonds. The abscissae of these diagrams give the span-lengths in feet, measuring from center to center of bearings; and the ordinates record the cost per lineal foot, measuring from center to center of piers. On each diagram is clearly indicated the span-length for greatest economy; and it is to be noticed by the flatness of the upper curves that a variation of twenty-five feet  or  more,  either  above  or  below  the  economic  length,  will  make  very