the several proposed crossings would affect the total earnings of the structure; hence this feature should receive special attention.

The grade and alignment for any proposed crossing are factors that must be included in the economic study, because they affect the cost of handling the traffic; and the matter of right-of-way may prove an important consideration.

The The property damages involved by the approaches to the structure and to by the shifting of existing tracks would differ materially in cost at at the various possible crossings, hence this feature is one involving economics.

The choice between single-deck and double-deck entails a consideration of economics that may prove to be of some importance.

After determining the various kinds of traffic to take care of, there remains the economic problem of deciding upon the live loads for the various parts of the structure. If these are made too high, there is a waste of material involved, and the bridge enterprise will forever after be burdened with an unnecessarily large annual interest to be paid on that account; but if they are made much too low, the life of the structure will be curtailed. The saving clause, however, in respect to this adjustment is that, ordinarily, a steel bridge does not have to be removed because of overloading until the metal thereof is actually stressed at least fifty (50) per cent more than the permissible intensities of working stresses given in standard specifications for design.

This general problem of the proposed New Orleans bridge, while not so complicated as that of the proposed San Francisco-Harbor structure, is of an intricate nature, and will demand for its solution engineering ability and experience of the highest order.

CASE III

There is given in "Bridge Engineering" in the chapter on "Reports" on pp. 1575 to 1581, inclusive, an economic study for the replacement of a bridge over the Mississippi River, which illustrates some of the economic questions that arise in a consulting bridge engineer's practice. In that case the point at issue was whether it would be best to build a single-track or a double-track bridge or to arrange for the conversion at some future time of a single-track structure into a double-track one. Five methods of doing the latter were suggested, and the estimates of their total first costs were made; then, at an assumed rate of interest, a table was prepared showing the total cost of each structure plus compound interest thereon for periods of five years, up to the limit of forty years. That table indicates at a glance the comparative economics of all five methods at any of the five-year periods. A diagram prepared from the said table, of course, would be preferable, as it would show more readily the comparison at any intermediate period.