some important bridge studies which he has been called upon to make in his practice, that there is needed by the profession a systematic investigation to determine in an authentic manner the economic span-lengths for simple-truss bridges to support the different kinds of live loads by piers resting on various types of foundation at all practicable depths, and to conform to changing market-prices for materials in place.

In connection with the series of economic studies on bridge design which the author has been making, especially of late years, and which he hopes to complete before he passes on, this question had to be settled sooner or later, consequently he has just spent three weeks in computing the actual costs of both substructure and superstructure for over two hundred cases of bridge layouts covering the following combinations:

Railway, Highway, and Combined-Railway-and-Highway Bridges on Concrete Pier-Shafts overlying Caissons or Cribs resting on Sand, Bed Rock, or Piles, and reaching to depths below low water of 50, 100, 150, 200, and 250 feet; also for low, medium, and high conditions of the material market.

The fact that all the computations were prepared by the author alone, and without a detailed check on the figuring, need not cause any doubt about the correctness of the results of his work, because all of them were plotted on cross-section diagrams, and, consequently, whenever any error of the least importance was made it was detected at once.

This investigation owes its existence to the fact that recently the author as a member of the Board of Advisory Engineers to the Public Belt Railroad Commission of New Orleans (appointed to study the question of bridging or tunneling the Mississippi River at or near that city), had occasion to make a large number of layouts with cost estimates for railway, highway, and combined-railway-and-highway bridges having sand foundations two hundred and fifty feet below the Gulf level. While the conditions precedent for those computations were used for certain of the layouts of this investigation, the actual results thereof were not incorporated, because all the calculations involved in this paper were special and had to be systematized. However, there were numerous deductions made from the New Orleans Bridge studies, which permitted the adoption of valuable short cuts in figuring.

A large portion of the data employed in making estimates of cost was taken from the various diagrams given in "Bridge Engineering," including live loads, impact, and weights of metal.

The following are the assumptions and conditions precedent adopted for the series of calculations:

Character of Structures

The different classes of bridges covered are Double-Track-Railway, Single-Track-Railway, Standard-Highway, and Combined Double-Track-