This question was investigated very exhaustively, considering every item of expense, including not only the cost of metal in place, but also that of concrete, excavation, back-filling, and pavement; also the possibility of expense for the moving of water pipes and other conduits. The investigation showed that, for plate-girder construction through private property, the economic span-length is about 40 ft. for intermediate spans and 23 ft. for tower spans; while for construction in the street, where towers are inadmissible, it varies from 47 to 50 ft., or even 3 or 4 ft. more, in case of cross-girders spanning wide streets from curb to curb.

The theory of true economy in elevated railroad designing, as far as length of bays is concerned, is simply this: "The cost of the longitudinal girders should be, as nearly as may be, equal to the cost of the bents and their supporting pedestals, in cases of doubt adopting the longer span."

Four-Column versus Two-Column Structures

Detailed estimates of cost show that, as far as economy is concerned, there is but little, if any, difference between these two styles of bent. Whether the total cost of the four-column bent will exceed that of the two-column one for a four-track structure depends upon the various schedule prices for metal, concrete, excavation, paving, etc., as well as upon the character of the soil. As there is no great difference in the cost of these two types of structure, and as the four-column bent is decidedly the more rigid of the two, it was adopted wherever practicable. Cantilevering an entire train load beyond the exterior column of a two-column bent is not conducive to rigidity, but this is the only method that will bring the cost as low as that of the four-column bent.

Braced Towers versus Solitary Columns

Where an elevated railroad occupies private property and crosses the streets by spanning from curb to curb, it is practicable to use braced towers and thus stiffen the structure and check vibration; and, moreover, this arrangement is very economical.

For the Northwestern Elevated, upon which it is proposed to run trains at a speed of 40 miles per hour on the inner tracks between the inter-track stations, situated about a mile apart, the consideration of the extra rigidity afforded by the braced towers is quite important. It was therefore, decided to use both longitudinal and transverse sway bracing, forming braced towers spaced about 150 ft. apart (or two towers per block), and to use the transverse sway bracing in all bents on curves, wherever practicable.

Two only of the three spaces between columns have transverse sway