the conclusion that ordinary differences of unit prices will not affect the economic layouts for simple-truss spans having piers on sand foundations. Similarly it might be shown that the conclusion holds good for any other type of pier foundation.

There is, however, a case in the late practice of the author which indicates that abnormally high variations (for the different component materials of bridges) in the changes in unit prices do sometimes affect materially the economics. At the present time the following pound prices for metal erected in suspension bridges prevail:

Wire Cables........... 23¢ per lb.

Carbon Steel..............8¢ per lb.

Nickel Steel.............10¢ per lb.

A fair average for the ante-bellum unit prices of these materials is respectively 13 1/2, 5, and 7 cents. Using the averages for the two kinds of structural steel as representative market prices, and applying their ratio to the pre-war price of wire cables indicates that a proper present price for the latter in place would be 20 cents. This shows that these cables are about 15 per cent too expensive. As explained in Chapter XIII, the use of existing unit prices instead of ante bellum ones raised the span-length of equal cost for highway cantilever and suspension bridges from 1,000 ft. to 1,200 ft., that length for like structures which carry a certain combination of highway and railway loadings from 2,190 ft. to 2,360 ft., and that length for similar bridges subjected to only steam-railway loading from 2,570 ft. to 2,630 ft. These changes are of some importance, hence one must conclude that the economics of certain types of bridges are materially affected by abnormally great variations in the ratios of rise or fall in the unit prices of their component constituents.

The location of a bridge may affect to a certain extent its economic layout, especially for American constructions in foreign countries. For instance, the freight and customs duties on superstructure metal might be very high and the importation of skilled erection-workmen might be a necessity, while the prices of substructure materials and of unskilled labor therefor might be exceedingly low; and with such a combination of conditions the economic span-lengths would be considerably shorter than those governing in the United States.