The economics of bridge design are not so greatly affected by variations in prices of labor and materials as is commonly supposed, because there is a tendency for all prices to rise and fall more or less uniformly. If they were to do so exactly, the effect on the economics would be absolutely nil. It is only when the variations in unit prices for the component materials are irregular that the economics of design are affected, and then in most cases but slightly. It is true that when there is a sudden rise or drop in the cost of superstructure metal erected, the proportionate change in substructure prices will lag behind; but it is generally not long before a state of comparative equilibrium is reached, and the variations in prices become more nearly uniform as compared with those that existed before the change occurred.

So far as economic span-lengths for simple-span bridges are concerned, the practical effect is small indeed; for, even though the theoretic economic length be considerably changed, there is always a rather wide range on either side of the minimum where the costs are but little higher.

Of more importance than this is the effect upon the relative economics of different types of structures, especially those of different materials such as steel and reinforced concrete. In normal times the base price of structural metal fluctuates from about 1.15 cents per pound to about 1.75 cents—it even ran up to 2.25 cents in 1899 and 1900. The cost of fabricated structural metal is subject to still greater changes. Freight rates in different portions of the country vary from about 0.1 cent a pound to over one cent. Local conditions may affect transportation and erection costs materially. For the foregoing reasons the price of structural metal erected is subject to a variation of about two cents a pound, entirely apart from causes influencing the prices of other materials. For instance, in January of 1920, the price of cement was $2.80 a barrel in New York and $2.43 a barrel in San Francisco, while the freight rate on structural metal was 0.27 cent a pound to the former point and 1.25 cents to the latter. At the same time, heavy construction timber cost nearly twice as much in New York as in San Francisco; and while structural iron workers received only 8 per cent more in the former than in the latter, common labor was paid nearly 50 per cent more. Sand, stone, and gravel were considerably cheaper in San Francisco than in New York.  At that period,  therefore,  structural  metal  was  about  one  cent

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