The settlement of the question as to which costs more, a steel bridge or a reinforced-concrete one of the same capacity, is a difficult task. For structures of ordinary span-lengths, under normal conditions of the material market, the first cost of the steel bridge is the smaller, but when the price of that metal takes a sudden jump the reverse is true. However, in a short time either the prices of concrete materials rise to correspond or else the cost of the metal gradually drops back until a normal ratio of unit prices for bridge materials once more exists.

But the question of the economics of the two types is not settled by a comparison of first costs alone, because the elements of maintenance and repairs must be considered; and these are much greater for the steel structure than for the reinforced-concrete one. The latter requires no painting, and there should be no renewal of parts called for, excepting only the pavements, while in the former these items are often large, especially that of painting, if the upkeep be properly performed. It is necessary, therefore, to capitalize the said items and add the results to the first costs, in which case under normal conditions there is generally but little difference; and as the painting of steel structures is very likely to be neglected and the metal, in consequence, to lose its normal areas, it will often pay to adopt the reinforced-concrete construction, even when the economic comparison indicates a slight disadvantage by so doing.

The fact that the annual cost of maintenance and repairs is less for reinforced-concrete bridges than for steel ones tends to render the former the more popular, especially amongst those persons who do not make a practice of comparing values strictly upon the basis of the principles of true economy. Such persons, too, are prone to say that the reinforced-concrete structure is superior to the steel one because of its longer life, not recognizing the facts that a properly designed, built, and cared-for steel bridge will do its work for a very long term of years, and that the longevity of reinforced-concrete structures as yet is a matter of surmise, in view of the uncertainty about the efficiency of the concrete in protecting the reinforcing steel perpetually against rusting-which, if allowed to continue, will quickly and inevitably disintegrate the said concrete and destroy the structure.

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