The comparison of the types is complicated also by the matter of personal equation in designing; for, with the most elaborate and rigid specifications ever written, two computers working independently on the same job would be likely to vary in their final results far more in reinforced-concrete work, than they would in steelwork. This is due partially to the fact that the science of reinforced-concrete bridge designing is newer and less highly developed than that of proportioning steel bridges, and also because in the former type equal strength can be secured with varying proportions of concrete and steel. As the most economical proportions are frequently not known, it is evident that the combined costs of the two materials in place may differ appreciably. Again, in arch bridges there is often a choice of ratio of rise to span or even of span-lengths; and as the effect on economics by variations in these features is not yet determined with accuracy, the final estimates of cost made by the two computers are liable to be still more widely divergent on this account.

The amount of attention paid to aesthetics when making the design generally affects the cost of a concrete bridge more than it does that of a steel one; hence this factor has to be given consideration when contrasting the two types in respect to the matter of economics.

The size of the live load, too, is likely to affect the comparison, because a diminution thereof cuts down the cost of a steel structure much more than it does that of a concrete one.

For short-span bridges, reinforced-concrete has an advantage over steel in respect to rigidity of structure; and under certain conditions such an advantage may be of importance, but ordinarily it is not.

There is another complication of the question, which, however, ought not to be allowed to exist, viz., the fact that many small reinforced-concrete bridges are designed by inexperienced and incompetent computers, who are hired by municipalities on account of the low compensation they are willing to accept.

Still another cause for divergence is the great variation in costs of excavation for foundations at different localities. This affects the substructure costs for reinforced-concrete arch-bridges far more than it does those for the corresponding steel structures, because of the larger footings and shafts of the former.

Is it then impracticable for a bridge engineer to settle quickly the comparative economics of the two types of construction for a proposed bridge? Not at all—only it will take more time than that required for the determination of most of the economic problems dealt with in this treatise. The method to be followed is this: Having obtained in advance all the unit prices for materials, figure the cost of the steel structure by means of the diagrams of quantities given in Chapters LV and LVI of "Bridge Engineering," then find that for the reinforced-concrete bridge by employing the rules, tables, and diagrams given for that purpose in the latter chapter, fixing by judgment, whenever necessary, an allowance for difference in