"In consequence, where it is desired in small and me dium spans that no tensile stresses shall exist, or, in other words, where only mass concrete work is employed, the superstructure cannot be kept too light. A light super structure is then justifiable only when demanded for architectural reasons, or when it is necessary to impose as little weight as possible on the foundations. . . .
"The arches can be constructed as restrained, or as three-hinged, by using suitable material. . . ." Prof. Morsch finally remarks that arched bridges with a proper arrangement of reinforced concrete and of large span, can compete successfully with steel construction.
The governing bodies of municipalities, counties, com panies, etc., are altogether too prone to accept the lowest bid in a competition, irrespective of the qualifications of the bidder. Many inexperienced men have submitted figures on work which they were unable to compass, when put to the actual test, often losing heavily, but being com pelled for business reasons to shoulder the loss and say nothing. In other and rarer cases, the work has suffered to a dangerous degree, and the municipality or other body for which the bridge was erected has lost much more through costs of repairs, than the difference between the lowest actual and the lowest responsible bid. Such bodies will usually save money in the long run by retain ing the services of an expert concrete engineer, who will either prepare a single design upon which all bidders c-an base their figures, or prepare specifications calling for detailed designs accompanying each bid, with the privi lege retained of accepting any bid which is considered as best meeting the requirements of the case. A waiver should also be exacted of each bidder, concerning any claim whatsoever for himself or any other if his bid is not accepted.
All the costs contained in Table IV are first costs, or the initial expenditure, which is almost invariably financed by an issue of bonds by the municipality, town ship, or corporation undertaking the erection of the struc ture. These bonds usually carry about four per cent in terest, and run for a period of from twenty to fifty years, provision for their retirement being made by the estab lishment of a sinking fund in which is placed a small yearly percentage raised by taxation or derived from income. The gross amount of these two annual obliga tions thus usually amounts to between six and ten per cent of the cost of the structure. This condition is the
same, irrespective of its type, the only point being the reduction of this yearly expenditure to a minimum by cutting down the first cost as much as possible, and ex tending the life of the bonds as much as advisable so as to reduce the annual amortization charge. Obviously, the bond period should not extend over a greater period than the life of the structure.
The question then immediately arises as to the prob able relative and actual lives of various types of bridges. Stone arch bridges have been in existence from the times of antiquity, but the advocates of steel structures will say "they were not built of concrete." In the south of France is a concrete arch bridge known as the Pont du Gard, which was erected in the year 56 B. C. The con crete in this was not composed of crushed stone or other small aggregate of the variety now employed in concrete bridge work, but was of the old style, consisting of alter nate layers of large and small stones, gravel, etc., and of cementitious materials. Vitruvius describes the materials and methods in use before the Christian era, and other writers like Alberti in 1485 and Palladio in 1570 accu rately describe the method which "the ancients" (as they call them) employed, "of using boards laid on edge and filling the space between with cement and all sorts of small and large stones mingled together." It is very improbable that the Pont du Gard would have withstood the rigors of climate of the Northern United States, but its actual state of preservation, as well as that of many other specimens of ancient concrete work, proves that if modern work is honestly executed, it will many times outlast any reasonable bond period, so that a very small yearly sinking fund per cent is all that is required for properly designed and erected concrete work. . . .
To steel or wooden structures, on the other hand, only a relatively short life can be assigned at best. Wood will be dismissed without further comment, except to remark that, in some parts of the West where wood is exceed ingly cheap, and where steel and cement can be obtained only after being transported very long distances, so that their costs are very high (from the heavy freight rates made necessary), it may be possible that wooden struc tures involve the smallest yearly charge, in spite of their very short life, even when treated with creosote or other preservatives.