Turning to steel then as the only real competitor of concrete, a remark ascribed to the late George S. Morison is often repeated, that, "Steel bridges are built to last fifty years, but they do usually last hardly half that time." This condition is due to insufficient or entire absence of maintenance. The writer has been told of one high way bridge in Wisconsin which had to be replaced after only eight years of service. Of course this is an excep tional case. Steel bridges over railroad tracks where loco motive gases can produce their full destructive effect, have been known to last just about the same period. Morsch cites an example of a girder erected in 1886, and found with its web entirely eaten away at many points, in 1907. . . .
Where highway bridges are properly maintained, even when subjected to the action of locomotive gases, they have been known to last much nearer the theoretical limit assigned them by Mr. Morison. . . . With careful inspection and maintenance there is really no reason why a well-constructed steel bridge should not last indefinitely, and such structures as Brooklyn Bridge (built in 1884), or the Niagara arches, may be expected to see many generations.
In the "Technograph" of the University of Illinois, for 1893, Mr. Albert F. Robinson, Engineer of Bridges and Buildings of the C., R. I. & P. Ry., states the princi ple that the life of steel railroad bridges is only fifteen years, because they must all be rebuilt to accommodate heavier traffic at intervals of about that length.
A conservative deduction from the above-mentioned and many other observations, would lead to the conclu sion that ordinary highway bridges should not be assigned an average life exceeding twenty years at most ; and fif teen would be a really better period to which to limit the life of steel highway bridge bonds. Railroad bridges usually receive better care, and their life may be con sidered as somewhat longer, except that from time to time they must be strengthened in order to make them safe for increased loads.
Mr. Robinson, in the paper cited above, presents esti mates of the costs of strengthening single-track struc tures of different lengths, assumed to have been erected about 1882 in accordance with the standard specifications of the Erie Railroad, and so rebuilt as to conform to the standard specifications of the Lehigh Valley Railroad. In condensed form they are as follows: These figures reveal the average percentage of cost of renewal to be 43.6. This is in addition to the regular maintenance costs, and is equivalent to about 3 per cent per annum, corresponding to a life of about thirty-five years for the original structure, computed on this purely theoretical basis.
In discussing this subject it is very difficult to sepa rate the annual depreciation from the maintenance costs. Lumping all such items together, the New York Central assumes 5 per cent per annum as the amount on which to base all maintenance and depreciation charges, when contracting with municipalities concerning railroad and highway crossing structures. Since this figure is to cover all charges of whatsoever kind, it is necessary to assign somewhat more than this figure, or about thirty years, as the assumed life of the bridge, and thus a somewhat arbitrary but fairly satisfactory rate of depreciation can be assumed for all steel bridges. On this basis, at least 5 per cent should always be assumed as the yearly amorti zation rate for such structures, against not to exceed 2 per cent, corresponding with a bond period of fifty years, for concrete ones, irrespective of first cost.
Inspection and Maintenance. Turning now to the sub ject of maintenance, pure and simple, nothing has yet been written better than the Preliminary Report of the Committee on Iron and Steel Structures of the Ameri can Railway Engineering and Maintenance of Way Association: "The maintenance of a metal bridge consists in the prevention of deterioration by rust and wear, the repair ing of all damage resulting from internal or external causes, and the necessary strengthening of the structure if called upon to carry loads heavier than those for which it was designed.
"To attain these ends, the structure must be kept free from rust by proper cleaning and painting; motion be tween the various parts must be reduced to a minimum; expansion joints must be kept working freely; all defects and injuries must be promptly discovered and repaired; and its capacity determined as accurately as possible.
"The purpose of an inspection is to ascertain the con dition and safety of the structure, and to determine the amount and rate of deterioration and the necessity for repairs or renewal. Ordinarily a thorough annual in spection made by a competent inspector will suffice for most cases where the bridge is properly constructed and is not overloaded. On the other hand, nothing short of a daily inspection will be sufficient to discover with suffi cient promptness the damage caused by an accidental blow.