CHAPTER XV.
LIFE OF BRIDGES—RELATIVE MERITS.
The life of bridges of differing materials has been incidentally touched upon by the examples quoted, in dealing with each class of structure. It will be useful to recapitulate some of the facts adduced, and to compare the terms of life so far as they appear to be indicated; but in doing this it is necessary to remember that the life of a bridge of any one material is inseparably connected with its own private history. The duration of any such structure may be limited by adverse conditions, peculiar to the case considered, by defects of design, material, or workmanship—present from the first—or by neglect, overloading, or accident, making up its later record.
With the exception of timber structures, it is difficult to find any class of bridges furnishing examples which have reached the limit of life, independently of the evils named, and as a result of unavoidable decrepitude. There are none the less influences at work tending to this condition, and which it is too much to expect can in all cases be foreseen or completely guarded against, such as the shifting or scouring of river-beds, settlement of foundations, natural decay, and minor faults in design, which even in the most capable hands may be expected ever to fall short of perfection. At the best, then, the life of any structure, though long, must have a limit. With bridges of more average or inferior qualities the life may be positively short, even without the destructive influence of overloading.
Dealing with instances of metallic bridges, the adjacent table gives the time each had been in existence when removed, and some indication of the reason for its condemnation. Those marked with an asterisk were cases of pronounced high stress. From a study of the table it appears that in actual practice, making no excuses of any sort, the length of life of the wrought-iron bridges specified varied between twelve and thirty-six years; but these figures applied to this collection of cases only. It is to be remarked that many other bridges outlasted these, and are likely to continue reliable. These results show, then, no more than that some wrought-iron bridges are short-lived, having, in fact, been selected as examples of this. Longer-lived exceptions are useful, as indicating that the durability of such structures is by no means so limited as the table would suggest. It is to be observed that, as design and maintenance are now better and more generally understood than when experience was largely wanting, it is to be expected that later examples will show no such poor results.
Of steel bridges little can be said, because of the limited time this material has been in use; but the generally acknowledged belief, quite in agreement with the author’s observation, that steel rusts more freely than wrought iron, suggests that such bridges will have a shorter lease of life, the more so that the surface-to-section ratio is also greater for higher unit stresses, though other adverse influences are much the same for one material as for the other.
Of cast-iron structures but few cases have been given; of these, cast-iron arches have been noticed as developing defects which led to reconstruction, or to limiting the loads to be carried. Plain cast-iron girders, on the other hand, have never, under the author’s direct observation, been removed for any other reason than because they were cast iron, or from over-stress, due to the growth of loads; never from defects or wasting, though it is not suggested no such cases exist. The author has no evidence which points to what may be the limit of life of a good cast-iron girder fairly treated.
Examples of Life of Metallic Bridges.
| Description. | Span. | Age. | Defect. | Reference. | |
|---|---|---|---|---|---|
| ft. | in. | Years. | |||
| Wrought Iron. | |||||
| *Plate girders | (?) | 12 | Loose rivets | ||
| *Ditto | 35 | 0 | 12 | Ditto | [p. 52] |
| *Ditto | 55 | 0 | 14 | Rust. Distortion | [pp. 78] & [97] |
| *Trough girders | 11 | 0 | 16 | Loose rivets. Cracked webs | [p. 50] |
| *Plate girders | (?) | 22 | Loose rivets | ||
| *Twin girders | 31 | 6 | 23 | Weak. Cracked webs | [p. 13] |
| *Ditto | 35 | 6 | 23 | Weak. Distorted. | [p. 74] |
| *Plate girders | 42 | 0 | 23 | Loose rivets. Cracked webs | [p. 21] |
| *Ditto | 72 | 0 | 29 | Weak. Loose rivets | [p. 53] |
| *Ditto | 47 | 0 | 24 | Distortion | [p. 9] |
| *Ditto | 32 | 0 | 32 | Rust. Cracked webs | [p. 14] |
| *Ditto | 25 | 0 | 36 | Weak | [p. 63] |
| Steel. | |||||
| *Trough girders | 15 | 8 | 32 | Weak. Rusted | [pp. 68] & [98] |
| Cast Iron. | |||||
| *Girders | 32 | 0 | 36 | Weak | [p. 141] |
| *Girders, cast-iron piles | (?) | 44 | Ditto | ||
| *Arches | 45 | 0 | 55 | Crack. Settlement | [p. 145] |
| *Ditto | 100 | 0 | 62 | Crack. Deformation | [pp. 80] & [145] |
With timber bridges the length of life appears to be about twenty-five years, but this is very largely dependent upon the question of maintenance, and may range from fifteen to thirty-five years. It is manifest that repairs, when extensive and consisting of the renewal of the more essential parts of the structure, border upon reconstruction, and may be continued indefinitely. The length of life in ordinary cases, and for the timbers commonly used in this country, may, for railway bridges, be taken as stated, though for highway bridges possibly longer.