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. |p. 50
| | |Cracked webs |
Plate girders | (?) | 22 |Loose rivets |
Twin girders | 31 6 | 23 |Weak. Cracked |p. 13
| | |webs |
Ditto | 35 6 | 23 |Weak. Distorted.|p. 74
Plate girders | 42 0 | 23 |Loose rivets. |p. 21
| | |Cracked webs |
Ditto | 72 0 | 29 |Weak. Loose |p. 53
| | |rivets |
Ditto | 47 0 | 24 |Distortion |p. 9
Ditto | 32 0 | 32 |Rust. Cracked |p. 14
| | |webs |
*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. Settle- |p. 145
| | |ment |
Ditto |100 0 | 62 |Crack. Deforma- |pp. 80 & 145
| | |tion |
-------------------------+-------+------+----------------+------------
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.
Of masonry bridges little is to be said but that it is only in cases of
bad work or material--with, perhaps, vibration or settlement--that these
have a shortness of life comparable with that of defective metallic
bridges. Where these adverse conditions obtain, heavy repairs may be
necessary before the structure is many years old; but, under reasonably
fair conditions, bridges of masonry may be expected to outlast
structures in any other material. Apart from road-bridges which are
admittedly long-lived, there are a large number of railway bridges and
viaducts of masonry which, despite heavy loads and vibration, have been
in use for the past seventy years.
Dealing with the cost of maintenance, this with bridges of wrought iron
or steel should result simply from scraping and painting, with such
other incidental work as may be necessary on the subsidiary materials
used in the structure. The cost of painting will vary with the height
and character of the bridge, and the amount of scaffolding, if any, and
may be from 5_d_. to 1_s_. or more per square yard; this if distributed
over five years, a not unusual interval between each painting, works out
at an appreciable figure, which may vary from one-third to one per cent.
of the first cost, per annum. The yearly cost of painting steel-work
will, for shorter intervals, come to a somewhat higher figure. Serious
occasional items of expense are those which should not be necessary,
repairs and possibly strengthening, which may raise the total cost of
maintenance very considerably.
Cast-iron bridges, being less liable to rust, cost less for painting
than other metallic bridges; and if the cast iron is closed in by
masonry, practically nothing; they do, indeed, involve very little
expenditure in the maintenance. Not being very amenable to repair or
strengthening, cast-iron bridges commonly remain very much as built, or
are reconstructed.
The proper care of timber bridges may become costly as the structure
gains in age, and soon grow to a very wasteful expenditure. This is
evident when it is considered that repairs may be necessary after ten
years, and that whatever may have been the cost of any part when new, it
cannot be replaced for the same amount, having regard to the labour
expended in removing the old member, and the special precautions to be
observed in dealing with an old structure carrying its load. In addition
to ordinary repairs, there will be paint or other protective coating to
be applied, though this is not always done.
The upkeep charges of masonry bridges will be practically nothing in
favourable cases; but, on the other hand, where extensive repairs become
necessary, may reach a considerable amount. Exceptional outlays are,
however, infrequent, and may be spread over a large number of years, in
those rare instances in which they become imperative.
_Durability._ _Maintenance _First Cost._
Charges._
Masonry Masonry Timber
Cast Iron Cast iron Masonry
Wrought iron Wrought iron Steel
Steel Steel Cast iron
Timber Timber Wrought iron
For purposes of ready comparison, placing bridges of the materials under
review in order of durability, they would appear as in column 1 of the
table above; in order of low maintenance charges, generally as in column
2; and in order of low first cost, as in column 3. With respect to the
question of first cost, the arrangement of the third column applies only
to small bridges, say, up to 70-foot span; and, being liable to
variation with the conditions, is but approximately correct. The less
costly descriptions of masonry are alone considered in this connection.
It may be added that the total yearly charge of interest on first cost,
redemption, and maintenance, appears to be for masonry bridges, about
one-half only of the corresponding totals for bridges of wrought iron,
steel, or timber; those of cast iron taking an intermediate place.
Summarising the above considerations, and dealing with the relative
merits of bridges in the different materials, it may be broadly stated
that for conditions at all suitable nothing seems to be superior to
masonry--including in this description first-class brickwork--whether
for road or railway bridges. One pronounced advantage of such bridges
with respect to length of life, is that they are but little affected by
increase of loads. The mass of a masonry arched structure is so great,
and the margin of strength commonly so liberal, that considerable
increments of load may have but little effect upon the reliability of
the structure.
Cast iron has, for bridges of simple design, a strong claim to the
second place, though its want of ductility is a demerit. It can,
however, have but a limited use in bridge construction, being applicable
only to small girder spans and skilfully-designed arched structures.
For bridges of moderate span in which the question of cost does not
control the matter, wrought iron should probably come next, steel being
best reserved for those of a larger size, in which weight of the
structure greatly affects economy.
Timber may be regarded as a material rarely to be used in this country
for structures to occupy a permanent place, unless for urgent economic
reasons of the moment.
While expressing this general view of the matter, it is to be admitted
that the propriety of these conclusions is somewhat discounted by the
difficulty there now is in obtaining cast iron of the desired
toughness, or wrought iron with promptitude and sufficient variety of
section at a reasonable price.
It is apparent, also, that the choice of material may be largely
influenced--even determined--by considerations of headway, construction
depth, or character of foundations; so that no very definite rules can
be usefully laid down, though the adoption of unsuitable materials has
not been so unusual as to make these suggestions altogether
purposeless.