CAST-IRON BRIDGES.
Cast Iron as a material for bridges has of late years fallen into
disrepute. It is now entirely tabooed by the Board of Trade for railway
under-bridges, unless of arched construction. This condemnation of cast
iron followed, and was apparently the result of, an accident which
occurred to an under-bridge on one of the southern lines, which bridge
had already earned for itself an ill repute by breaking down on a
previous occasion. The ultimate issue was, however, good, inasmuch as it
led to a thorough overhaul of all railway under-bridges in this country,
and the renewal of a great number no longer in a condition suited to the
carriage of heavy or of passenger traffic; yet there is little doubt
that, in the author’s judgment, many excellent cast-iron bridges were
then removed at considerable cost, to be replaced by others of wrought
iron or steel, which will not last so long as many of those displaced
had done, or would still have lasted had they not been dismantled.
The earlier cast-iron bridges were commonly made of cold-blast iron, a
material of such strength and toughness as to give an extraordinary
amount of trouble in breaking up the heavier parts, when the time
arrived to do this, and with which material ordinary hot-blast iron is
not to be compared for reliability.
[Illustration: FIG. 79.]
As illustrating the very considerable stress to which cast iron may be
subjected, without of necessity leading to any mishap, two cases may be
cited. The first, a bridge of 32 feet effective span, carrying two
lines of way, each pair of rails being supported upon Barlow rails,
forming the bridge floor, the ends resting upon the bottom flanges of
inverted [T]-shaped girders, 2 feet 3 inches deep, as shown in Fig. 79.
The extreme fibre stress works out at 2·9 tons per square inch in
tension, and 5·9 tons per square inch compression, calculated as it
would be in ordinary office work; but for the actual loads, at a span as
above, exceeding the clear span by 6 inches only, and without regard to
the effects of eccentric application of the load. The girders when taken
out showed upon examination no sign of overstrain. The practice of
loading cast-iron girders in this manner cannot, however, be too
strongly condemned, notwithstanding that in this case no ill resulted.
It is evident that a piece of the lower flange being broken out from
this cause, as occasionally happens, might so reduce the section as to
result in complete failure.
[Illustration: FIGS. 80 and 81.]
The second example is that of a small railway under-bridge of two spans,
continuous over the central pier, each span being 16 feet 6 inches. The
rails were supported upon longitudinal timbers lying within
trough-shaped girders, as shown in Figs 80 and 81.
The stress over the pier, in the extreme fibres of the top flange, is
estimated at 4·7 tons per square inch in tension, but it should be noted
that the effect of the timber longitudinal and rail has been neglected
in arriving at this result, which might possibly on this account be
reduced to near 3 tons per square inch.
The case is noticeable because no evidence of high stress was apparent.
The author saw nothing to suggest sinking of the central pier, the
effect of which, within limits, would be to further reduce the stress as
calculated; but it is quite possible some slight settlement had
occurred; this, as the spans were so small, would have a sensible
effect. While too much reliance should not, it is clear, be placed upon
any estimated result about which there is a lingering doubt, it should
be remarked that, as it would be necessary the pier should sink 3/16 of
an inch, for each ton of reduced stress, it is not probable that the
results quoted are in excess to any material degree; they are, indeed,
more probably low, as no notice has been taken of impact.
Though cast-iron girders for railway under-bridges are now prohibited in
this country for new works, there are still uses to which they may be
applied, and it may be well to insist that girders of this material
should be fairly loaded, the weight being brought upon them in such a
way that there shall be no serious secondary stress, such as arises when
wide flanges are made to carry concentrated loads; the author has,
indeed, met with no instance of a cast-iron girder breaking down under a
load fairly applied. Preference is now given to steel or wrought iron
for columns; while this is often quite justifiable, there remain many
cases in which nothing better need be desired for this purpose than good
cast iron, provided only that the column be loaded in a suitable
manner--i.e., axially, and that the arrangement and details of the
super-structure are such that there shall be no cross-breaking efforts,
or rocking of the column due to temperature or other causes; unless,
indeed, such cross-breaking or rocking is definitely taken into account
in designing the work. The same care observed in the detailing of
cast-iron work that is not infrequently taken in the design of
structures made of rolled sections would, in suitable cases, the author
has no doubt, yield results just as reliable in practice, with the
advantage of greater resistance to rust, and a reduced cost in
maintenance.
Good cast iron is, in fact, when used with discretion, a most excellent
material, popular predjudice notwithstanding. The oldest metallic bridge
in this country at the present moment is of that metal.
The one chief respect in which cast iron is at a disadvantage compared
with wrought iron or steel is that it does not give premonitory warning
of failure--it remains intact, or it breaks. The indications of
weakness, which may be read by an experienced inspector of other
metallic bridges, are in a great measure absent. There is also an
objection which may exist, but is to be avoided by good design and care
in the foundry--viz., internal stress due to unequal cooling. In extreme
cases this may lead to fracture before the work has left the maker’s
hands, but it can only occur by neglect of ordinary precautions.
[Illustration: FIGS. 82 and 83.]
In a case which has already been referred to in the chapter on
“Deformations,” page 80, an outer rib of a cast-iron arch fractured near
the crown after fifty-four years’ use. Owing to the nature of the
design, and the fact that the near abutment had closed in slightly,
bringing the linear arch of necessity near the lower edges of the arch
segment in question, it was possible to estimate, with a probability of
truth, the extreme fibre stress (tensile) due to the load forces, at the
upper edge where fracture commenced. The result was very far from
explaining the occurrence of the break, but an examination of the
details shown in Figs. 82 and 83 will make it apparent that, in addition
to the tensile stress, as calculated, there was probably a severe
initial stress of the same character due to irregular cooling in the
foundry half a century before. The sum of these stresses, it is
suggested, placed this particular casting in a critical condition, such
that operations in the construction of a new bridge adjacent either by
producing a small further settlement of the foundations, of which the
author saw no evidence, or, as is more probable, the attachment of a
rope to this rib for the purpose of keeping a barge in position, which
certainly did occur, gave the arch rib just such an additional strain as
to result in the break shown, though no one of these causes acting
singly would have been sufficient to induce fracture. The inner ribs
were of a much less objectionable section.
[Illustration: FIG. 84.]
Cast-iron arches, though still allowed by the Board of Trade rules, are,
indeed, liable to be seriously affected by settlement, or yielding of
the abutments, unless hinges at the crown are introduced. As an instance
of this may be quoted a bridge of some 45 feet span, in which the arches
were cast in two pieces abutting, and very efficiently bolted together
at the crown, the springing and vertical abutment member of the
spandrel being bolted and built solidly into heavy masonry. The arch
sank at the crown, caused by, or itself the cause of, a movement of the
abutment, with the result that the lower bolts at the crown joint broke
away, rupturing the casting, as shown in Fig. 84. The arch must then
have acted as though hinged at the crown, as effectiveness of the
connection was destroyed. It had been better, evidently, if a proper
hinge had originally been provided. The break happened to occur so as to
leave a sufficiently good bearing face at the crown; there was, indeed,
no tendency for one surface to slide upon another; but in the accidental
fracture of cast iron this cannot be assured, and the liability to it is
a risk which should be eliminated if possible.
A second case of very much the same character has also been under the
author’s observation, though in this the ends of the spandrels were not
built into the brickwork of which the abutments were composed. Other
instances of fracture either in the arch proper or in the spandrel work,
have come under notice, though particulars cannot now be adduced; but
the examples cited are by themselves sufficient to justify the
conclusion that it is imprudent to construct a cast-iron arch without a
central pin or its equivalent, unless the abutments, being exceptionally
well founded, may be relied upon as free from any liability to move. It
is, however, to be borne in mind that movement in the abutments of a
small arch of any given absolute amount is more injurious than the same
amount of movement in the abutments of large arches of similar design,
so that what may be negligible in the latter case would perhaps be
destructive in the former.
To the absence of ductility and liability to initial stress must be
added yet another disadvantage to which cast-iron work is prone--viz.,
the possibility of concealed defects, blow-holes or cold-shuts; these in
good foundry practice are not very likely to occur, but, as they are
possible, cannot be overlooked in considering the suitability of cast
iron for bridgework, or, indeed, any structural work liable to serious
stress, and particularly tensile stress. With these remarks by way of
qualification, the author reiterates his opinion that there is still a
use for cast iron in bridgework.
With respect to the repair of cast-iron bridges, but little is to be
said; the possibilities in this direction are very limited. Occasionally
it may be desired to deal with the fracture of some member in the
spandrel bracing of an arch, when it is commonly sufficient, and even
preferable, to limit the repair work to confining the fractured parts in
such a way as to prevent displacement.
Rarely it may happen that an arch fractures as a result of settlement,
or other movement, when, if it is decided that safety of the structure
is not imperilled, it will in this case also be preferable to confine
the parts simply by flitch-plates or other contrivance, with no attempt
rigidly to make good the break, the consequences of which treatment
would probably be to induce fracture in some other place. Effective
strengthening of a cast-iron structure is seldom practicable, though
something may occasionally be done by the negative process of lightening
the dead load, or by remodelling the permanent way. Arches may, however,
be rendered much more reliable by the introduction of suitable bracing
where this is either wanting or inefficient.
In scheming such additions it is desirable to arrange for as little
drilling of the old work as is possible; where this cannot be altogether
avoided, the position of the holes should be carefully chosen with
regard to the effect they may have upon the strength of the old work.