MASONRY BRIDGES.
Masonry bridges, in which description it is intended to include
structures both in stone and brick, are, when well built, amongst the
most durable and long-suffering of any which come under the care of a
maintenance engineer; yet when developing the faults peculiar to their
kind, they may be the occasion of much anxiety, and render necessary
frequent inspection, or even continuous watching.
Apart from decay of mortar or material, defects may very commonly be
traced to the foundations, or to earth-slips. Sinking, when uniform, may
be quite harmless, though possibly inconvenient; irregular sinking of
piers or abutments is quite a different matter. It is, however,
remarkable to what a degree sinking may be evident, without of necessity
rendering a structure unsafe. Movement of an amount and kind which would
be fatal to the connections of metallic bridgework is endured by bridges
of stone or brick; not, it may be, without damage, yet with no occasion
for alarm. The superstructure of metallic bridges may often, however, be
restored to the true level before the mischief has become serious,
whereas in the case of masonry arches this is not practicable.
Spreading of the abutments is very seldom the cause of any great injury
to an arch, though it is common enough to find old and flat arches
slightly down at the crown; but the contrary case of abutments closing
in is not very unusual when these are high, or terminate a viaduct over
a deep valley. Such an abutment may move during or soon after
construction, throwing up the crown of the end span affected; or, if the
arches are very solid and heavy, the abutment may slide forward at the
base, with no sensible reduction of the opening.
When a viaduct connects the two ends of a high embankment, it may happen
that the end piers are not clear of the embankment slope, in which event
a pier may, should the bank slip, move with it, as to that part not in
solid ground; with the result, in a bad case, that it is broken across
and the superstructure imperilled.
[Illustration: FIG. 90.]
A case of abutment movement is illustrated in Fig. 90, which represents
the end arch of a masonry viaduct, one abutment of which had moved
forward in the manner already referred to. From the springing upwards
the arch retained its form to within a short distance of the crown,
where it was forced up in the way indicated. When the movement became
pronounced, heavy timber centering was introduced, with the object of
preventing any mishap, the damaged portions being ultimately cut out and
made good. The structure was thirty-five years old.
The practical utility of stop piers in long arched viaducts is, perhaps,
rather in checking movement of the tops of piers under moving load than
in arresting actual failure of a series of arches. That the tops of
piers do move very sensibly need not be doubted. The author has
attempted to measure this in the case of piers about 60 feet to the
springing, by means of a theodolite placed below, but has reached no
more definite result than that a movement existed, of which he was not
able to determine the amount. If in a viaduct some arches are more
heavily loaded than others, each spreading slightly, the end piers of
the group will move amounts which together equal the sum of the
individual span spreads, with a tendency in the arches beyond those of
the group overloaded to rise.
This rocking may be detrimental both to the piers and arches, and helps
to account for the disintegration of mortar in arches and piers, which
not infrequently happens. The soffits will sometimes be seen with a
thick incrustation of lime, which has washed out of the joints, or from
limestone ballast above, where this has been in use. Arches of tall
viaducts may, indeed, become in so bad a condition that pieces of stone
or brick will drop out, necessitating repair at heavy expense, of which
scaffolding is commonly a large part.
Tall piers may be found badly out of the upright due to sinking of
foundations. A marked case of this kind came under the author’s
notice--a viaduct of fifteen semicircular arches, in which, though many
piers were wanting in truth, one in particular was about 1 foot 4 inches
out of vertical, making one side of the shaft plumb, and doubling the
normal batter of the other. Inquiry showed that in this instance the
pier had never been upright from its earliest history dating back
thirty-six years. This makes clear the desirability, to avoid hasty
conclusions, of ascertaining, when it is possible to do so, the complete
record of any structure.
A bridge fifty-eight years old, of three skew spans, carrying a railway
over a canal, and having somewhat flat brick arches with stone quoins
upon low piers, developed the somewhat unusual defect, as to the centre
arch, of splitting along its length for about 10 feet, parallel to and
some 7 feet from one face. In this case there was reason to believe that
there had been considerable local settlement of the piers on that side
of the bridge. The arches were otherwise in bad condition, the brickwork
poor, and the mortar decayed. Each arch was down at the centre, and
displayed a fault not unusual where bad brickwork joins up to good cut
stonework, the quoins showing a tendency to separate from the brick
rings. Below the bridge were coal-workings.
Brick arches built in parallel rings sometimes separate one ring from
the other, demonstrating the known propriety of bonding the rings
together properly, and of carrying the arch round, when building, at its
full thickness.
[Illustration: FIG. 91.]
An instance of bridge failure from a somewhat peculiar cause may be
quoted as of some interest, largely because the structure was very
ancient, having been in existence some 400 years. This bridge, carrying
a road, was of the type usual in old masonry bridges over a river,
having small arches, thick piers, and solid backings to the arches. Two
flood-openings at one end had, by sinking and want of care, become
partly closed. The centre arch had, however, been widened about 140
years previously. During a severe flood, the swollen river, overflowing
its banks, trespassed upon a timber yard a little above bridge, and
washed down into the stream a large quantity of sawn timber; this,
unable to get through the main arch with freedom, compacted into a
serious obstruction. The flood water, thus checked in its passage, seems
to have scoured below the timber, and robbed the piers of such support
as they formerly had (see Fig. 91). The bridge stood in this condition
till the water lowered, when the middle part of the structure broke up,
and subsided into the hole which had been washed out. But for the
monolithic character of the old work it is probable the bridge would
have failed long before, as the gravel bed on which the piers stood had
been partly undermined for very many years. The case is instructive, as
showing how a slight accident--powerless by itself to work mischief--may
be very damaging when allied with so powerful an agent as running water.
[Illustration: FIG. 92.]
The enduring character of even the roughest class of masonry arch, if
only the material be good and abutments stable, is shown when it becomes
necessary to destroy old work of this character. Fig. 92 represents a
short length of “cut and cover” arching in process of demolition, just
before it fell in. The masonry was of hard sandstone rubble and had been
cut away, as shown, till at the point A only a very small piece of the
arch remained, when the length finally broke up and dropped. Arches have
commonly a great reserve of strength; tunnel linings are, indeed, often
badly out of shape, closed in, and sunken; yet continue, with close
watching, and occasional repairs where the work has decayed or bulged,
to serve the purpose intended.
Though the equilibrium of masonry arches has been the occasion of much
profound study, and the nicest calculation has sometimes been applied to
the design of such work, yet it appears that when an arch is well backed
up, the theoretical linear arch need have but little connection with the
figure of the intrados; a statement consonant both with common-sense and
the teachings of experience. With solid backing, this would indeed seem
to be more important than any part of the arch ring below the top of the
backing, the lower part of the ring serving chiefly to preserve the face
of the solid work. Arches are frequently to be met with so out of their
true shape that but for the consideration named, failure would seem to
be inevitable. The masonry or brickwork does not always show evidence of
damage, if the distortion has been slow; suggesting that structures of
this kind have a power of accommodation with which they are not
generally credited.
A noticeable cause of deterioration of masonry structures, which may be
quite independent of settlement, is serious vibration. This is well
known in connection with church belfries, and is also locally apparent
when telegraph or other poles are attached to masonry parapets.
Vibration, when caused by heavy railway traffic, acting upon arches
light or originally bad, may demoralise the structure to such an extent
that repair becomes exceedingly difficult, because of the extensive
character of the mischief; but masonry bridges substantially built, and
particularly those carrying ordinary roads, and not subject to much
vibration, have great lasting powers, if repaired with skill, or even
let alone. Distortion of the arch may be quite consistent with practical
stability, if the movement or decay with which it originated is not
progressive, or has been arrested. In this connection a distinction is
to be made between arches well backed, to which the foregoing remarks
apply, and in which the two halves of each arch may act as separate
monoliths meeting at the crown, and the case of a true arch ring
independent of any outside resistance, such as backing or spandrels may
give, and depending almost wholly upon the proper balance of its
component voussoirs for its stability. With the latter class of
structure no liberties may be taken; whilst with the former there is
seldom cause for fear, if the foundations do not give way, and the work
is dealt with judiciously, if at all. It must, however, be understood
that there are limits as to what may be done effectively, short of
rebuilding, in dealing with structures in which, perhaps, brickwork is
rotten and mortar decayed and crumbling, the whole being little better
than a broken mass of rubbish.
In cases where it may be prudent to introduce safety centring, as in an
instance already referred to, it is commonly expedient to refrain from
causing this to take any sensible part of the load till all movement has
ceased, the centres being at the outset largely precautionary. The
requirement with an arch in bad condition is to avoid disturbing it for
the worse. If the centres are wedged up whilst movement is still going
on, the effect may be to cause the arch to break up upon the centring,
and precipitate repair work which might otherwise have been left to a
more convenient time, when all movement had stopped or been checked by
suitable measures. Viaduct arches in a bad condition, but not
necessitating the use of relief centres, are commonly dealt with
piecemeal by cutting out the bad places, a small part at a time, and
making good. The work requires the greatest care of experienced men.
Pointing masonry or brickwork is effective for little other than
protective purposes, and to check further weathering; it has obviously
no effect upon the interior work, and if made to cover up the evidences
of internal decay, is even misleading and objectionable. In extreme
cases it may be desirable to open out the road and deal with the
filling, to relieve or to strengthen the outer spandrel walls, which
sometimes bulge, or for other purposes, as, for example, for rebuilding
inner spandrel walls, grouting up or otherwise repairing solid backing,
in which operations some regard must be had to the effect of the work
upon the balance of the opposing halves of the arch.
Of the different classes of masonry commonly used in bridgework, it may
be well to remark that good coursed rubble, or preferably that variety
bonding both vertically and horizontally, of a durable stone, perhaps
quite unfit for any but rough dressing, may make a most lasting
structure, the mortar, of course, being good. Each rough-dressed stone
presents a durable piece, fragments removed separate from the block,
probably along some line of relative weakness--there is no “nursing” of
weak corners; whereas with stones reduced to a perfectly regular shape
by chisel work, the plane surfaces and geometrical angles are made with
partial regard only to the natural grain of the stone.