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Here
are more pictures of the same bridge. One shows places (arrowed)
where new bricks have been added to repair eroded ones. The new
ones are ordinary bricks, which break the line of the chamfer. This is a simple example the effect of specialist construction on the
difficulty and cost of maintenance.
A
Bridge Needing Repair
These
pictures show the same bridge at a later date, after some voussoirs had
fallen off. If even one voussoir falls out, the compressive force on
all the others in its row are to some extent relieved, and those nearest
to the gap are held in place only by the adhesion of the mortar. The
release of stress means a release of strain, and that implies that the
relieved row of voussoirs tends to become slightly longer than the
neighbouring row. There will thus be shear stress between the broken
row and the unbroken row. This shear stress will in fact tend to
share the compressive stress between the two rows, especially far from the
gap.
There
are implications for a repair. If any structure contains more than the
minimal set of parts that will keep it standing, loss of a part may not be
disastrous: the stresses are simply shared among other parts. But
when a repair is attempted, simply fitting a new part is not enough: in
theory, the gap should be jacked until the stresses are similar to their
values before the break. Sometimes the loss of stress is
irreparable. When the Britannia bridge over the Menai Straits caught
fire, the internal stressing put in by Stephenson was lost, and the spans
now have arches to support them. In the example of the skew bridge,
any one row of voussoirs is so lightly stressed that simple cementing new
bricks is probably an adequate repair. An example where repair is
difficult is a large piece of pottery that breaks, and the pieces are
found not to match when held together. This happens when internal
stresses and strains were locked in during the cooling process after
firing. They are released by the breaking, and the parts take
slightly different shapes.
This
picture shows that
the bridge pier was originally pierced by two arches, which have been filled
in. Behind this pier is another arch of the bridge, which is
occupied, as is common, by a small workshop.
Between
Bristol and Gloucester, the railway line and the A38 road have to cross,
because the railway curves around high ground. The crossing angle
is about 48 degrees from the perpendicular in both places, resulting in extremely skewed
bridges. The chamfer mentioned above is here much wider, to avoid
the very sharp 42 degree corners that would otherwise have been
built. The pictures were taken on a very dull day, and are
consequently very poor. The first picture shoes the angle of skew,
while the second shows the chamfered soffit, and also huge amounts of
distortion, though the bridge still carries much heavier traffic than
that for which it was designed. Why is the chamfer employed?
The next diagrams show horizontal sections through a bridge with a skew
of 45 degrees, near the springing. The chamfer eliminates a narrow
nose of brick that would be vulnerable to weathering and crumbling,
which would be accelerated if the bricks were in compression. If
they were not in compression they would be redundant in any case. Such a small area can contribute little to the strength of the bridge.
The
pictures below show an unusual bridge near the Cotswold Water Park. It crosses the line of a disused canal, and has a skew of around 10º.
The builders actually set the masonry courses parallel to the piers, and so
they make slight zig-zags at the surfaces, as the fourth picture shows. The steps are of about the same size throughout, though if
the arch were a complete semicircle, the steps would decrease to nothing
at the springings. In a
genuine skew arch construction the steps would vary from zero at the
crown to a maximum at the springings. Like the previous bridge
this one has arches that are only a fraction of a semicircle.
The
holes in the piers were built with skew, which would seem to be
unnecessary. This makes their surfaces all a part of a single long
tubular shape.
This
picture shows clearly how the holes are parallel with the road above,
and not with the piers. It also shows that, as with the main
arches, their bricks are not built on a skew basis, but parallel with
the holes, leading to the same uniform zig-zags as in the main arches. The middle picture below shows that subsidence has occurred.
It is
a tilted and squashed version of the left hand picture to show the dislocation in
the line of the bridge. Within the same area there are two other
bridges of the same construction.
Near
Andoversford, Gloucestershire, two genuine skew arches with a skew of about 35° mark the route of
a dismantled railway line. The bridges are similar, and have
zig-zags at the ends. The one nearer to Andoversford has been
repaired and pointed, but the other is showing signs of distress, with
numerous broken and missing bricks, cracks, and even growing plants. Here are some pictures.
All
the bridges shown so far encompass only about a quarter of a
circle. The next set of pictures show a bridge near Swindon,
carrying a disused railway track (now a footpath) over a disused canal
(now a very long pond). Much of the original brickwork has been
replaced, and this has not been done with complete regard for the
alignment along the faces of the arch. This bridge is
located at a sharp bend in the canal. Had it been built only a few
yards along the canal, it need not have been built on a skew.
Note
that the angle at which the bricks of arch meet the bricks of the side
walls is the same as the angle between the line of the bridge and the
perpendicular to the railway or road below. If the bricks meet at
an angle of 35º, the bridge is at an angle of 65º to the road. The
bridge has one more surprise for us. The first picture at left, squeezed
horizontally, shows that the bridge is distorted. Remember that as
a railway bridge, it should be dead straight. The vertical yellow
lines show two kinks in the masonry, and the two others show the
misalignment in the courses. The second picture has circles added
to show the distortion of the soffit from a single smooth curve. We have to be careful with this kind of thing to be sure that our lens
has not caused some or all of the distortion. Using a wide-angle
lens can also change the shapes of curves very significantly, as shown
by the third picture, a poor example of a bridge picture. |
