Skew
Bridge, Monkhide
Skew Bridge, Monkhide, is a unique feature; it is the most askew
canal bridge on the entire national canal network. In his book, “The
Hereford and Gloucester Canal”, David Bick suggests that it was
built by Ballard, simply to prove his technical expertise and this
was surely the case.
Later in the same book, Bick points out that before Ballard started
building the second section of the canal, between Ledbury and
Hereford, he was given £25 in 1829, for a fact-finding visit to the
Liverpool and Manchester Railway, which was at that time, under
construction. It is recorded that he walked ten miles along the
railway, accompanied by Mr. Stephenson, the engineer.
Interestingly enough, I read recently in Derrick Beckett’s book,
“Stephenson’s Britain”, that there were three skew bridges on the
Liverpool and Manchester Railway. One of these skew bridges was the
Rainhill Bridge, which had been completed by the end of 1828. When a
conventional stone bridge is constructed, once the abutments or
foundations for the base of the arch have been constructed, wooden
frames or centring are constructed springing from these abutments
and the stones or voussoirs which form the soffit or arch of the
bridge are then laid on this supporting framework. The wing walls
and spandrels are then constructed; burden is laid on top of the
arch of the bridge and after this has consolidated the centring is
removed. Stone was the first durable constructional material used by
man and as Hopkins points out in his book, “A Span of Bridges”,
providing it is used where the stone blocks are pressing against
each other, or where they are subject to compressive forces, it is
very strong - even relatively soft stone. However, if stone is
subject to tension, it has little strength and this imposed
limitations on early civilizations, who only used post and lintel or
trabeated constructional technique; Stonehenge being a good example.
The origin of the construction of the arch is open to doubt; Robert
Furneaux-Jordan states that it was known to the Greeks, but not
exploited by them, perhaps because they were not engineers. It was
of course the Romans who were the first to fully exploit arcuated
construction and in particular, the semi-circular arch, which is the
simplest and safest arch to construct. The disadvantage with the
semi-circular arch is its height, or rise, in relationship to its
span. Consequently engineers sought to construct flatter arches or
even elliptical spans. A good example of the latter is Telford’s
1829 bridge at Over, near Gloucester, which carried the A40 across
the River Severn until quite recent times.
However, the flatter the arch, the greater the risk of the bridge
collapsing, since unlike the semi-circular arch, the thrust is not
evenly distributed. Here again Over bridge
is a good example, for although it has not collapsed, it is recorded
that, when the centring was struck, the arch dropped ten inches,
much to Telford’s consternation. This can still be seen today in the
flatness of the parapet of the bridge.
When building a skew bridge, similar problems of instabitity arise,
since the line of thrust is no longer at right angles to the
abutments. The most common solution to this problem is to lay the
stones or bricks of the arch in a spiral form, a technique devised
by William Chapman and explained in Ree’s Cyclopedia of 1813.
However, before building the Rainhill Bridge, Stephenson was so
unsure that Beckett relates that he commisioned his masons to
construct a full-size timber model.
The second skew bridge was Haggerleases bridge, built towards the
end of the Haggerleases branch line, which was opened in 1830 and
another wooden model was made before the construction of this
bridge. Apparently, Haggerleases bridge has since been demolished.
Bearing-in-mind the date of Ballard’s visit to the Liverpool and
Manchester Railway, I would suggest that he probably saw Stephenson
grappling with the construction of the Haggerleases bridge and
returned home looking for an opportunity to emulate his feat - hence
the construction of Skew Bridge, Monkhide.
NCJ 7/96.
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