Northwich Town Swing Bridge
in Northwich, Cheshire
It is one of five large swing bridges taking public roads across the River Weaver. The others are:-
It replaced a wrought iron plate girder bridge built in 1858. The swing bridge weighed about 300 tons, but over the years modifications increased the weight to 460 tons. The bridge was apparently operated more than 55,000 times in the first 15 years.[1]
Designed by John Arthur Saner, and built by Andrew Handyside and Co. It is one of the first electrically-driven swing bridges in Britain. It is also one of the earliest 'floating' swing bridges in Britain, a large proportion of the weight being supported by a tank floating in a chamber (pontoon tanks floating in a caisson). To clarify: the bridge is not actually floating. There is a cylindrical buoyancy tank (the pontoon) attached to the bridge, on the same axis as the rollers on which the bridge turns. The pontoon sits in a water-filled chamber (the caisson), and its buoyant force acts to greatly reduce the weight borne by the rollers.
The bridge is turned by an electrically-driven winch acting on a wire rope, an admirably simple arrangement used by J. A. Saner on all his large swing bridges.
1899 'The work of completing the Northwich swing bridges has been pushed on with during the year — that adjacent to the head offices called Navigation bridge having been placed in full working order in February last; while the new town bridge has been turned by hand for the first time, and is expected to be in complete working order very shortly. The old town bridge belonging to the trustees, for which the new swing bridge has been substituted, was handed over to the County Council by arrangement last year, when the trustees agreed at the combined request of the County Council and the Urban District Council, respectively, to take down, remove, and place it over the river Dane, near Crosss-street, the expense of the work (which was completed in June, 1899) being jointly borne by the two Councils. It is now known as the Victoria bridge. ... New works:— The Navigation Bridge, Northwich, has been fitted with the electrical machinery, and has been successfully operated some 100 times by day and night. On November 14th. 1898, the road traffic was suspended over the old Town Bridge, Northwich, and work of removal commenced. Owing, however, to a subsidence in Leftwich-road, there was some delay in getting fairly to work. The bridge was, however, eventually removed and the pontoon chamber, &c. prepared and handed over to the outractors at Whitsuntide. Since then, the bridge has been so far completed as to enable it to be turned by hand on the 14th inst., or exactly 12 months after the first closing of the old bridge. The electric machinery will very shortly be ready, and in all probability the bridge will be sufficiently in order to be utilised for road traffic before the annual meeting. On the completion of these bridges, the only "bar" preventing masted vessels and steam barges of much greater capacity than those now using the river reaching Winsford will be Hartford Bridge. .... ;[2]
In 1998 the bridge had to be upgraded to accommodate 40 ton vehicles, and to repair corrosion damage. A mobile crane was used to lift the superstructure, weighing 356 tonnes and the pontoon assesmbly (180 tonnes). 19 of the 76 rollers were replaced at this time.[3]
Hayhurst Bridge, located 1/3 mile upstream, was practically identical, and was constructed slightly earlier. However, the support arrangements were changed at some point.
The formidable gates have been retained. The pair at the tail end of the Town Bridge being particularly impressive. See photos 11 - 13. They run on rails, and are driven by electric motors via rack and pinions
From The Engineer 26 January 1900:-
'THE INSTITUTION OF CIVIL ENGINEERS.
SWING BRIDGE OVER THE RIVER WEAVER AT NORTHWICH.
At the ordinary meeting on Tuesday, the 23rd January 1900
Sir Douglas Fox, President, in the chair, the paper read was on
"Swing Bridges over the River Weaver at Northwich," by John
Arthur Saner, M. Inst. C.E.
In 1893 the trustees of the river Weaver, to whom the author
was engineer, decided to apply to Parliament for powers to convert the so-called fixed bridge at Northwich into one capable of
being opened for river traffic. An Act was obtained, only, however, on condition that two bridges were built some little distance
apart, in order that one might always be available in case of
breakdown.
When called upon to solve the engineering portion of the
problem, the author considered all the different forms of movable
bridges at present in use, but could not find any which exactly
suited the case. The problem to be solved was somewhat unusual
owing to the nature of the foundations in the salt district which
as was well known, were seriously affected by the abstraction of brine
and salt from the subsoil. The average subsidence at the Northwich bridge had been about 4 1/2in. per annum during the last seventeen years, necessitating the raising of the girders to give headway
for the river traffic ; and it being impossible to raise the street in
the immediate nerghbourhood without partially burying or raising
the adjoining houses, the road gradients had become as steep as
1 in 11. To obviate this inconvenience, and to provide for the
more efficient carrying on of the salt and other trades on the
Weaver,and also with the idea of eventually passing coasting
vessels with fixed masts, the before-mentioned decision was arrived
at; and after a strenuous contest in Parliament, the Bill received
the Royal assent, resulting in the building of two exactly similar
bridges.
These bridges - the only difference between which was that one
had 10ft. and the other 16ft. headway - were of 54ft. span with a
19ft. 6in. roadway, and two 4ft. 6in. footpaths, and were too heavy
to be worked without mechanical power of some kind. Steam or
hydraulic machinery would have been costly, and almost constantly
out of repair on such a foundation ; It was therefore decided to
adopt the following plan:- First, the superstructure of the bridge, which might, for argument sake, be taken as weighing 300 tons,
was supported by a roller path and rollers, which in turn were
carried upon a set of piles, strongly braced together. Connected
with, and exactly under the centre of gravity of, the superstructure
was a circular pontoon or buoy, divided into two chambers. This
pontoon had the appearance of being suspended from the superstructure, and in reality would be if the water were not present, as
it was entirely clear of both bottom and sides of the chamber in
which it was placed. Of this large buoy the lower chamber, which
had a displacement equal to 250 tons, was perfectly water-tight and
always submerged, so that its displacement was practically constant.
The upper chamber was open at the top, and either served as an
access to the lower chamber, or, by varying the amount of water
allowed to enter it, increased or decreased the buoyancy of the
whole. It would be seen that the downward pressure on the
rollers and paths, due to the weight of the superstructure, was
partially counteracted by the upward tendency of the pontoon, and
was thus reduced, in the case under consideration, to 300 - 250 =
50 tons. By emptying the upper part of the pontoon this might be
further reduced within certain limits. There were three natural
impediments to such an arrangement of bridge, namely, floods,
drought, and ice; and in the Northwich case, there was the un-natural impediment of subsidence.
These disabilities were met thus:-
First, the lower portion of the pontoon, being water-tight had a constant buoyancy, and therefore the pontoon would not rise with the flood ; and if there should
be the least danger under exceptional circumstances, it was only
necessary to open a valve and partially fill the pontoon with water,
when it would soon become immovable. Secondly, in the case of
drought, even if the water fell below the level of the upper chamber, it meant a few extra tons on the rollers, which they were well
calculated to bear. Such droughts are exceedingly rare on the
Weaver, and there was ample extra turning-power in the motor.
Thirdly, to prevent the water in close proximity to the pontoon
freezing, and hindering it from turning, the chamber in which the
pontoon was placed would be covered in, and it would be a comparatively easy task to prevent the small body of water in the
chamber from freezing. Fourthly, special arrangements at the
pile-heads and in the wedge-gearing were made to compensate for
the exigencies of subsistence.
To convince the trustees, and also Parliament, that the scheme
was feasible, the author had a model constructed at a scale of 1in.
to 1ft., and also converted to this principle an occupation bridge,
at that time under repair; both these afforded ample proof of the
feasibility of the scheme. The difficulty presented by subsidence
entailed careful considerntion as to the motive power to be
adopted. Pressure pipes of any kind being inadmissible, the
author decided to adopt electrical power, although, so far as he
knew, it had never been tried before for a similar purpose. He
also decided to use wire rope for turning, as giving the most flexible
connection between the bridge and motor. The local company
undertook to supply current at 4d. per Board of Trade unit
throughout day and night, and guaranteed to pay any cost incurred should the supply fail. The outer mains of their three-
wire system were drawn from for power purposes at 440 volts, and
a 20 horse-power motor working a vertical drum for turning, with
two 4 horse-power motors direct-coupled at either end of the
bridge, but fixed on the abutments for withdrawing the wedges,
wore used.
The switches were specially designed, and had only one set of
resistances for both the opening and closing motions of the motors.
The bridges were moved with remarkable facility and the consumption of current after they had been working a short time
and all the bearings, &c., were free, only amounted to 1/4 a Board
of Trade unit for the complete cycle of operations, viz. withdrawing wedges, opening and closing the bridge, and replacing the
wedges.
In conclusion, the author desired to express his indebtedness
to Mr. John J. Webster, M. Inst. C.E., for his suggestion of the
pontoon principle, and for his assistance in making the original
designs; and to Sir Benjamin Baker, who, with the late Mr. Thomas Hawksley, assisted in the passage of the Bill through Parliament.
See here for the Discussion Paper relating to Saner's I.C.E. Paper
James Price (1831-1895), the Dublin Harbour Engineer who had built two swing bridges with pontoons over Spencer Dock in 1873, was contacted about the possibility of his giving evidence during the passage of the Bill through Parliament. A trial swing bridge which used a pontoon was built at Weston Marsh in 1893.[4]
An interesting account of the restoration work carried out in the late 1990s includes schematic drawings showing the arrangement of the pontoon.[5]. It is stated that the structure was mainly made from wrought iron, but the pontoons were made of mild steel, and the deck beams were replaced in steel at various dates. The roller track is made of cast iron, and has suffered virtually no wear. The refurbishment work resulted in a change in the centre of gravity, requiring alterations to the counterbalance weight (kentledge).
The pontoon is a steel cylinder, attached to the underside of the bridge. It is actually assembled from four quadrants. The quadrant-shaped plates at the bottom are dished outwards, so as to resist the external pressure of the water. The water is contained in a concrete caisson (Photo 8). This caisson supports a triangular support frame (photos 4, 6, 7), which in turn supports the the lower ring beam upon which the 76 rollers run (Photo 5).
See Also
Sources of Information
- ↑ [1] Northwich Guardian: Northwich town bridge is swinging again after repair work. 5th November 2021
- ↑ Northwich Guardian - Friday 24 November 1899
- ↑ [2] 'Conservation of Bridges' by Graham Tilly, Gifford & Partners, 2002, pp.274-5
- ↑ [3] canalworld forum, Sutton Weaver Swing Bridge
- ↑ [4] Bridge Management 4: Inspection, Maintenance, Assessment and Repair, edited by M. J. Ryall, J. E. Harding, G. A. R. Parke: Refurbishment of town bridge, Northwich, Cheshire by T. Jenkins, Parkman Ltd.