I first heard about this project when it was reported on Central News on 28/08/2007 I visited the site one year later. Work was progressing on the redevelopment of the area but work on the bridge had been delayed by the discovery of un-mapped electrical cables and junction boxes during excavation of the site. It had been planned to site the bridge during the Spring of 2008, but it did not open to the public until the 19th March 2009.
On 14th May 2009 I was invited to attend the spring meeting of the Midlands branch of the Institute of Structural Engineers, held in the Derby Industrial Museum housed in the old Silk Mill. During the meeting a joint lecture about the design and construction of the new Cathedral Green Swing Bridge, sited adjacent to the Silk Mill, was presented by Project Architect Stephen James, Project Engineer Stephen Wilson and Mechanical Engineer David Price.
The design team for the bridge included Stephen James of Whitbybird (now part of the Rambolls Group), Stephen Wilson of Rambolls (a division of Balfour Beatty) and David Price of M.G. Bennetts Ltd. (now Atkins Bennetts). David Price has since formed a new consultancy -EADON Consulting -with fellow engineer James Hill.
There are several unique features to the Cathedral Green Bridge. The first arises from the reasons for the bridge being built as a movable structure. The bridge does not move in order to allow the passage of boats, neither is it intended as a defensive structure – which are the most common reasons for constructing movable bridges.
The design brief presented two constraints. The first was that the proposed bridge should not have a detrimental visual impact on the Cathedral Green Area, so the bridge had to be built level with the river banks without high approach ramps. However the River Derwent suffers from wide fluctuations in water levels, especially following heavy rainstorms in the surrounding Peak District. Placing a new bridge at low level therefore incurred the risk that the bridge would – at times of very high water levels – obstruct the flow and this would endanger the bridge itself.
The solution therefore was a swing bridge that could be swung aside to allow continued free flow of water at all times and in order to protect the structure itself from damage.
The anchor point of the bridge was to be sited on an island that had originally been created by the construction of the Mill Race serving the Silk Mill. In the original brief there was to be a fixed bridge across the Mill Race and then a second bridge from the island across the River Derwent. The winning design offered the alternative solution of a single bridge that spanned both water courses with a main span of 40 metres across the river and a backspan of 20 metres across the Mill Race.
At present I cannot think of another swing bridge in the British Isles that spans two water courses using a single continuous deck structure.
The architectural team then set the structural engineers a major challenge. Constructing the bridge as a continuous straight span – as is usual – would have brought users crossing from west to east face to face with a dark space between two existing buildings. In the process of considering the historical significance of the area – especially the Mills along the Derwent that formed an early part of the Industrial Revolution – the team developed the concept of a kinked bridge reflecting the shape of a single scissor blade with the sculpted river bank forming the other half of the scissors. This structural form created a more natural flow between the bridge and the riverside walk on the east bank.
The kinked asymmetric structure did however present complex problems for the structural engineers in the calculation of stresses, which influenced the choice of material for the bridge and the internal form of the deck. A torsional box girder structure was eventually chosen, with the deck tapered in the transverse cross section, with tensions in the deck structure balanced by a cable stayed superstructure.
Other features of the bridge also attempt to reflect the historical links with the local textile manufacturing trades. The bridge mast reflects the long tapered shape of a needle – and also creates a visual triangle with the cathedral spire and the pump house tower at the Silk Mill – and the cabled parapets and open form of the seating on the bridge emanate from the production of woven material.
As far as the designers are aware this is the only kinked cable stayed bridge in the world – and I have certainly not come across anything like it.
Although at one time local planners were considering reinstating the Mill Race in its entirety the final decision was that this was not feasible within the overall plan for the redevelopment of the Silk Mill site. The downstream end of the Mill Race was retained as a historical feature – hence the need for a backspan on the new bridge. Between the retained section of the Race and the Silk Mill the path and scale of the Race has been depicted in contrasting paving across the concourse at the lower part of Cathedral Green.
The next unique feature is that the bridge has been designed so that it can still be used as a pedestrian and cycle route whether open or closed. In its open position users can cross the Mill Race and the River - while in the closed position users can still cross the Mill Race and then continue along the main span as part of the riverside promenade.
This has been achieved as part of the design requirements to move the bridge structure out of the flow of the river at times of flood. The main span of the bridge swings over an apron that is recessed into the west bank, while the sweep of the backspan leaves it still within the confines of the Mill Race. Once the bridge is at rest in this closed position gates in the riverside fencing can be opened allowing a free flow of pedestrian and cycle traffic along it.
The design team was anxious to create a slender structure that would blend easily with the general re-development of the Cathedral Green area. Asymmetric swing bridges are commonly counterweighted – weight being added to the tail section to balance the weight of the longer nose section. This leaves the two sections in equilibrium about the pivot point. Such a solution here would have detracted from the elegance of the bridge, and an alternative solution was sought.
Without any counterwieghting the main span was inevitably going to be subject to downward force while the lighter backspan would be subject to an upward force. To control these forces the design team have effectively inverted the bearings and mechanism that control the tail of the bridge. In most swing bridges with a tail wheel the wheel bears down on a radial track set into the abutment beneath the bridge. In the Cathedral Green Bridge structure the tail wheel bears upwards onto a radial track set underneath an overhanging abutment. The radial track has been extended outwards from the overhanging abutment with the edge of the track engineered as a toothed rack. The slew drive is also mounted beneath the tail of the bridge and engages with the toothed rack to rotate the bridge.
To the best of my knowledge I have not seen this inverted design on any other swing bridge – so this appears to be another unique feature of the Cathedral Green Bridge!
The centre wheel which rolls around the pintle as the bridge moves has been tapered – a common feature in swing bridges. David Price explained that a non-tapered wheel would tend to try and roll in a straight line causing scrubbing and creating a high wear rate. A tapered wheel rolls naturally in a circle and therefore reduces wear with a subsequent positive impact on maintenance costs and also the efficiency of the system.
The bridge can be operated from a control panel within the control housing although the operator would have no view of the bridge and its approaches. A remote control point has therefore been installed above ground level. The operator can bring a control pedant to the remote control point. Once the pedant has been plugged in the operator can use the push button controls on the pedant to operate the bridge from a position that provides 360 degree surveillance of the bridge, the river and the approaches to the bridge.
(Since the bridge does not need to open for boats as would be the case on a navigable waterway there is no need for there to be a system for operation by members of the public.)
As the bridge will only be opened once per month for maintenance purposes and on infrequent occasions when floods threaten the structure there was no great need to build in a high speed drive system. The motor therefore operates at just 3 rpm and it takes around 5 minutes to sweep the bridge from open to closed positions and visa versa.
It was inevitable that the main span would bend under its own weight once it was hung on the pintle. To avoid the main deck twisting downwards from pintle to nose it was therefore built off-site with a pre-camber – that is with an upward curve towards the nose. When brought to site the main span was placed on pre-prepared trestles -assembled on the swing apron – which supported it in the pre-cambered shape. The backspan was then placed against the main span and the two sections welded together. The mast and cables were then introduced and tensioned. The temporary trestles were removed and as the whole assembly came into tension the self-contained weight of the deck pulled it straight. After fine tuning the tension on the cables the longitudinal form of the main span was within 20mm of the target shape over its 40 metre span – an accuracy of 99.95%.
I learnt a great deal from this excellent presentation – not only about the Cathedral Green Bridge itself but about the whole design process and the way in which the partners in the design and construction process work together. The bridge is a striking example of the coming together of art, science and innovation to create a landmark structure that is also fit for purpose.
I must express my thanks to the Institute for welcoming me as a guest, and to Stephen James, Stephen Wilson, David Price and James Hill for their willingness to talk to me - a non-engineer – after the lecture about their work.