Restoring and strengthening heritage bridge structures

Ferry Bridge on the River Trent in Staffordshire was identified as under strength and in need of significant restoration, and then renovated and strengthened.

The Grade II ‘self-anchoring’ suspension bridge now remains in service and fit for purpose.
The Grade II ‘self-anchoring’ suspension bridge now remains in service and fit for purpose.

Ferry Bridge is a Grade II listed, 75m, three span Victorian wrought iron suspension footbridge across the River Trent. A few years ago the full extent of corrosion to the structural wrought iron was measured and subsequently the bridge was found to be understrength when checked for full theoretical loading.

History of the bridge structure

Built in 1889 to replace the existing ferry boat service, Ferry Bridge connects the communities of Stapenhill and Burton upon Trent and was originally gifted to the town by Michael Bass, who later became Lord Burton.

Its opening was a cause of great celebration and this iconic bridge has always held a special place of importance for the people of Burton upon Trent. It is now owned by Staffordshire County Council (SCC) and maintained by Amey.

In 2014 a full inspection was undertaken to fully quantify the extent of corrosion and provide data for a full structural assessment. This analysed the capacity of the bridge to carry pedestrian footbridge loading in combination with wind loading, in addition to its dead load.

A two pronged approach was adopted to secure the long-term future of this well-used bridge. It was decided to halt the ongoing deterioration by giving it a new four coat, HE-approved protection system to see it through the next 30 years or so before any major maintenance would be required and undertake strengthening works to bring it up to current design and loading requirements to secure the future of this fine example of heritage.

Bridge analysis and design of strengthening works

The bridge is a very early example of a self-anchoring suspension bridge, comprising a wrought iron lattice truss deck hung off riveted plate catenary cables by solid bar hangers. Rather than relying on anchorages located off the bridge to resist the tensile forces in the catenaries, the catenaries are connected directly to the ends of the longitudinal lattice truss deck which carries the forces in compression.

The structural analysis was further complicated by the out-of-plane catenaries and inclined hangers which, unusually, are attached directly to the top of the parapet edge girders rather than the deck.

It was agreed at technical approval stage that inter-related, 2D computer models would be created and analysed. This was to replicate the approach that was no doubt taken in the original design and avoided the overly complex 3D computer model that would have resulted from analysing the bridge in its entirety due to the amount of redundancy.

The bridge was found to be structurally substandard and the final strengthening scheme comprised the novel use of inverted portal frames installed all along the bridge deck to connect to, and work with, the existing deck structure.

Additional strengthening of the longitudinal girders catered for the increased self-anchoring forces resulting from the greater dead load. New internal deck bracing to replace the original greatly simplified the installation of the new structural elements and timber deck. Carbon fibre wrapping was used to reinforce the cast iron pier legs.

Listed building consent was secured in order to undertake the work by ensuring all the new structural elements were designed to complement the existing bridge and remain within the depth of the deck. A new, English Heritage-approved cycle-compliant parapet allowed the bridge to be re-opened as a combined cycle/footbridge.

Construction work and timescales

Access to undertake the works was via a fully independent scaffold system, founded on screwpiles in the river bed so as to avoid overloading the understrength bridge. The screwpiles were carefully located to avoid a large water main in the river bed.

Self-supporting scaffold on Ferry Bridge
Self-supporting scaffold on Ferry Bridge

It was important to maintain the support of the local community by minimising inconvenience for them. Given that there was no alternative but to close the bridge for the duration of the works, the decision was made to start work in autumn hence avoiding disruption of the activities and river regatta planned for the summer.

The scaffold was encapsulated in a phased sequence from one end of the bridge to the other. It prevented river and air pollution during the extensive grit-blasting operation, protected the workforce from the winter weather and allowed optimum temperature and moisture levels to be maintained for the repainting operation.

Continual monitoring of temperature and moisture levels within the enclosure and the film thickness of each of the four coats of paint ensured the final system met the specification, despite the storms experienced throughout the winter. The scaffold was set high enough to allow the river to remain navigable for the small craft that regularly use it.

Top coat of Ferry Bridge
Top coat of Ferry Bridge

Once each phase of painting was completed, strengthening works followed on behind. This comprised the U-frame installation, additional strengthening plates and new cross bracing.

Installing new deck U-frames on Ferry Bridge
Installing new deck U-frames on Ferry Bridge
New deck on on Ferry Bridge
New deck on on Ferry Bridge

Statutory undertakers' equipment

One particular complication of the job was the number of BT ducts crossing the bridge, most of which were badly corroded and needed replacing or moving. Amey worked in partnership with BT at the design stage to encourage them to align their maintenance budget to re-route their supplies via new chambers and ducts installed as part of the works.

Working with the local community

During the bridge closure, pedestrians were diverted via another crossing 100m downstream. Interested members of the public were able to view progress of the work from a scaffold viewing platform that was erected at one end before work started.

The bridge was repainted in its previous livery of black and white which was the popular choice of the Ferry Bridge Working Group - the group having been set up during the design stage to consult and inform interested parties. It comprised officers from SCC, Amey and other authorities, local councillors, English Heritage and members of local interest groups including the Civic Society. The group met regularly throughout the design process and contacts were maintained during the works.

One significant result of this consultation was the support received from the local community. The 'Friends of Ferry Bridge' and the Burton Civic Society were particularly supportive as they recognised the importance of the work. These two groups even went to the extent of commissioning replica full-sized plaques bearing Lord Burton's coat-of-arms to replace those taken off the bridge many years ago.

Amey supported their Listed Building application and fixed the plaques to the outside of the bridge towers prior to re-opening.

The restoration work progressed well and both the works and the story of the plaques were favourably reported by the local media.

Conclusions

This project brought together many parties who worked to their strengths to collectively progress this complex scheme from establishing the problems and finding the right solution to undertaking the works in order to provide the client and all stakeholders with the best long-term solution to ensure the future of a valuable piece of heritage.

The decision to involve the community from the outset by the creation of a working group was time and effort well spent as it reaped the reward of their continued support throughout the scheme.

Project details

Location: Burton Upon Trent
Value: £1.5M
Date of completion: October 2016
Duration: 12 months
Client: Staffordshire County Council
Contractor: Amey/Currall, Lewis and Martin Ltd.
Designer: Amey/Inertia Ltd.
Project Manager: Amey

Challenge summary: Restoration and strengthening of heritage structure. To ensure an early example of a grade 2 'self-anchoring' suspension bridge remains in service and fit for purpose.

Challenge solution: Co-ordinated approach to provide long-term restoration. Sympathetic strengthening and refurbishment, including the installation of an internal stiffening frame, replacement sway bracing and new timber deck.

Contributors

David Hughes, Senior Structures Engineer, Staffordshire County Council.

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