Ballingdon Bridge, Ballingdon, Suffolk

The first stage in the creation of the structure was a competition organised by the Royal Institute British Architects to allow the people of Sudbury and various influential groups associated with the town to be involved in choosing the design of a new bridge. The winning design was that from Brookes Stacey Randall (architects) and Ove Arup (civil engineers).

Design and construction

The form of the new bridge complements the rich architectural history of Sudbury. It seeks to maintain construction excellence in the town while responding to the diversity in the building fabric of the Conservation Area and has been designed to provide a quiet and gentle profile. It has been built from materials with a design life of 120 years and so will require minimum maintenance. Its design and construction paid particular attention to the quality of the River Stour landscape.

From the engineering viewpoint, it is a monolithic, integral reinforced concrete structure with no movement joints or expansion joints. As with all successful projects, delivering the design demanded close collaboration between the engineers, the contractor, the client, architects and specialist subcontractors: in other words, true multidisciplinary teamwork, facilitated by partnering. The completed structure was officially opened on 18 July 2003.

Precast concrete superstructure

The bulk of the bridge is formed from 12 high-quality, exposed precast units constructed off site and stitched together on site. Using precast concrete cut construction time considerably as the sections were assembled on site quickly, reducing potential disruption to local traffic. To produce a finished colour that would match the limestone of local churches, a mix was developed and refined by producing 1m^sup 3^ samples representative of the geometry of the bridge.

Precasting was carried out by Buchan at its factory in Accrington, which has the capability to lift units weighing up to 120 tonnes. The company not only has considerable experience of achieving the architectural quality finish required at Ballingdon but also has the necessary in-house mould design and manufacture capability.

Morphed geometry

Brookes Stacey Randall's design for Ballingdon Bridge combines a number of radii to produce a gentle curvilinear profile with a fine edge and constantly changing three-dimensional soffit geometry. No two adjacent sections are the same. The bridge was designed to be asymmetrical rather than symmetrical, as this gave a more 'natural' form that suited the situation with an extended tail to the Sudbury Bank. By carefully tuning the profile and resultant levels, the precast units produced a diagonal symmetry. As a result, only six moulds were required to cast the 12 units of the bridge superstructure.

Digital data of the form of the new Ballingdon Bridge were used to develop mould geometry. Drawings were carefully checked by all members of the design team and a 3D model of the bridge was adopted to calculate the weight of each unit and its centre of gravity. This information enabled the location of the lifting sockets and temporary support/cradle attachment points to be determined accurately.

Moulds

Key to the production of the precast elements was the fabrication of the moulds at Buchan's Coleshill Works in Birmingham, where they were regarded as the most exacting the company had produced in the past 50 years. Here, the digital geometry started to become a physical reality with the production of 6mm plywood templates. Two types were used: internal templates, which supported the timber surface of the mould, and external templates. These, the key cross-sections of the bridge, were used to check the surface of the mould.

In addition to being required for mould construction, the internal plywood templates were also needed to produce the reinforcing links. The mould surface was Jelutong, a readily worked hardwood with no knots or surface imperfections. Moulds were planed and sanded into the smooth final form, checked and inspected before varnishing. After manufacture, moulds were mounted on steel frames and transported to Accrington for the 12 units to be cast, finished and delivered to site. Each mould was protected with hardboard and the reinforcement cage built within it. On arrival, the reinforcement cage was lifted out by crane and the protection removed. After casting, the unit was inspected and then finished by acid etching. Mixes were carefully batched using white cement and granite aggregate to ensure a consistent colour on all units of the bridge when viewed together.

Assembly

Units were assembled by Costain in February 2003 on a temporary steel structure. Once utilities crossing the bridge had been laid, the in-situ bridge deck was cast. Finally, the temporary support structure was removed in April 2003.

Judges' comments

The form, finish and colour of this bridge are entirely appropriate to its situation and function, and the complex asymmetrical geometry is most appealing. All in all, it is an excellent and well-executed design. In particular, the quality of the precast concrete is first-rate and it is clear that great care has gone into the manufacture. The rendering of a very complex mould deserves special praise. The only disappointment, it could be argued, is that the bridge is not visible in elevation, except from a pub garden - but perhaps that's a good vantage point from where to toast its success!

Ballingdon Bridge

Ballingdon, Suffolk

Client: Suffolk County Council

Architect: Brookes Stacey Randall

Consulting engineer: Arup

Main contractor: Costain Ltd

Precast concrete: C V Buchan Ltd

Formwork and reinforcement: Perform Construction

Ready-mixed concrete: Hanson Premix Central