Tennison Road Bridge is the largest bridge replacement project over the rail network in the UK’s South East region in a decade. In addition to providing the London Borough of Croydon with a much improved bridge, this project employed several innovative design approaches and demonstrated that with technology and engineering expertise, it is possible to keep trains running and passengers travelling whilst a new bridge is being launched into position.
The original 1920’s Tennison Road Overbridge
Tennison Road Bridge is a strategic road over rail crossing point spanning 10 operational rail lines at Norwood Junction, Croydon, in South London. The original bridge, built in 1922 and partially replaced in the interim, carries a suburban road and footways over the main line between London Bridge and Gatwick/Brighton. The bridge is a strategic crossing point for Statutory Undertakers Equipment, and carries two main electric feeds for the London Borough of Croydon as well as medium and low pressure gas mains, a water main and telecommunications and highway lighting.
The railway interface below the bridge is complex and comprises of a total of 10 lines, comprising five commuter lines and five other lines linked to Selhurst Train Depot. At the southwest corner of the bridge, Selhurst Train Depot operates 24 hours a day all year round except Christmas Day and part of Boxing Day.
In early 2014, Network Rail commissioned the replacement of the existing bridge, following inspections that rated the superstructure of the existing 9-span bridge as fair to poor. The new Tennison Road Bridge has been designed to serve commuters for the next 120 years with wider road and footpaths, unleashing it from its three tonne weight restriction.
Project summary and Ramboll’s contribution
The scheme comprised of installation of a temporary footbridge to carry pedestrians and temporary utility diversions, followed by staged demolition of the original 9-span bridge and replacement with a new 100m long 3-span bridge.
The 53m main span section of the replacement bridge consists of a half through steel girder design, carrying a wider carriageway and footways over the operational tracks. A two span prestressed concrete beam structure carries the eastern approach spans over redundant rail land. Contiguous bored pile walls retain the approach embankments on the approaches to the bridge.
In an impressive feat of engineering, the 53m 565 tonne main span was launched longitudinally into place over running trains and live tracks, ensuring there was no disruption to passenger services.
Ramboll undertook all permanent bridge works and approach wall design for the scheme, including sequencing for demolition of the existing bridge and checking of the main span during launching. Works also included highway design and environmental co-ordination.
Demolition of the original bridge
Preparation for the demolition of the existing structure began with inspection of the existing bridge. The bridge deck was found to be propping the abutments. Available rail possessions dictated that the structure be removed sequentially, span by span, over a period of a month. This meant that the structure would potentially be unstable at each stage during demolition.
A methodology was devised to stabilise the structure at each stage of the demolition works by constructing the new bridge abutments in advance and tying back both the existing abutments and the bridge deck to these new elements.
Design of the new bridge
The new 53m weathering steel through girder main span is at the limit of the span range for this type of construction. In order to minimise the steel tonnage, Ramboll designed the structure using innovative 3D non-linear finite element analysis techniques, taking into account material non-linearity and geometrical imperfections. This minimised the weight of the steelwork, giving approximately a 30% tonnage saving over conventional analysis techniques, and contributed towards a reduction in the load requirements during bridge launch with added cost and safety benefits and a reduction in the temporary works.
Working in collaboration with Graham Construction from tender stage, several other innovative techniques were discussed and used to value engineer the design, such as pre-setting of the permanent bearings and part casting of the deck to act as a counterweight during launching.
BIM and laser surveying techniques were also used to aid the design and construction of the scheme. The original and new structures were modelled, ensuring build-ability of the proposed design at each stage and aiding in clash identification.
Due to propping requirements, the approach span piers and foundations were constructed in advance of the existing bridge demolition and around existing structure foundations and services, using a low headroom piling rig. The close proximity of adjacent residential buildings required careful consideration and contiguous bored pile retaining walls were built to retain each approach embankment, faced with architectural panels.
Longitudinal launch over operational rail lines
The 300 tonne main steel span was fabricated off site and assembled on temporary supports on the western approach embankment. The concrete deck was then part cast and steel reinforcement added to act as counterweight, a temporary launch nose attached, and the span was then launched longitudinally over live rails into its final position using a hydraulic strand jack system.
See Graham Construction’s time lapse video (below) of the launch of the replacement bridge taking place over passengers’ heads while trains remained running.
Rory McFadden, Graham Construction Project Manager, said: "Launching a 565 tonne bridge over 10 operational rail lines is quite a feat of engineering but the innovative methods used allowed for a controlled and safe installation”
Network Rail's Scheme Project Manager, Shane O'Connor said: "By using technology and engineering expertise we were able to keep trains running and passengers travelling while we undertook what would otherwise have required a complete closure of the railway whilst the bridge was being launched”.
"It's a great step forward for Network Rail and it's also a big leap forward for Croydon, as it will soon have a much improved bridge with extended paths and a wider road."
Project benefits
The new bridge design is wider, stronger and more resilient, eliminating the need for a constraining weight restriction that recently prevented commercial vehicles crossing the old structure. It also boasts improved lighting, wider and safer footways and wider carriageways, improving safety for cyclists.
This is a significant investment in Croydon’s transport network and will provide long-term benefits to drivers and local residents.
Awards
- Winner of ICE London: Evening Standard's People's Choice award 2016
- National Rail Awards, Civil Engineering Achievement of the Year 2015 – Highly Commended
- BCIA, Civil Engineering Project of the Year 2015 (up to £10m) - Shortlisted
