Finzels reach footbridge is a new £2.7million footbridge across Bristols floating harbour, which allows pedestrians and cyclists a route to the new Finzels Reach devleopment in the city.  The 91m bridge, 4m clear width structure adds a new dimension to the Finzels Reach project.  It twists its way gracefully across the Avon waterway linking the Finzels Reach development with Castle Park.

The bridge has an usual sinuous shape and was designed to maximise the length of the structure to allow the minimum gradient of the bridge form either side while maintaining the necessary clearance over the navigable section of the watercourse.

The architect (Bush Consultancy) and client (Dickson Powell) approached CTS directly to create a structural solution from their architectural aspirations which gave ealier budget confidence and identified critical structural constraints at the outset.  From the initial meeting the Client and Architect committed to work with CTS, confident in our abilities to develop a scheme which would offer them their desired vision and meet their budget.

CTS undertook all structural design of the scheme, its value engineering, manufacture and installation.  In brief, the structure comprises a series of curved steel sections, braced by bolted top and bottom members to create a torsion box which can be assembled on site.

The complex geometry posed challenges for design with regards to both analysis and concept.  The plan curvature generated a low natural frequency structure easily excited by joggers.  CTS's early involvement meant that vibration was identified at concept as being a critical design parameter.  As such, CTS employed a vibration specialist Full Scale Dynamics whom we partnered with to design, manufacture and install and commission a bespoke tuned mass damper to position in the longest most eccentric span to ensure comfort parameters were met for walkers, joggers and crowds alike.

The rising S shape of the bridge brought about challenges during the design, modelling and cladding process of the project.   Modelling the double curvature of each of the beam sections and the bridge as a whole presented a challenge due to the spiralling effect that can be realised in manufacture but cannot be fully reaslised in the model.  Therefore careful detailing and consideration of each of the subsequent elements had to accommodate increased tolerance to account for and manage unknown offsets and interfaces for all parties.  The Tekla Award recognised the hard work involved in the project and how the structure was developed using 3D facilities.  As the model progressed and when it was complete, CTS were able to send BIMsight files on to the Architects and the cladders 'Mouldcam'.  From this model, the Architects were able to see the bridge and allowed them to coordinate all the different teams working on the project and ensure the structure worked alongside the substructure work carried out by Andrew Scotts and existing structure interfaces on site.  The Architect and Cladding team relied upon CTS's complex model to allow accurate interfaces.  CTS worked closed with the Architect to advise on finishes, buildability details to allow their vision to be realised.

Installation

The onerous access constraints required the bridge to be designed such that it could be assembled in progressive stages.  The installation methodology was seen as critical and was established at very early stages and informed the design.  For example, due to the plan curvature, road delivery required the structure to be delivered to the assembly area as single beams.  Then it had to be partially construction into 5 moveable steel frames which could be transported down the river on barges and fit beneath the space limitations imposed by historical arch bridges en-route.  The structure required a design that could be assembled from a kit of parts to create a weaving stiff torsion box spine beam to allow safe progressive installation but also one which could be installed quickly over the river without extensive scaffolding or plant.  Each section was between 8 and 24m long and was limited in size by lifting plant and pontoon capabilities.

Detailed sequencing plans were required to ensure plant and pontoons were not boxed in while simultaneously utilising pontoons for sequential temporary propping to ensure stability and using as travel barges.  The primary spine beam was installed in a week, to programme, despite several set-backs outside our control.

Once the spine beam was installed, finally working from pontoons CTS installed each of the cantilevering outrigger arms to complete the structure to its full width.  Co-ordination between our works, the cladders and the lighting suppliers dictated sequencing to ensure we all met the programme.

This bridge construction project presented many technical and logistical challenges due to working in the harbour but the proactive input and team effort from all stakeholders produced an outstanding finished result.