Summary

Dukes Meadows Footbridge is a new pedestrian bridge in Chiswick, West London, positioned beneath the Grade II listed Barnes Bridge. It joins two previously disconnected sections of the Thames Path encouraging recreation and sustainable modes of travel. 

The distinct alignment, span configuration and consequent structural form directly respond to complex geometric constraints. The result is an elegantly efficient structure that sits comfortably below Joseph Locke’s historic Barnes Bridge. This bridge is very much ‘part of its surroundings’, expressing a connection between context and bridge. The design challenges it overcame include: a wide tidal range exacerbated by frequent flooding, close proximity to a nature reserve, live railway and listed structure and competing stakeholder requirements.

A light touch approach to the sensitive site supported a ‘vision of sustainability’ encompassing both longevity and climate resilience. During the prolonged design stage of this bridge, the way we considered carbon in our structures evolved significantly. It was an early example of carbon benchmarking at key milestones, using methods designed specifically for this project that are becoming industry standard. This process reduced the bridge’s carbon footprint while influencing future designs by the team and their peers.

Introduction

Dukes Meadows comprises 187 acres of parkland located in the London Borough of Hounslow, adjacent to the river Thames. The site is synonymous with public playing fields and sports facilities — with the southern perimeter also hosting many Thames rowing clubs. The site is split by the main Wessex route railway, which crosses the river Thames at Barnes Bridge.

Pedestrian access across the Dukes Meadows site has long been hampered by the northern abutment of the Barnes Bridge railway crossing. This directly blocks the main Thames path requiring pedestrians to walk 300m inland to a railway underpass to circumnavigate — effectively hindering public access, particularly after dark, between facilities located in the western half of Dukes Meadows and nearby public transport links.

In 2016, the London Borough of Hounslow launched the Dukes Meadows Masterplan, aimed at delivering an improved public realm and regeneration of sporting facilities across the wider Dukes Meadows site. A major component identified by the masterplan was the need for a new footbridge around the existing Barnes Bridge railway abutment to simplify public access to facilities in the western half of Dukes Meadows.

Conceptual Design

The London Borough of Hounslow appointed a team led by CampbellReith consultants to develop a footbridge scheme. CampbellReith developed the civil, structural and environmental engineering aspects of the conceptual design with Moxon architects, Marmus ltd (marine engineering) and Slender Winter Partnership (electrical engineering),  

Initial planning surveys and discussions with relevant stakeholders regarding the viability of a new footbridge scheme identified many different aspects that would need to be addressed for a successful scheme to achieve planning / licencing approval and be buildable within the available budget: 

• Appearance: The existing Barnes Bridge railway crossing, including the masonry abutments, is Grade II heritage listed. A new footbridge scheme would need sympathetic architecture to the existing structure, meet DDA disabled access requirements and to be constructed within a limited budget.
• Wildlife: Immediately to the west of the existing railway abutment is located the Dukes Hollow Nature Reserve, one of the few remaining natural tidal habitats in London and home to many forms of local wildlife, including two protected rare species of snails. A new footbridge would need to mitigate adverse implications on the Nature Reserve and its resident wildlife.
• River Authorities and Users: The need for the footbridge deck and piers to be located in the River Thames would cause implications on river flow, navigation and marine life - in particular the needs and concerns of local rowing clubs, who regularly access the river at the proposed bridge site, would need to be addressed.
• Flooding: A new footbridge should not create any adverse implications on the existing flood defences nor exacerbate flooding risks at any location.
• Robustness: The new footbridge would need robustness against accidental impact forces from either errant river vessels or large floating objects carried on floodwater.
• Inundation: To provide adequate headroom for pedestrians under the existing railway crossing whilst minimising encroachment into the river, the deck of the footbridge would need to be beneath the maximum flood level of the river and would therefore need to be designed to withstand partial inundation during extreme flood events. In addition, the design should be capable of being quickly opened back to the public after a flooding event without needing extensive repair or maintenance.
• Network Rail: The new footbridge would need to provide facility for Network Rail to still access and maintain their existing bridge abutment and to not create any adverse implications on the existing abutment or surrounding Network Rail infrastructure– in particular with respect to potentially exacerbating scour risks.
• Archaeology: The new footbridge would need to respect the Thames foreshore with respect to the archaeological significance of the site and meet Historic England’s requirements .
• Construction: The new bridge should be buildable within complex tidal river site, taking account the 4-5m daily tidal water level variation and the limited construction headroom under the existing railway crossing.

Final Alignment

The final alignment of the new 4 span footbridge structure provides a solution that best satisfies the   differing stakeholder requirements for the site. The main 34.3m span under the overhead railway bridge is located to provide a circa 3m clearance between the new footbridge and the face of the existing railway bridge abutment – providing an acceptable trade-off between adequate clearance to maintain the existing abutment without significant encroachment into the river. 

The deck height of the main span is set at +4.900m AOD. This is dictated by the need to provide sufficient headroom under the existing Network rail bridge arches to meet current standards and to maintain security to the overhead structure. Whilst this deck height is above the river Mean High Water Springs (“MHWS”) tide level for the site it is unavoidably beneath the 1 in 100year flood level predicted at +5.185m AOD – needing the deck to be designed for partial inundation during extreme flooding events. 

Connecting side span alignments and lengths are defined by the combined need to continue the new route around the protected nature reserve, whilst maintaining access to existing rowing club slipways and complying with allowable DDA footbridge gradients in tying the new route into existing topography. 

Local widening of the deck above the two river piers gives bridge users the opportunity to enjoy the river views at these locations without restricting pedestrian flow.

River Piers

Following discussions with stakeholders, it became apparent that a successful scheme would need a minimum footprint on the river Thames foreshore. 

A minimum footprint gave many advantages: reduced obstructions and hazards to river users, reduced implications on foreshore archaeology, reduced scour risks, reduced implications on the existing railway, reduced construction costs – as construction within the river tidal zone would incur a substantial cost premium on account of the extra temporary works needed.

The footprint of the river piers and their foundations was further reduced by specifying rigid connections to the deck at the top of the pier. A significant challenge was created by the stakeholders’ requirement for the new footbridge to be able to withstand appropriate river vessel impact loads. A site-specific river vessel survey and risk assessment found that CEMT Class 1 impact loading to BS EN 1991-1-7 would be an appropriate representation of worst-case river vessel impact loading for the piers. 

Initial studies of resulting foundation options found considerable cost benefit taking advantage of the bridge alignment parallel to the river flow in conjunction with a rigid connection between the deck and the piers. Whilst complicating construction, the rigid connection creates a portal frame with respect to river vessel impact forces, dramatically reducing the overturning moments needing to be resisted at the supports. Compared to the extensive foundations needed to resist river vessel impact loads on the piers without the rigid connection between deck and pier (and the piers acting as individual cantilevers) the effective portal frame dramatically reduced the extent of foundations needed in the river tidal foreshore location where construction costs are at a premium. 

The resulting two river piers are formed from elongated circular sections, formed from precast concrete rings with an in-situ core. The piers are positioned parallel to the river to minimise impact on the river water flow, deliberately located away from the existing railway abutment to minimise adverse implications on the existing rail infrastructure and to enable access to the abutment face. Scour implications caused by the new piers were checked with a specialist hydrodynamic assessment and found to be acceptable. 

Approach Span Piers and End Abutment

Additional support piers are provided adjacent to the existing floodwall on the eastern landing – (termed Pier P1) and an intermediate support in front of the nature reserve on the west side (termed Pier P4). 

As these piers are still within the tidal zone, a foundation solution was developed for both using load bearing sheet piles. The sheet piles combined the temporary works requirement to provide a cofferdam to keep excavations dry at high tide, which could then be left in place to transfer permanent works footbridge loads down to adequate bearing strata. Site-welded shear studs provided the load transfer mechanism between pilecap and sheet piles, with long-term corrosion concerns addressed by use of a sacrificial thickness in the sheet piles combined with a reinforced capping beam to prevent water ingress between the piles and pilecap.

The same principle adopted for the construction of an end abutment at the west side and a short approach structure, using load-bearing sheet piles with a reinforced concrete capping.

Superstructure

To minimise encroachment into the river, half-through construction was adopted for the superstructure. Following consideration of many different layouts prior to planning, a warren truss configuration was chosen as the preferred option. Architecturally this best complimented the existing language of the overhead railway crossing whilst maximizing views of the river and surrounding area through the open truss profile.

Final Design

Upon completion of the conceptual design and granting of planning permission, Knights Brown contractors were awarded the contract to complete the final detailed design and carry out the construction of the bridge on behalf of the London Borough of Hounslow. As part of the Knights Brown team, COWI were appointed as civil and structural engineers with Moxon as architects. 

Design Development

The first part of the construction process was to review the conceptual design to ensure that the final detailed design met the bespoke requirements of the Knights Brown supply chain. The following significant developments were part of the final design:

• The foundations for river piers P2 and P3 were changed from a single 1050mm diameter mono-pile proposed in the conceptual design to a pair of bored in-situ 900mm diameter piles connected by a pilecap in the final design. This development was a consequence of a different preferred construction method for the river pier foundations, working at low tide times from a temporary causeway constructed in the river. The original conceptual design had assumed that the river pier monopiles and pier structure were to be constructed from a jack-up barge.
• The main truss tapered bracings were to be formed from trapezoidal bent plates welded together as opposed to the profiled solid plates proposed in the conceptual design.  This change enabled more efficient fabrication of the final superstructure and used less steel.
• Following a detailed review of the logistics, access and stakeholder constraints to the site by Knights Brown’s supply chain, it was concluded that the superstructure spans between P1 to P2, P3 to P4 and P4 to P5 could be more economically constructed by delivering the sections to site by road and lifting into place using land-based cranes. The conceptual design had assumed that these spans were be floated to site on a river barge and lifted into place by a floating crane. The construction team decided that the main span P2-P3 under the existing railway bridge still needed to be constructed by floating into place, as also originally proposed in the conceptual design.

Carbon & Climate

Whilst minimising embodied carbon was not an initial criterion for the project, COWI nevertheless tracked the carbon implications of decisions throughout the design process, calibrating and making use of new tools they have developed in this area. An extensive value engineering exercise delivered a leaner design to achieve construction cost savings but was also shown to reduce the embodied carbon of the final detailed design. As the design progressed to construction the team further reduced embodied carbon through material specification. By quantifying potential carbon savings, cement replacement optimizing use of GGBS adopted for all concrete mixes was justified and delivered. Steel sourced with a target embodied carbon was also proposed but unable to be achieved due to procurement constraints in the market at the time. Overall, the Designer's optimisation of structural arrangement, material specification and construction technique between design and completion have enabled a 20 to 30% reduction in the footbridge’s overall carbon footprint compared to the Client’s reference design.

The bridge is climate change resilient: it can withstand frequent submergence during extreme flooding events. The main structure is designed to withstand the hydrodynamic and buoyancy forces and the aluminium decking allows free drainage. Vulnerable details are minimised through specification of stainless steel and aluminium and the adoption of fully integral connections to the substructure at Piers 1, 2, 3 and 4. Laminated elastomeric bearings are only used at Pier 5, which is well above the most severe flood levels.

Design Automation

The final design structural analysis model was generated parametrically using grasshopper software to tackle the complex geometry, plan layout refinement and truss configuration that were all analysed and evolving during the design process.  Modern structure analysis techniques were used to verify the top chord against U-frame buckling and plate buckling in the pier box due to ship impact.

A modal analysis python module developed internally by COWI was used to efficiently verify user comfort during dynamic loading. It was able to quickly analyse the response to all possible speeds of walker and jogger groups with the codified intensity of loading at all locations along the bridge. This set up enabled the tuning of the positioning and density of the grout amongst other design decisions to minimise vibrations. 

Advanced analysis of the foundation soil-structure interaction enabled final detailed design of the integral bridge piers formed from load bearing sheet piles tied to concrete pilecaps.

Construction

The main Contractor engaged with the community extensively during construction, with regular public updates on local notice boards and a project website. Their engagement extended to local schools throughout the construction period to strengthen community ties, culminating in a steel pan band, choir and flash mob performances from local children at the bridge opening. The result was a high level of local support for the new footbridge, evidenced throughout, with the positive local interest expressed during construction and following opening.

The main span of the bridge runs along the river Thames underneath the Barnes Railway Bridge.  An innovative and unusual construction method was developed to tackle the restricted headroom beneath the existing bridge and the large tidal variation at the site. . 

Firstly, the main span was fabricated as three separate sections and transported to Tilbury dock by road. At Tilbury it was assembled into a continuous length on the dock side and then craned onto a purpose-designed marine pontoon, procured and piloted by the Marine Subcontractor. 

After completing its 36-mile journey up the Thames from Tilbury to the Dukes Meadows site, tug-boats moved the pontoon span into position above its newly built piers at the start of an approximate 30-to-45-minute high tide window. 

100mm diameter steel locating pins, supported on temporary davits at either end of the steel span were lowered through holes in the steel deck into receptacles cast into the top of the piers. Conical heads welded to the upper sections of the locating pins acted as centralisers to force the span into its desired position as it lowered with the tide. The lower sections of the pin were designed to bend plastically under any accidental impacts as the span lowered into its final position. This creative innovation ensured that the magnitude of any horizontal impact forces imparted to the piers were within acceptable levels. 

As the span lowered it eventually engaged with jacks supported on temporary columns which provided interim vertical and torsional restraint to the span. At this point the span was no longer supported on the pontoon which could then be manoeuvred away on the falling tide. 

These interim jacks provided temporary support to the span until a permanent connection to the pier was made with grouting and the stressing of connection PT bars. The remaining spans were then added conventionally – by delivering the deck sections to site by road and lifting in with a crane. 

In summary, the final erection process was completed without issue – a testament to the planning carried out beforehand by the entire construction team in developing the innovative construction methodology.

The superstructure was created by the Fabricator employing state of the art digital 3D modelling techniques to inform fabrication of the complex geometry. The process employed doubly curved SHS chords, RHS cross beams, bespoke fabricated diagonals and purlins, facetted steel boxes with openings for temporary works and electropolished stainless steel parapets. All the structural steelwork delivered to site with a very high standard of workmanship.

Discussion and Conclusion

Dukes Meadows Footbridge is a sophisticated structure entirely suited to its place. The design responds to a complex site and a long list of (at times competing) stakeholder requirements. Fabrication and installation influenced the design at the conceptual and technical stages. Collaboration led by a committed client involved the design and construction teams as well as a range of specialists. In addition to formal stakeholders, the general public were consistently engaged and generally delighted with the outcome. The impact of this footbridge can be felt far beyond the project’s boundaries as it enhances the already cherished Dukes Meadows.