Projects: The story of The Tide, the Greenwich Peninsula High Line

New York’s High Line park is one of the most successful high-profile urban regeneration projects of the 21st century. Designed by US architect Diller Scofidio + Renfro and US-based English landscaper James Corner Field Operations, the 1.45-mile elevated linear park is situated on former disused railway line in Manhattan’s Lower West Side. Since its opening in 2009, the park has been visited by millions, has spurred the regeneration of adjacent districts and has generated several imitations across the globe. And the latest of these – by the same architect – has now opened in London.

The Tide may not be as long or as high-profile as the High Line but it builds on the same format of an elevated parkway route, and hopes to emulate the New York version’s runaway success. Designed by Diller Scofidio + Renfro in conjunction with engineer AKT II, The Tide is situated in Greenwich Peninsula beside the Millennium Dome, and connects Peninsula Square by North Greenwich tube station to the North Greenwich riverboat pier on the River Thames. 

“There were lots of obvious loading limitations that prevented placing a conventional brute force steel construction above the station”

Emmanuel Verkinderen, AKT II

Although it, too, is a linear park, The Tide presents an entirely different approach to design, engineering and construction to the High Line. While the latter requisitioned disused railway lines, the London model is entirely new build. The High Line is primarily supported by existing cast iron girders, while The Tide is set atop a parametrically designed steel monocoque structure. And at The Tide, the structure had to be built directly on top of the North Greenwich underground station and an adjacent basement car park, which had massive consequences for the material, structural and fabrication solutions developed for the project. Here, Building examines how was it built – and assesses its wider engineering significance.  

Structure

Although The Tide’s full walkway is 1km long, only 250m is currently elevated in the manner of the High Line. It also forms part of an ambitious and as-yet unrealised plan for a 5km riverside route of gardens, public spaces and parks. This would stretch along the full perimeter of the peninsula and is part of east London’s vast Greenwich Peninsula masterplan developed by Hong Kong-based developer Knight Frank and featuring mixed-use buildings by SOM and Santiago Calatrava. So while the northern end of The Tide stops at the Thames, its current southern end has been designed to be extended in the future.

The Tide’s elevated walkway is formed from a series of sculptural “islands”. Four of these are single level and 6m high with a fifth, two level island 9.5m high. Each island is created from between two and eight elements bolted together in a variety of configurations to give each one an individual character. Each element consists of a column or pier which broaden out towards the top in the same way the stem of a wineglass broadens out into the goblet. These piers are set angles which gives each cluster of elements the appearance of series of abstractly cusped arches. But above it becomes clear that the reason why the piers broaden into broad canopies is to accommodate subsoil for the generously planted and landscaped areas incorporated onto the “islands”, and root balls for its mature trees, some of which will grow to be over 8m high. Structurally each element functions as a monocoque, each island is connected seamlessly to its neighbour via a footbridge to create one long walkway.

Each pier contains concealed drainage and services piping for the discreet dispersal of rainwater and for the distribution of power to the walkway, which is dramatically illuminated at night. The location of each pier and cluster was also carefully determined so as to minimise the walkways’ sub-grade impact on the London Underground infrastructure beneath. The twin lines of the Jubilee line run directly under the site and throughout the length of The Tide: North Greenwich station is just 2m or 3m below the ground. There was intense consultation with London Underground throughout the project and onerous restrictions were placed on where and to what extent The Tide’s structure was allowed to permeate below ground depths.

One of the most obvious restrictions involved the size and nature of The Tide’s foundations. Piled foundations were impossible and in the end an expansive but very shallow raft foundation system was used in order to spread rather than concentrate loads. The foundations were solely restricted to the areas underneath the attendant tripod pier clusters above. 

Lightweight

Another structural consequence of the walkway being built above a station was that it needed to be a very lightweight structure in order to limit the size of the foundations and ensure that no undue or unnecessary weight was placed in the ground above the station box and tunnels. As AKT II technical director Edoardo Tibuzzi explains, the monocoque solution emerged very early in the project.

The monocoque structure involves an outer welded steel shell enclosing further welded plates of inner steel ribs. The engineering is similar to that used on aircraft wings and boat hulls and offers the benefits of a jointless and robust structural system combined with lightweight, largely hollow “sandwich” construction. As AKT II technical director Emmanuel Verkinderen (himself a former aircraft engineer) explains, it offered a number of advantages over more conventional engineering solutions.”

“There were lots of obvious loading limitations that prevented placing a conventional brute force steel construction above the station. So right from the start we were attracted to a monocoque solution for a number of reasons. First, it departs from a cladding solution, which makes it more cost effective and means structural skin and architectural finish can be efficiently combined as one. It’s also more slender than conventional steel and enabled the walkway to assume its dynamic geometry. It’s also lightweight, waterproof and its hollow composition easily allowed for the installation of services within the structure.”

One aspect of the monocoque solution is that its lightweight structure and slender form makes the walkway more susceptible to lateral vibration, similar to that famously endured by Foster + Partners and Arup’s wobbly London Millennium Footbridge 19 years ago. There, viscous dampers and tuned-mass vibration absorbers were eventually installed to address the problem but at The Tide that is pre-empted by the addition of 750kg tuned mass dampeners to three of the longest bridges. During the construction stage the footbridges were also tested with accelerometers under factory conditions to ensure the dampeners were effective.

The mass dampeners stabilise the footbridges from excessive vibration by exerting a counteractive force to the natural sway of the structure, effectively acting like a more sophisticated version of a shock absorber or suspension system on a car. Despite their effectiveness, they are used relatively infrequently on bridges and walkways with AKT II associate director Alessandro Margnelli blaming this on widespread industry preference for the “brute force steel construction approach” and general reluctance to “engage in the rigorously detailed analysis required to make mass dampeners effective”.

Prefabrication

Another key aspect of the monocoque was its suitability to prefabricated assembly. Partially because of the contractor’s reluctance to entertain temporary works on site, the walkway was constructed entirely offsite in the steel fabricator’s factory near Venice in northern Italy. Taking full advantage of its waterside location, the components were then shipped by barge from the Adriatic Coast and right up to the Thames. Not only did this offer the environmental benefit of eradicating lorry transport from the project’s logistics but it also meant that size of the smallest transported walkway component was significantly larger than that which could have been accommodated on the back of a lorry.

As AKT II’s Tibuzzi explains, the prefabricated approach also offered a number of advantages. “We enjoyed an enormous level of quality control – by the time the components had been delivered on site we were fully confident that they had been rigorously tested and with regard to both aesthetics and performance. The components also came fully fitted with servicing, drainage, power and finishes, all that was required on site was for them to be slotted together like a jigsaw. And very importantly, prefabrication allowed streamlined fabrication of the components from the parametric 3D model from which the design itself developed.”

This parametric 3D model is the final crucial element of The Tide. The engineering and structural solution was developed in conjunction with the architect right from the concept design section of the project. The pure parametric model not only helped deliver some of the walkway’s complex geometries but it also enabled the consequences of any design changes to be instantly absorbed and then incorporated onto the model. As well as regulating the complex relationship between aesthetics and structure, the model also incorporated all manner of vibrational, climatic and environmental data, which could also be instantly accessed to assess how they would be impacted by design changes.

So much so that Tibuzzi describes the design of The Tide as being based on “the very best optimisation of structure required to realise the architectural vision of a slender elevated walkway.”

This is ultimately its defining legacy. For in using prefabrication and parametric design, The Tide essentially digitises architecture in order to infinitely expand its possibilities.