Pivoting bridge in the Dinosaur Island. Swing Bridge by Tonkin Liu

Description of project by Tonkin Liu

Swing Bridge to Dinosaur Island, Crystal Palace Park

The unique circumstances of the Crystal Palace Park Dinosaur Islands and the need for a secure crossing inspired the structural artwork of Swing Bridge. Informed by Tonkin Liu team’s decade-long immersion in biomimetic studies, the project delivers three specific innovations: swinging to access the banks, undulating geometry for structural strength, and the comb construction technique.

The 167-year-old, Grade I Listed concrete Dinosaur sculptures were configured to depict extinct animals in the lost worlds of deep geological time. The team chose the bridge’s location to mark the start of this sequence, helping to tell a story of evolution, which in turn inspiring the reference to the prehistoric bony fish, the precursor to the Dinosaurs and the evolutionary timeline depicted on the islands.

To protect access to the Dinosaur Islands, the bridge has been designed to remain in the water and only make its connection to land when access is given for education and for maintenance. This negates the need for a large protective barrier, keeps its distance from the Dinosaurs, and requires only one central foundation. Crystal Palace Park in turn enjoys, when the bridge is not in use, a sculptural artwork floating in water.

Form gives strength through geometry in each of the component of the structure. A triangular spine beam delivers load to the central bearing, its tailored form is minimised in response to the bending moments. The skeletal deck structure projects out from the beam’s backbone like form. The bridge form gains overall strength from being widest and tallest over its central support. 

Minimal thickness of the steel sheet is achieved by optimising resistance through the geometry. In the evolution of the first bony fish, an undulating movement produces a force that propels it forward. The undulating form of the balustrade and deck resists forces applied to the handrail through a push-pull action. The balusters lean backward and forward along the length of the undulating deck, the outer acting as a strut and the inner acting as a tie. The multiple bent and welded prongs act together like spokes in a wheel to give the undulating form lateral stiffness. The frequency and pitch of the oscillation have been engineered to find the optimum form.

Responding to budgetary constraints and drawing on the iconography of the bony fish, Tonkin Liu proposed the laser-cut skeletal comb structural technique. The plate is laser-cut to prongs of three different lengths. One prong remains flat to form the bridge deck, one is bent up to form the balustrade, one bent down to form a strut to the lower edge of the central beam, utilising the whole surface of the 10mm steel sheet. This minimises material cost and wastage, reduces the amount of welding by 50%, and allows for the direct expression of strength in sculptural form.

A story-telling process that searches for inspiration in nature has delivered technical innovation in response to the unique circumstances of the site. The project was instigated by the long-standing commitment and insightfulness of the Friends of Crystal Place Dinosaurs, who raised funds from the GLA and crowdfunding, and delivered, through a close collaboration with architects Tonkin Liu, engineers Arup, and locallybased fabricators, Cake Industries.

- Client statement
Ellinor Michel, Friends of Crystal Palace Dinosaurs.

Safe, secure access to the Crystal Palace Dinosaurs is vital for conservation monitoring, repair of the sculptures and grounds maintenance to protect this magnificent, internationally famous Grade-1 listed historic site that is on the national ‘Heritage At Risk’ Register. As a local heritage asset, we are also keen to enhance volunteer, education and outreach opportunities. All this was made significantly more challenging when the original bridge to the islands was removed in 2017.

As the world’s first reconstructions of dinosaurs and other extinct animals, these lifesized sculptures were created at a pivotal moment in 1854 as the first ever “edutainment for science”. Set in landscapes that attempt to reflect 400 million years of geologic time, the ensemble of plants, geology and sculptures is embodies a turning point in history, the democratisation of scientific knowledge. The Dinosaurs are also remarkably beautiful and strange as works of art, and continue to capture visitors’ imaginations in many ways.

Architects Tonkin Liu, engineers Arup and metal fabricators Cake Industries completed a beautiful and fit-for-purpose pedestrian bridge. As a charity we crowdfunded its design and manufacture, building a sense of ownership among the many hundreds of small and large scale donors who gave not only money, but also organised events, donations and pro bono work on the project. In November 2019 we were thrilled to receive unanimous approval of the bridge design from Bromley Borough Council’s Planning Committee, with the Chair remarking that she’d never before seen an application in which every interested party supported the proposal.

The bridge has not only solved a practical challenge, but has added to the beauty of the park, brought together the community and raised our capabilities as a charity. Visitors love the bridge, giving overwhelmingly positive, deep appreciation.

- Engineer Statemen
Stuart Chambers, Structural Engineer, Arup.

Growing up in Australia I had an obsession with Dinosaurs. I can recall seeing the sculptures in the books I read, but never knew they were at the Crystal Palace, so I found it amusing that I should find myself now as an adult working on a project to help extend their legacy.

As an Engineer I’m aware of how despite our best interests we can sometimes stunt creativity. I see the start of a project as hugely important, it’s where ideas are given the opportunity to grow from a delicate beginnings to grand outcomes. At Arup we focussed on providing technical insight to the design team without being restrictive. Together with Tonkin Liu, we explored a number of opportunities that ranged from simple bridge types spanning bank to bank, through to a collection of complex moving bridge forms. We found a way to use a relatively inexpensive, replaceable bearing product, as the project converged on the idea of the bridge as rotating gateway to the dinosaur island. We then started to utilise the technical tools that help us refine our designs. Using Rhino, Grasshopper and Geometry Gym and GSA analysis software a number of parametric models were created and tested in a short space of time allowing us to fine-tune the form, efficiency, and buildability of the bridge. The team experimented with the frequency and number of structural elements, hierarchy of thicknesses and specific form of the bridge curvature. With the input from Cake Industries we were able to precisely align what was most structurally efficient with what was easiest to build. What we built was a structure that like a bicycle wheel has many slender elements but through their shared strength and form deliver an efficient outcome.

- Fabricator statement
David Knight, Structural Engineer, Cake Industries.

As a local fabricator based about a mile away from the site, this project was an ideal opportunity to support a local charity and create something that we are proud of.

The scheme spans the gap between precise digital manufacture and craft-based skills. Cake Industries came on board and developed prototypes and models to feed into the design, to ensure that the innovative processes were achievable. A full object-based 3D model of the structure was then generated by Cake Industries to define the cutting patterns for the laser cut steel. Once laser-cut, the steel sheets arrived in flat pack form ready to be manipulated by the fabrication team to create the undulating geometry. Individual areas were heated with an oxyacetylene torch to allow the comb to be bent into shape. The fully welded assembly was brought together into one piece, before being dipped in the largest galvanising bath in the UK to provide a durable surface finish.

Cake Industries also acted as the main contractor, coordinating the various subcontractors on site as we battled through the uncertainties of building during the pandemic and delivering a finished project. Many of the team use the park regularly and so this has been a personal project for us all, and we are delighted to be able to support FCPD in looking after the Dinosaur sculptures.

- Working detail
Mike Tonkin and Matthew Burnett, Tonkin Liu.

The requirement for supervised access only led to a number of typological studies that fell into two broad types; deployable structures such as gated crossings and forms of folding structures, together with moving structures such as sinking deck and swing bridges. The idea of the swing bridge seemed the right choice, as it also controlled access and had minimal impact on the historical island. The great advantage of the centre swing bridge is that it would be a water-bound swivel bridge, with no permanent connection to land, which with a chain could be locked in place surrounded by deep muddy water, removing temptation to cross to the protected island. This solution also needed only one central foundation submerged and hidden from view.

Ideas in form were tested and proved through a process of digital analysis, through evolving variations the form, to achieves the ambition that form and structure should be as one. A single swivel footing with a single cantilevering beam has the greatest structural forces at the central support so the triangular profile of this spine is optimised to be deepest and widest at the centre and curves upwards and outwards to the two landing points. This curving triangular profile cross section gives the beam maximum strength and stiffness for the minimum amount of structural material as it delivers loads directly to the central bearing.

Many years of developing Shell Lace Structure taught us that laser-cutting was costeffective, and could aid in the process of minimising material usage. Economy and material economy were both key. The first model that explored the deck was a simple paper model cut by scissors, rather like a paper lantern, that made an unfolding comb. Being able to optimise the use of one sheet of metal to be deck, balustrade, and strut, was an exciting direction. To optimise strength in the overall geometry, a quick art straw model tested the notion that curving profile of the comb would add stability to the balustrade and deck. To further add strength, we then increased the amplitude of each undulation, whilst reducing the number of undulations in the whole plan, to decrease visual complexity. Observation of this model prompted us to add a further refinement, a gentle arch to the line of the deck and to the underside, reflecting the true bending moment diagram in the beam, whilst enriching the visual and spatial experience. The material thickness was able to be at its absolute minimal-10mm thick- due to the optimisation of the bridge’s macro and micro geometry.