Design Awards: 2013: Award

Twin Sails Bridge, Poole

twin sails bridge
© Dave Morris Photography

Architect

Wilkinson Eyre Architects

Structural Engineer

Ramboll

Steelwork Contractor

Cleveland Bridge UK Ltd

Main Contractor

Hochtief (UK) Construction Ltd

Client

Borough of Poole

The Twin Sails Bridge is at the heart of Poole’s plans for the future and will unlock one of the largest regeneration sites in south west England. The five span lifting bridge crosses Poole’s Backwater Channel and is a dramatic new landmark.

Traffic regularly ground to a standstill when the existing bridge opened for maritime traffic. With the new bridge operating in sequence with the existing bridge, there will now always be a route for vehicles over the Backwater Channel, significantly reducing disruption.

The bridge, with its soaring landmark carbon fibre masts and high standard of finishing and sophisticated lighting, ensures that each operation of the bridge is a spectacle. When closed the low level span mirrors the flat surrounding landscape of Poole town but, when open, visually echoes yachts and sailboats – an acknowledgement of the area’s maritime heritage.

The deck in the pivot zone contains observation windows allowing pedestrians and vehicle occupants to see across the Channel to the opposing sail and enjoy dramatic views of the structure’s operation.

The bridge was designed to nationally accepted standards as defined by the Highways Agency (HA). Due diligence was paid to the Construction Regulations and included assessments of risk to the health and safety of those constructing, maintaining, using and demolishing the structure.

An Environmental Impact Assessment was undertaken during the project leading to the development of a waste management and recycling strategy during construction. All design decisions relating to structural form and materials were assessed in relation to their environmental impact during construction and throughout the bridge’s life.

Materials were selected to maximise durability and minimise maintenance requirements throughout the life of the structure. The lift span deck utilises weathering steel to remove the need for maintenance painting of the internal surfaces within the box. The footways decking is lightweight aluminium and the parapets comprise corrosion resistant stainless steel. The main deck of the approaches is a steel composite construction with concrete deck.

The structure is a double leaf bascule bridge in method, but untraditional in that there are no counterweights. The operation is managed by two hydraulic rams for each sail at normal operational speed – each ram providing sufficient strength to enable the bridge to continue to operate during maintenance possession, but with a reduced operational speed. The rams and hydraulic pumps servicing them are housed in plant rooms within the main supports either side of the navigation channel.

The adaptability of steelwork and its ability to create robust yet lightweight sculpted forms made it the ideal material for fabricating the triangular ‘sails’ of the lifting span. In addition, the use of steelwork acting compositely with a concrete deck for the approach spans also was an ideal solution to produce the desired shallow deck profile. Overall, this enabled the creation of an elegant yet unassuming bridge in the closed position, in keeping with the surrounding landscape, yet creating a dramatic and iconic vision each time the bridge opens for marine traffic.

Tapering transverse steel beams cantilever from the longest catheti edge to support the lightweight perforated deck and steel parapets, drawing the eye away from the deck itself and serving to reduce wind loading when in the open position.

The bridge is an exemplar of innovation. The diagonal cut of the central lifting span creating the impression of two passing sails is unique. The diagonal joint enables the leading point of each triangular leaf to land on a permanent bearing on the main support. This significantly reduces differential deflection as vehicles pass over the joint and removes reliance on hydraulic nose pin interlocks between adjacent leaves.

The diagonal joint format and bearing approach has enabled a passive wedge interlock arrangement between leaves along the joint to be developed; one which requires little maintenance.

The use of weathering steel for the lift span boxes removes the requirement for painting of the internal surfaces; a significant benefit for health and safety and the long term maintenance strategy. External steel surfaces were coated with a modern protective paint system compliant with the latest HA standards ensuring extended life to major maintenance in excess of 25 years. The curved form of the superstructure will also serve to extend the life of the paint system.

The bridge features two distinct structural forms; the approach spans constructed as composite steel boxes with curved soffits, and the triangular lifting spans comprising fabricated steel boxes with orthotropic deck plates, again with curved soffits. The abutments and piers sit on top of 53 foundation piles, mostly 1.2m in diameter and sunk 31m into the Channel bed. The deck and sails were made up of 41 sections of steel.

The construction process itself was inventive. The most economical method of constructing this bridge was to fabricate much of the steel superstructure offsite. Modular sections were brought to the site, assembled to form individual deck spans and welded together on the quayside adjacent to the Channel.

Once assembled each span of the superstructure was jacked up off temporary supports on self-propelled modular transporters (SPMTs), driven onto a barge and floated into position. Making use of the falling tide and fine adjustment from the SPMTs, each deck element was lowered onto permanent bearings allowing the barge with transporters to be removed. This was repeated for each of the approach spans and each lifting leaf, negating the need for significant craneage and reducing the amount of temporary works and assembly work over the water.

Judges’ Comment

This ingenious design for an opening traffic bridge combines elegance and functional efficiency. When closed, the slender deck blends with the surrounding landscape, and when open the tapering arms create a dramatic landmark, reflecting the masts and sails of passing boats. The detailing and fabrication are of the highest standard, and ensure easy maintenance.

The client and the public are delighted with this iconic and vital bridge.

Emirates Air Line, connecting Greenwich Peninsula and The Royal Docks

greenwich docklands
© LUSAS

Concept Architect

Wilkinson Eyre Architects

Delivery Architect

Aedas

Structural Engineer

Buro Happold

Steelwork Contractor

Severfield-Watson Structures Ltd

Main Contractor

Mace Ltd

Client

Docklands Light Railway Ltd

From early discussions with the Mayor of London through to the opening in June 2012, the Emirates Air Line took just over 15 months from contract award – a remarkable achievement – illustrating how a dedicated team, with skills spread across the design and construction disciplines, can work together to meet the most challenging of timescales and deliver an iconic structure.

The project team’s experience in delivering complex structures with all the added pressure of working at height, over the river Thames and in difficult weather conditions proved invaluable. The finished project is a new landmark for London.

The Emirates Air Line is the UK’s first urban cable car system. It was commissioned to close a gap in the transport network by providing a direct link between the Royal Victoria Docklands Light Railway (DLR) station on the north side of the Thames and the Jubilee line station at North Greenwich on the south side of the Thames.

The system consists of two ground stations for boarding and disembarking and five towers supporting the cable. The gondolas hold up to 10 passengers each travelling at speeds up to 6m/s with a journey time of just under five minutes. The total maximum capacity of the system is approximately 2,500 passengers per hour.

The three main structural steel towers of up to 90m high were required to be attractive in their own right and provide a perfect opportunity to demonstrate the flexibility and benefits of steel.

The towers rise up to 90m above the Thames and each comprises a shaft tapering towards a narrow neckline with a flared tower head supporting the cable car sheave assembly. The overall geometry and form was aesthetically driven, as was the form of the shaft itself. This had been conceived as an interwoven assembly of two helically spiralling wide plates known as ribbons and four narrower plates known as helices spiralling in the opposite direction. Structurally the shaft works as a Vierendeel formed from the two interwoven differently pitched and spatially offset ‘springs’ of the ribbon and helix plates.

The geometry was constrained by a maximum footprint of around 10m diameter at the base, a minimum neckline dimension for structure and access and a maximum head width to permit clear passing of the gondolas.

Severfield-Watson Structures developed a unique fabrication method for the tower sections using a series of highly accurate jigs combined with 3D surveying techniques. The first operation was to form the longitudinal ribbons using the double curved plates which formed the outer surface.

A series of support plates were developed from the 3D model and constructed on the shop floor so they mirrored the required 3D geometry of the ribbons. The double curved plates were then placed into the jig with the inner face upwards. This process acted as a double-check on the geometry because had the bending not been correct, the plates would not have fitted in the jig. The longitudinal stiffeners and the nodes were then added to the inside face of the ribbon.

The construction strategy was based on limiting the site welding operations, wherever possible, to an assembly area with all insitu connections at height or over the water being formed by bolting. Assembly areas were therefore created adjacent to the North Tower and on the bank adjacent to the South Tower which is located some 70m into the Thames. The North Intermediate Tower sections were delivered ready for erection and did not require any pre-assembly.

To assist with the assembly a temporary steel base frame was provided under each section, which allowed the tower sections to be assembled in their vertical orientation, and were accurately pre-drilled to receive the individual tower ribbon sections.

One of the primary objectives was to reduce risks and simplify the erection process, which was achieved by developing an end plate bearing flange at each tower splice, which had been proved during a trial fit up, so that the sections could be simply stacked on top of each other without the need for site adjustment. This proved to work very successfully.

Another key consideration was the access required for the steelwork erectors to each splice location. The permanent access ladders and rest platforms were designed where possible to align with the splice positions so that the use of temporary access platforms was minimised.

The erection of the top ears and cable car support frames was very challenging due to the difficult access and tight tolerances that were required. The final position of the head frame had to be aligned to ensure that the true centre line of the cable car was within +/- 15mm after the effects of accumulative tolerances and dead load deflections had been taken into account.

In order to achieve this tolerance and provide the necessary rigidity in the connections the top support brackets were surveyed and then site welded.

Detailed lifting studies were carried out on each of the tower sections which were erected with large mobile cranes located as close as possible to minimise the lifting radius.

Erection of all the towers took place over the winter period between November and March. This combined with the exposed location, the tower heights and lifting restrictions due to the adjacent DLR railway and London City Airport, meant a lot of the operations had to be carried out at night.

Judges’ Comment

The extraordinary demands of constructing complex steel structures to tight tolerances, across the navigable Thames and London City Airport flightpaths, were successfully met in the short 15 month programme.

A structural steelwork success story contributing to London’s outstanding year 2012 and an elegant legacy for ongoing regeneration.

The Cutty Sark, Greenwich

CuttySark

Architect

Grimshaw Architects

Structural Engineer

Buro Happold

Steelwork Contractor (ship steelwork)

S H Structures Ltd

Main Contractor

Gardiner & Theobald
Ellmer Construction

Client

The Cutty Sark Trust

The Cutty Sark epitomises the great age of sail; she is the last surviving tea clipper. It is hoped Cutty Sark’s innovative design and triumph of engineering will set a new benchmark of how historic ships are preserved from now on.

The biggest overhaul of the Grade I listed landmark for 50 years commenced in 2004 with a comprehensive programme of conservation, with the ship’s reopening planned to take place in 2009.

However, the project was brought to a dramatic halt when a fire in 2007 swept through the wooden structure, causing extensive damage to the centre of the ship. This disaster initiated a major fundraising campaign, resulting in the project restarting at the end of 2009 with the design brief enhanced.

When the Cutty Sark was brought to Greenwich from Shadwell Basin she was housed in a dry berth, created in the 1950s, and purpose built in mass concrete on a former bomb site. The ship was floated down the Thames, and manoeuvred into the berth before the end was sealed and the water drained to allow her to rest on the floor of the berth.

The new design and conservation solution proposed to raise the 963t Cutty Sark three metres within the constraints of the dry berth, and demanded that the new interventions had to respect, repair and adapt to the original fabric of the ship.

A complex but elegant pre-stressed system hangs and stabilises the ship in its new position. This steel structure will preserve the shape of the ship’s iconic hull and has enabled an additional public space to be created in the dry berth below, allowing visitors to walk underneath and admire the ship’s form.

The Cutty Sark project features innovative use of structural steel for the conservation and interpretation of a unique Grade I listed ship. In order to achieve this, precise three dimensional surveys were carried out and every new detail was digitally modelled.

Work was carried out to tight tolerances and demanding geometries, at times twisted and curved. Every element of the new steelwork was costed throughout the process, allowing optimisation and management of the design.

When the timbers were removed the true extent of the iron frame’s corrosion was exposed. All of the salt induced corrosion was painstakingly removed and sections of new steel ribs were installed. At the bottom of the ship where the ribs connected to the keel, this involved substantial work as the iron had completely dissolved. The original conserved ironwork is identified by the white painted vertical ribs, horizontal keelsons, deck beams above and their supporting posts. In contrast, all new strengthening steelwork is painted grey.

Redistributing the weight of the ship required the vessel to be lifted at regular intervals along its length so 12 new triangulated steel frames could be installed. These frames take the form of an inverted coat hanger, with two tie rods from the ship’s keel running diagonally up to each end of a horizontal strut that spans the width of the ship immediately beneath the Tween Deck. This inverted hanger frame carries the weight of the ship’s keel and masts back up to the new external support points.

The 12 sets of horizontal beams and diagonal ties form a triangle between the strake plates and new box keelson which encases the ship’s original keel, fixing its vertical position and preserving the ship’s iconic shape from within.

A thirteenth cradle completes the system by connecting the stern of the ship to the keel. The new steel cradles have been integrated with the existing fabric of the ship wherever possible.

The cradle system is fully adjustable via giant turnbuckles set within the primary ties and struts, which ensures a perfect fit to the existing ship fabric and dry berth. The 24 inclined struts carry the weight of the ship and its visitors down to the mass concrete of the original dry berth.

The intense collaboration between designers, specialists and contractors has resulted in a cost effective, robust and elegant solution that will preserve the iconic ship for generations to come.

The journey through the interior of the ship begins in the hold, the storage space used for Cutty Sark’s cargo. A new steel deck is provided to act as both a passageway and a viewing point for the interior of the hull.

Visitors can then walk through the main exhibition space (the Tween Deck) where their route continues from the weather deck across a new suspended bridge to the access tower, a new ancillary structure on the starboard side of the ship.

The visitor then reaches the opening to the dry berth, arriving below the lifted hull of the Cutty Sark for the first time. Resting three metres above, the contours of the muntz clad hull are visible as if the ship is suspended in water, with the glass canopy enveloping the space to meet the hull at its highest waterline. Proudly displayed, and conserved for a further 50 years, the last surviving tea clipper maintains her rightful position in the heart of Greenwich as a an emblem of Britain’s rich and fascinating maritime history.

Judges’ Comment

The steelwork enables this famous ship to be preserved and, importantly, to be viewed internally and externally from below the hull.

The inclined steel struts support the ship, and the timber and refurbished iron members and internal steels maintain the hull’s original shape. The steelwork has been very carefully and sensitively detailed by the use of extensive 3D modelling.

Ingenious steelwork is key to this remarkable visitor attraction.

Air W1, London

AirW1-London

Architect

Dixon Jones

Structural Engineer

Waterman Group

Steelwork Contractor

William Hare Ltd

Main Contractor

Sir Robert McAlpine Ltd

Client

The Crown Estate

The refurbishment of the Regent Place Hotel, originally built in 1915, included gutting the shell of the building and retaining the original Grade II listed façade. The new development (renamed Air W1 – in reference to its postcode and the adjacent Air Street) includes seven floors of offices, a ground floor incorporating shops, restaurant units in the three level basement and nine apartments.

The original building was built for J Lyons and Co and consisted of 1,100 rooms, making it the largest hotel in Europe at the time. It was said to have had the opulence and scale of a transatlantic liner.

By the end of the 20th Century the buildings and their surroundings were in need of significant investment; the Regent Place Hotel was no longer fit for purpose as a modern hotel, and the buildings were in poor condition with size and mix of retail units inappropriate for an international retail destination.

Work on Air W1 began in 2008 and continued through the recession. The go-ahead was given at a time when most schemes were being pulled. By the time the building opened this decision was rewarded by a lack of competition from alternative schemes.

Within the building the 1930s art deco interiors of the Atlantic Bar and Grill and the Titanic Bars, designed by Oliver Barnard, were dismantled and later reinstated. Timber and plaster features were carefully recorded with latex squeezes taken from all profiles and have been fully restored within the redevelopment.

The project has been described as a textbook example of recycling an old steel frame. Wherever possible the existing structural elements were used and adapted for use in the new structure. Where this was not possible the steels were removed and sent for recycling and replaced with new elements. The end result is a melding of the old and the new and demonstrates both the adaptability and sustainability of steel construction.

Waterman arranged for test samples to be cut from elements of the existing structure in order to verify the material properties. These results were then passed to William Hare for incorporation into the final connection design.

One of the more unusual uses of the existing steel structure was to weld one of the tower cranes to it. This was done to protect the still partially intact Atlantic Bar while disassembly took place.

Structurally the building features a larger floor plate than is usual for a building of this type. This was facilitated by some large plate girder transfer structures at the lower levels. In total the job features about a dozen large plate girders each weighing over 10t with the largest being 3.2m deep.

In several cases these plate girders exceeded the capacity of the tower cranes. The solution was to introduce additional splices to break the members down into smaller pieces which were held in place by temporary propping steel until fully erected.

To minimise work at height each of the buildings six atrium bridges were delivered to site as a prefabricated unit and all edge protection, decking, reinforcement and edging fitted at ground level prior to installation.

The roof structure presented an additional set of challenges. The first point of note was the complex geometry of the mansard/atrium roof, which placed the spotlight on the fabrication accuracy and the management of the delivery and erection sequence so as to ensure that the build could proceed smoothly.

Further challenges were encountered, as it was not possible to gain access to survey the roof connections until quite late in the day. In some cases this was overcome by completing the final connection design and fabrication on site.

The layout of the new slabs against the retained façades also placed constraints in the sizes of the members used, and this resulted in the provision of stiff fabricated boxed sections around 200mm deep.

Sir Robert McAlpine developed a special protocol for the exchange of information between various parties for the project. The level of communication between William Hare and the M&E contractor was such that not a single clash occurred between the service holes in the beams and the M&E plant – almost unprecedented for a project of this size and complexity.

The hard work done by the project team at the front-end of the project is recognised to have contributed to the fast build on site and delivery to the end client ahead of time and under budget.

Two combined cooling heat and power units, photovoltaic cells and thermal stores provide 22% of the energy and heat for both this building and the adjacent Café Royal. One of these units is said to be the largest and most efficient fuel cell in Europe, reducing CO2 by 40%.

The topping out ceremony took place on 24th September 2010 and the project was completed in October 2011.

The completed building has already been the recipient of multiple awards including a BREEAM ‘Excellent’ rating.

Judges’ Comment

For this redevelopment over 1,000t of existing steelwork, including massive plate girders, was carefully assessed and then refurbished, adapted or enhanced behind the Edwardian façade.

This is an exemplar for imaginative incorporation of existing steelwork in a striking modern office/retail/residential development.