2020 Awards

Structural Steel Design Awards

Tintagel Footbridge, Cornwall

  • © Hufton and Crow
  • © Hufton and Crow
  • © Jim Holden

PROJECT TEAM

Lead Architect:
Ney & Partners

Co Architect:
William Matthews Associates

Steelwork Contractor:
Underhill Engineering Limited

Main Contractor:
American Bridge UK

Client:
English Heritage

Tintagel Castle is one of the UK’s most important historic sites - a Scheduled Ancient Monument, Site of Special Scientific Interest, Area of Outstanding Natural Beauty and a Special Area of Conservation. Managed by English Heritage on behalf of the Duchy of Cornwall it draws over 200,000 visitors a year and up to 3,000 a day in the peak summer season.

The Cornish name Din Tagell means literally the Fortress of the Narrow Entrance. An important trading port in the fifth to seventh centuries, it achieved international fame in the 12th Century when Geoffrey of Monmouth’s History of the Kings of Britain described it as the place of the legendary King Arthur’s conception. In 1230 this inspired Richard, Earl of Cornwall, to build a castle on the natural land bridge linking the headland (also known as the island) with the mainland. The land bridge has since eroded significantly and destroyed large parts of the castle which is now split into two and separated by a void 65m wide. For centuries visitors crossed a wooden bridge at the foot of the void and climbed a series of vertiginous steps onto the island. This restricted access for many people and caused significant congestion in the summer months, ruining the experience for many visitors.

In 2015 English Heritage held a design competition for a new bridge reconnecting the two halves of the castle allowing visitors to experience the historic approach and entrance to the castle’s inner ward. From 135 entries six teams were shortlisted, and the winner was announced in early 2016.

The design is relatively simple - two 33m-long cantilevers which reach out from each abutment and don’t quite meet in the middle. The central gap serves two functions; technically it allows each bridge half to expand and contract with variations in temperature; and poetically it creates a threshold between the mainland and the island. A series of 16m-long rock anchors tie the bridge halves into each cliff face.

The palette of materials is equally simple - painted mild steel for the main chords; duplex stainless steel for the cross bracing, deck trays and balustrading; Delabole slate laid on edge for the deck finish; and untreated English oak for the handrail. Each material was selected for its durability in an extremely harsh marine environment, and the manner in which it would weather with as little maintenance as possible. Architecturally, the aim was to create a bridge which was resolutely contemporary in its design and fabrication, but also timeless and complementary to its setting.

The steel element was chosen as a lightweight solution, and one that can be fabricated offsite into deliverable pieces. Getting the steel elements to site was just one of the challenges that needed to be overcome, as the gatehouse can only be accessed by one narrow lane. A multi-axle vehicle was used to deliver the steelwork and navigate the winding road.

Lifting the steel into place was another significant challenge, with no room or access for a crane in the gorge, which is more than 60m-deep. A cable crane was installed, more commonly used in mountainous regions such as the Alps, to supply materials and even personnel to otherwise inaccessible locations. The cable crane had a 5-tonne lifting capacity, could pick-up steel elements from a small holding area on the headland and subsequently fed the construction of the bridge’s two cantilevers. None of the bridge’s steel elements exceeded the cable crane’s capacity, while the largest two pieces, each 10m-long × 4.5m-deep and installed at either end of the cantilevers where the structure meets the abutments, were within a size that was transportable on the access route.

All of the steel elements were fabricated into fully-assembled and erectable pieces, that included top and bottom chords, bracings and cross members. A total of six pieces were needed for each of the cantilevers. As the bridge is in a very aggressive environment with plenty of wind-borne sea salt around, mild steel was chosen for the parts which can be easily repainted and stainless steel, which is more resistant to corrosion, for the areas where painting would be more difficult.              

The connection points between each individual steel assemblage (two on each piece) are also were fabricated from stainless steel and were welded to the main chord members during the fabrication stage. The connections consist of finger joints that interlock with opposite members on the adjoining section, similar to a woodwork dovetail joint. Once the individual sections were lifted and manoeuvred into place during the erection programme, the connections were then bolted up. The fabrication process required some precise engineering and each section was trial erected with its neighbouring piece to ensure the two cantilevers could be seamlessly erected on-site.

Construction started in September 2018 and was completed in August 2019.

A highly crafted and timeless structure: daring in its concept yet modest and sympathetic to its historic a nd natural context. Every steel component has been carefully detailed for constructability and durability, elevating the graceful aesthetic. The project is a triumph: a credit to English Heritage’s vision and the entire team which employed mostly local fabricators, supported by alpine construction specialists.

Judges’ comment

52 Lime Street, London

  • © Simon Carr Photography
  • © Simon Carr Photography

PROJECT TEAM

Architect:

Kohn Pedersen Fox

Structural Engineer:
Arup

Steelwork Contractor:
William Hare

Main Contractor:
Skanska

Client:
WRBC Development UK Limited

Located in the heart of the City of London, 52 Lime Street is also known as The Scalpel, a moniker awarded due to the building’s striking, iconic and sharp design.

The client developed 52 Lime Street as a location for their UK Headquarters, as well as providing lettable commercial office space for additional tenants. Their main objectives were to maximise the rental space on this key site; to create large uninterrupted floor spaces providing an efficient and collaborative working environment; and to deliver exemplary sustainability performance and building performance metrics that exceed British Council for Offices (BCO) standards.

The Scalpel is sleek and geometrical with a series of interesting reflective planes giving it a distinct identity which sets it apart from its neighbours. It features an inclined northern façade, which has a diagonal fold line running from top to bottom. This façade is formed by a series of cranked plate girder columns, spaced at 6m centres. For the double-height ground floor these columns are vertical, but from the first floor they are cranked and slope inwards all the way to the building’s pointed top.

At 42 storeys, including 36,966m2 of internal office space over 35 floors, 52 Lime Street is a tall, yet sympathetic addition to the City cluster, designed with particular regard to distant and local views.

The structural frame consists of a composite design with steelwork supporting metal decking and a concrete slab. All of the floor beams are 670mm-deep fabricated cellular plate girders to allow service integration within the structural floor zone.

Unlike many commercial buildings, the Scalpel’s main core is offset and positioned along the south elevation which provides shade from solar gain. In this way, the structure’s available floor space has been maximised and internal spans of up to 20m have been achieved. Having an offset core coupled with an inclined north elevation means that the loads on the building are eccentric from the main stability-giving core. To counteract this, the north elevation, as well as the east and west façades, were designed as large perimeter moment frames to add stiffness to the building. The frames also allowed erection to continue with minimal temporary works, reducing cost during construction.              

As always, cost was a key driver of the project and the use of a BIM model on this scheme helped to ensure the steel frame is as efficient as possible. Considerable weight

savings were made by using beams with varying flanges and webs depending on the relevant loadings, which was determined automatically using the BIM model.

Another key driver on 52 Lime Street was a complex construction sequence, which involved optimised connection and main member designs, the installation of damper systems and a trial erection of an iconic 10-storey triangular attic at the peak of the building, which houses the plant and maintenance walkways. To ensure the smooth erection of the attic structure on-site, this portion of the building was trial erected in the fabrication yard where the complex fabrication and tight tolerances were tested and proven.

Following the trial assembly, the structure was dismantled and transported to London in the largest possible pieces to reduce the piece count and allow for erection on-site by tower cranes. The use of a complex pre-set strategy ensured the final position of the structure was within design tolerances. The attic was designed to be erected floor-by-floor, with each floor level immediately stable upon erection. Designing the attic in this way was vital as there was no core this high up the building to provide lateral stability and no internal floors to provide horizontal diaphragm action.

The triangular shape of The Scalpel and the prevailing south-westerly winds that hit the structure’s narrowest point, meant that a total of seven viscous dampers were needed to be installed within the north elevation of the steelwork. These hydraulic devices dissipate the kinetic energy of the building and stop the build-up of uncomfortable side-to-side accelerations in high winds, improving user experience and the durability of the building as a whole. By building the viscous dampers into the stability system of the structure they provide damping at a fraction of the cost and use less space than more traditional ‘Tuned Mass Dampers’.              

The environmental successes demonstrated by 52 Lime Street, acknowledged by the first ever Design Stage Certificate under the BREEAM UK New Construction 2014 Scheme, were a direct result of the strong collaboration between the architect, main contractor and structural engineer from the outset. Examples include a 25% operational carbon reduction, a Green Guide rating A+ for embodied impacts, a 45% reduction in potable water use, a biodiverse green roof and support for sustainable commuting with the incorporation of secure, covered storage in the building for almost 400 bicycles, along with extensive locker and shower facilities.

Taking its place within the cluster of prestigious tall buildings in London’s financial centre, the distinct inclined outlines of ‘The Scalpel’ complement the surrounding profiles. Ground-breaking savings in both costs and embodied carbon have been achieved by innovative solutions including integral damping and advanced digital design. Advanced use of BIM, along with full-scale trials enabled compact integration, maximising letting areas.

Judges’ comment

The Curragh Racecourse Redevelopment, Kildare

  • © Gareth Byrne
  • © Aecom
  • © Gareth Byrne

PROJECT TEAM

Architect:
Grimshaw Architects

Structural Engineer:
AECOM

Steelwork Contractor:
Kiernan Structural Steel Ltd

Main Contractor:
John Sisk & Son

Client:
The Curragh Racecourse Ltd

Located in the heart of the protected grasslands of the Curragh plains in County Kildare, Ireland, The Curragh racecourse is steeped in history and tradition. It is internationally recognised as one of the best racecourses in the world, as well as the spiritual home of flat racing in Ireland. However, to maintain its competitive position, a full redevelopment of the site was needed to meet the demands of the future.
The brief required a new racecourse grandstand with supporting infrastructure facilities for 6,000 people, within a masterplan that was designed to accommodate a crowd flux of up to 30,000 within the wider grounds.

The design responds to the site’s unique context in an elegant, yet unobtrusive manner that embraces a spirit of place. The new grandstand comprises stacked horizontal forms, crowned with a dramatic soaring roof that recognises the planar landscape in which it is set. The copper colour of the roof references the rural, Irish vernacular and agricultural heritage of Kildare, whilst the contemporary panelled roof structure, comprised of aluminium sinusoidal panels, provides a striking yet empathetic appearance amidst the rolling countryside.

In the world of sports structures, it the scale and sense of drama that usually impresses. Up close, the structures can appear somewhat utilitarian. The Curragh is different. Through meticulous attention to detail both in the design and construction, there is a sense of exquisite quality at every scale, a rare combination of grace and grandiose, a place where structural artistry meets architectural vision.

The dramatic 7,200m2 cantilever roof design was key to creating the architectural vision of a “planar building in a planar landscape”, with the envelope surfaces tuned to mask the depth of the structure and create a gravity-defying illusion with cantilever spans ranging from 27m in the central area to 45m in the double-cantilevered corners.

This vision has been delivered by focusing on structural elegance and simplicity. The roof structure, supported on the exposed precast concrete grandstand frame below, consists of a regular arrangement of steel cantilever trusses tapering into open plated sections at the tips to create the razor-sharp leading edge as well as simplifying fabrication. Additional spine trusses follow the diagonal hip line of the roof corners, creating a two-way lattice frame with optimised planar geometry.

Adopting very shallow steel trusses would have yielded a very heavy and uneconomical structure, but it was found that the illusion of the sharp edges and super-thin roof surface could be achieved using relatively modest span-to-depth ratios for the vast majority of the trusses, because the double-clad top and bottom surfaces would never be viewed together. This also allowed the MEP plant to be concealed within the roof space with no detriment to the overall form. The result is a total steelwork mass of approximately 115kg/m2 for the majority of the roof area, a fine achievement for an environment with dominant wind loads and the need for compatibility with the fully clad surface and the glazed box suites that are suspended over the main grandstand from the roof structure.

Integrating the structural solution with the building envelope was key to the success of realising the design team’s mutual vision. For the long-span double-clad roof the structural engineers and façade engineers worked hand-in-hand to deliver a holistic design solution, minimising the overall quantities of structural steelwork by ensuring all steel surfaces were fully coordinated with the cladding fixing requirements. This included integrating with the MEP, lighting and rainwater collection systems without compromising the structural or visual integrity, which allowed the overall depth of the structure and envelope to be minimised whilst also delivering an economic solution.

The extensive use of bespoke digital modelling tools throughout the design and delivery process allowed for the MEP building services and architectural finishes to be efficiently integrated with the structure. Regular exchange of digital models and visualisations combined with continuous dialogue and interaction between all parties was key to the success of the project.

The parametric model of the structure informed early discussions with the client and later the main contractor in relation to crane siting, lifting capacities, splice details, locked-in stresses and movements and tolerances between the adjacent frames. By pre-empting these issues, the design team could react quickly, having a detailed knowledge of the constraints and options available from the outset. In particular, a detailed construction stage study of the cantilever roof pre-setting was critical to achieve a seamless and crisp roof profile upon completion of the cladding installation.

The collaboration between all parties during both design and construction phases, and the development of the original concept design into the completed building through creative and innovative structural design, is an exemplar of bringing best value to a scheme without losing the essence of the original vision.

The completed project opened to the public in Spring 2019.

A blade like aerofoil roof is now the dramatic centrepiece to this open landscape and world-famous sporting venue. Behind this bold architectural statement lies a highly accomplished level of detailed design, precise fabrication, and accurate construction to the most demanding of tolerance requirements. A great team effort.

Judges’ comment

Bath Schools of Art and Design

  • © Chris-Wakefield
  • © Chris-Wakefield
  • © Paul Raftery

PROJECT TEAM

Architect:
Grimshaw Architects

Structural Engineer:
Mann Williams

Steelwork Contractor:
Littleton Steel

Main Contractor:
Willmott Dixon

Client:
Bath Spa University

The existing building, designed by the Farrell/Grimshaw Partnership, was completed in 1976 as Herman Miller’s primary furniture factory in the United Kingdom, as it remained until 2015. Bath Spa University purchased the building in 2016, by which time it was Grade II-listed, in order to relocate the Bath School of Art and Design. The re-purposing builds on the original design ethos: allowing for the ongoing flexibility of the building, it will continue to adapt to the changing needs of the building users and engage with the wider community.

It was a key ambition to retain as much of the existing building as possible. The steel façade frame remains, which supports a flexible modular system of glazed and solid panels, as well the primary structure of continuous secondary roof beams at 2.5m centres which span the three main 20m bays, and are supported on primary beams on a 10m grid of columns. Beyond the challenges of retaining and refurbishing the existing, new steel structure raises the roof by 1m, supports a new roof deck for extensive plant, supports a rooftop extension above the existing building and encloses two wings of flexible workshops and studios, as well as providing a substantial new mezzanine level.

Reflective areas encourage students to distance from their practice enabling discourse outside of disciplines. Communal open spaces encourage ‘bumping into’ of staff, students, professionals and visitors, providing unknown opportunities. While spaces such as the Gallery, Art Shop, rooftop pavilion, a publicly accessible café and riverside landscape, are designed to actively engage enterprise activities and the local community.

With an emphasis on ‘thinking through making’ the students and their creative practice were placed central to the overall design. The immense workshop facilities were located in the centre of the building to enable focused support from specialist technicians, and the ability for students to move seamlessly between materials and processes, whilst then allowing the mezzanine to be open and highly flexible.

It was well known that the courses taught at the building will evolve over time in an unpredictable way. To facilitate this the new roof is raised by new Vierendeel steel trusses, allowing a flexible network of ‘plug & play’ services to run at high level. This allows the spaces below to be reconfigured as required. The modular façade system also allows the elevation to be easily reconfigured to respond to changing internal requirements.

The mezzanine floor beams have additional web openings to allow for future servicing and both the mezzanine and rooftop pavilion are designed to allow the internal layout to be reconfigured to suit future needs. All structures are framed to be independent of the existing to allow for future removal or adaption without detriment to the original.

More than retaining significant embodied carbon within the building, a key outcome for the project was to ensure that the energy performance was brought up to modern standards and beyond, safeguarding its long-term future. The entire envelope was upgraded to provide dramatically improved thermal performance through new double glazing, additional insulation and much improved air-tightness. The new roof provides over 100 rooflights, reducing the reliance on artificial daylighting, and photovoltaic (PV) solar panels providing over 10% of the building’s energy consumption.

The steel structure allowed the 20m spans to be retained, leaving a flexible volume below with minimal interference from structural columns. It was also in-keeping with the ethos and industrial character of the listed building. The efficiency of the structure also allowed space for the services to run between it at high level.

The new raised roof structure spans above the existing beams, moving the load of the roof closer to the columns. This maximised the capacity of the existing structure so the roof could support the increased insulation, PVs and rooflights.              

The new Vierendeel steel trusses were fabricated offsite in two parts, and craned into position, before being bolted together. The majority of the structural steel relies on bolted connections, to both facilitate future deconstruction and to protect the integrity of the existing listed structure. To this end the new rooftop extension is supported on steel columns and cantilevered trusses that thread between the existing steel beams, as an independent structure, with half the columns making use of existing pad foundations that were provided to allow for a future extension. The new rooftop plant deck structure also follows this same ethos.

The project posed many unique challenges in restoring, enhancing and extending an existing building. Its structure, servicing and architecture all work together to deliver an outcome that safeguards the future of this listed building, whilst significantly enhancing the environment for the current users and the energy performance. It has been a true collaboration, from designers to constructors and client, and in maintaining its flexible principles, the building will be enjoyed by generations to come.

This project involved a major re-purposing of a Grade II-listed industrial building, thus validating key concepts of the original 1970s design - adaptability and sustainability. Structural additions were separated from the existing, requiring careful installation and the façade sensitively upgraded to improve performance. The result is a building of exceptional quality ideally suited to its new use.

Judges’ comment

A14 Cambridge to Huntingdon Improvement Scheme

PROJECT TEAM

Structural Engineer:
Atkins, CH2M Hill Joint Venture

Steelwork Contractor:
Cleveland Bridge

Main Contractor:
A14 Integrated Delivery Team

Client:
Highways England

The £1.5 billion scheme to improve the A14 trunk road between Cambridge and Huntingdon required a number of new bridges along the new route. These included the scheme’s showpiece bridge; a 750m-long viaduct over the River Great Ouse, and two identical 1,050-tonne bridges to carry a major roundabout at Bar Hill Junction over the new A14. Weathering steel was used for all three structures to provide the required durability with minimal future maintenance.

The River Great Ouse viaduct required 6,000 tonnes of steel, comprising 76 separate main girders and 800 cross girders. The ladder deck bridge spans the river itself and a large area of floodplain on either side. Supported on 16 pairs of piers, most of the main girders required were 40m-long, 2m-deep and weighed 50 tonnes. The section of bridge that crosses the river has a longer span requiring more complex girders, with larger, deeper haunches to carry the greater load.

In order to produce the cross girders more efficiently, a new welding procedure was devised that involved modifying the T & I machine with two welding heads on each arm, instead of the usual one, allowing twice the amount of weld metal to be placed per metre per minute.

A time-saving construction method adopted on this viaduct was the use of precast concrete slabs for the deck rather than the more traditional insitu concrete deck slab. This meant that the concrete deck units could be installed simultaneously while steelwork erection was still in progress further along the bridge. This construction sequence demanded close coordination and also meant that every piece of steelwork had to be fabricated to extremely tight tolerances to ensure a precise interface with the precast concrete slabs.

A temporary platform on the floodplain under the length of the bridge provided a solid base for cranes and lorries, but a different crane was offered to the one originally specified. By using a 600-tonne capacity crawler crane in place of a 300-tonne capacity crawler crane, the installation team could install all girders from one position at each ‘bay’, as well as being able to install all the heavy girders at the river span section. By minimising the crane movements, an installation rate of one bay per week could be maintained and even accelerated during periods of good weather.

The viaduct was completed on budget and ahead of schedule.

The installation of the twin bridges at Bar Hill Junction over the new A14 maximised the advantages of offsite steel fabrication and rapid assembly to improve programme times, reduce environmental impacts and minimise disruption to road users.

The multi-girder bridge decks, each measuring 47.5m in length comprise three pairs of braced main girders supporting GRP permanent formwork and an insitu concrete deck slab. Overall, each deck contains 330 tonnes of steel and 720 tonnes of concrete.

The original plan was to erect the bridges piece-by-piece using a crane. This would have involved closing the A14 for a number of weekends, causing significant disruption. However, a more cost-effective scheme was developed that allowed both bridges to be constructed offline prior to installation, and then installed using self-propelled modular transporters (SPMTs).

Following unforeseen programme delays, the site erection scheme was modified to reduce the time required on site even further. Instead of delivering the bridges as part-length paired-girders, they were delivered as 12 full-length single girders. This removed the need for main girder joints to be welded on site, which reduced the overall programme by three weeks. This also significantly reduced the number of deliveries to site from 18 to 12, minimising environmental impacts from transportation.

All steel components were fully trial assembled at the factory prior to delivery to ensure accurate fit-up. The girders were then delivered to a large temporary set-down area that was created close to the bridge site. Upon delivery, the single girders were placed onto specially constructed trestles and braced together. GRP permanent formwork and cantilever edge formwork were then added and the insitu concrete deck slab cast. The offline deck slab construction significantly reduced the overall construction programme.

Civil engineering works on site, including the construction of the concrete abutments, were undertaken in parallel with girder fabrication, so close collaboration was essential to ensure that both elements interfaced perfectly.

The A14 was closed to traffic at 9pm on a Friday to allow the sections of the existing A14 carriageway to be infilled and surfaced. The fully concreted bridge decks were then lifted from the trestles onto the SPMTs, and manoeuvred at less than 1mph onto and along the carriageway. The decks were positioned by the SPMTs and lowered precisely onto the concrete abutments.

Both bridges were installed during a single 11-hour period and the road was clear for reopening at noon on Sunday, an incredible 18 hours ahead of schedule.

One of England’s largest road improvements included a 750m long viaduct over the River Great Ouse and two new bridges at Bar Hill interchange, incorporating over 9,000 tonnes of weathering steel. The judges were impressed with the innovative solutions the design team employed to minimise disruption and optimise the programme. Flawless execution on site included installing two bridges, each weighing over 1,000 tonnes, in just 11 hours.

Judges’ comment

Brunel Building, London

  • © Jack Hobhouse
  • © Guy Archard
  • © Dirk Lindner

PROJECT TEAM

Architect:
Fletcher Priest Architects

Structural Engineer:
Arup

Steelwork Contractor:
Severfield

Main Contractor:
Laing O’Rourke

Client:
Derwent London

The architectural brief was to create a landmark building which provided high-quality, innovative, people-centred workspace and which would re-engage the site with the canal. The site faces Brunel’s Paddington Station and this legacy of engineering inspired the building’s design. Carefully refined and consistent structural steel details are a key aspect of the aesthetic of the project.

The structure and services are exposed internally to maximise flexibility and workspace volume. This logic is continued externally with an exoskeleton positioned outside the façade, which extends beyond roof level to create glazed, wind-sheltered gardens on the 15th and 17th floor levels. The exoskeleton also shades the large expanses of glazing, affording scenic panoramic views across the city skyline. This approach brings many benefits to the building, including tall, open column-free workspace, 25% solar shading and a dynamic appearance.

Despite the bespoke nature of the building, a regular 6m floor beam spacing was used with precast lattice slabs set down into the web zone of the supporting steel plate girders. The services and structure are seamlessly integrated, thus enabling a more efficient use of the available structural depth and maximising floor-to-ceiling heights.

A semi-unitised curtain-wall cladding with an insulated strong-back system provided a considerable amount of repetition together with flexibility where required.

The benefit of this adaptability can be seen in the fact that the building had been pre-let by the time of practical completion, with eight different tenants who all do different things and who have all fit-out their spaces in different ways.

This adaptability and high architectural quality should also allow the structural design life to be met and exceeded. A detailed whole-life carbon assessment forecast a 71% operational energy improvement over a typical office fit-out and a 7.5% embodied carbon reduction against a typical new-build office building.

Generally, floor beams span directly from core wall out to the exoskeleton. One consequence of this is that the location of the floor beams on each level varies so as to meet the exoskeleton support, which also means that beam spans and service opening locations vary on each floor. This pushed the project team to use digital workflows to optimise and communicate plate thicknesses, weld sizes, connection designs, precambers, movements and fabrication and installation information. For example, close collaboration between all parties allowed the various stiffnesses, tolerances and construction sequence impacts to be considered and individual precamber values agreed for each beam in the building.

This also provided a challenge for the contractor and MEP sub-contractors which they solved, in part, by projecting the MEP sub-contractor information onto the relevant ceilings whilst the operatives installed the relevant equipment and service runs.

Where floor beams pass through the façade, they are haunched so that the head of the glazing can be raised, increasing daylight penetration. At the core end, they are haunched to allow air distribution ducts to pass beneath. This notch was unstiffened at the request of the architect, and justified by using plastic design including non-linear finite element modelling. The beams meet the external structure with a thermally broken connection encased in an insulated stub collar, which is removable to allow for future inspections.

The exoskeleton defines the character of the building and is a visual focal point, with the inclined columns and braces carrying the gravity loads at the perimeter of the building and bridging over constraints below ground level - the development is adjacent to the canal and sits atop two London Underground tunnels. However, the conscious design decision to incline the perimeter structure and make it external leads to inevitable complexity and risk, which was mitigated in several ways.

Full use was made of the latent structural capacity in the concrete core, which allowed freedom in the diagrid topology with minimal impact on structural tonnage. Tuning the diagrid structure and connection releases controlled the load-paths and optimised the diagrid tonnage. Some elements which are slotted and ‘inactive’ in the permanent design were ‘locked’ during construction by the use of specially machined shoulder bolts which were then replaced with standard bolts. Highly advanced modelling of key connections (both structural and thermal) achieved compactness and purity of detailing.

Generally, for fire protection, the steel structure is either insulated by concrete or protected with intumescent paint. The typical floor build-up includes lattice planks and topping sitting flush with the steel floor beams. This meant that the top flanges would be exposed during construction. Significant time and cost savings were achieved by not coating the exposed top flange with intumescent paint in most locations. This was assessed and justified using 2D computational heat transfer analysis.

The realised project could only have been achieved with a steel frame, which facilitated long spans, integral MEP service runs, optimisation of the exoskeleton and floor beams, and the overall design aesthetic of the building.

This project shows how a proactive client working with a talented team can produce a commercial office building of the highest integrity. Expressed structural steelwork in the external frame and floor structures is dramatic and dynamic; all is detailed with great care and attention. A roof garden overlooking Paddington station and the canal basin provide a welcome extension to the public domain.

Judges’ comment

.