Structural Steel Design Awards
Wimbledon No.1 Court
Installing a moving roof over No.1 Court involved the adaption of the 1997 building without interrupting the annual tennis Championships. This extraordinarily complex work was carried out over three seasons with minimum public awareness. Large movable steel trusses installed to very exacting tolerances over the existing building provide a roof that can shelter a match from rain within minutes.
Thornton Tomasetti Ltd
Sir Robert McAlpine
The All England Lawn Tennis Club
Constructed over a three-year period, the steelwork element of the re-development of the No.1 Court included the demolition of the existing roof built in 1997, infilling and extension of the level 4 and 5 floorplate steel and new fixed roof, followed by the construction of a moving roof that will provide full rain cover during the Championships.
The main client requirement was that the works to the No.1 Court would still allow the Championships to take place with a minimum of disruption. This resulted in one of the more interesting aspects of the build and necessitated the development of a staged site programme that incorporated into the working methods and temporary works the appearance and character expected of the All England Lawn Tennis Club.
Other objectives for the design were to extend the terraces to maximise the seating within the existing structure and provide new, enlarged and improved debenture/hospitality facilities, whilst optimising grass growing conditions throughout the year both on the No.1 Court itself and the adjacent grass courts. In addition, the movable roof had to be deployed, and the internal environment conditioned, both to eliminate condensation on the playing surface and provide a comfortable spectator environment, all within a 30-minute stoppage of play.
The structure for the east and west sections of the static roof is largely symmetrical. There are long span prismatic trusses spanning 47m between the existing cores, and 80m between new supporting columns installed in the corners. The exception is the NW corner, where the truss is supported on another transfer truss spanning between the cores. The top and bottom chords follow the profile of the cladding surfaces and are large tubular sections. These main elements provide sufficient vertical stiffness to support the weight of the static and moving roofs, good lateral stiffness to resist the lateral forces imposed on the structure from both wind and the moving roof, and create a large open internal space within the truss that facilitates the integration of the MEP equipment.
A secondary structure provides support to the rails that carry the moving roof. The inclination of the prismatic trusses, and the positioning of this secondary structure, enables the load from the moving roof to be located over the bottom chord of the prismatic truss for structural efficiency.
The southern roof utilises two planar trusses spanning across the width of the bowl. The inner truss spans between a planar truss in the corners which transfers the load back to the new corner columns and the existing cores. The rear truss is located such that it is spanning between the cores.
This arrangement allows the inner roof structure to cantilever out over the bowl, with the forces being resolved as a push-pull on the two trusses.
The north quadrant of the static roof differs from the south as it carries a large quantity of plant. To achieve the necessary structural strength, the last moving roof truss, that is actually always static, has been incorporated into the static roof structure. This truss is much deeper than the southern roof trusses, and therefore offers a more efficient structural form.
The retractable roof for No. 1 Court is a concertina system using fabric panels tensioned between long span triangular trusses, which in turn span between the east and west sections of the static roof.
The triangular trusses, approximately 6.6m-deep and 1.5m-wide, span in an east-west direction over the court, and are supported on bogeys that run on rails supported by the east and west primary prismatic trusses. Bearings on the bogeys at each end allow thermal expansion and contraction in the plane of the truss and the leading trusses of the two halves have bearings on the east side that are fixed in the east/west direction to provide some stability to the group.
The complex geometry and heavy loading led to the development of jigs that were used in the workshop to achieve the requisite accuracy of fabrication of highly complex connection assemblies, some of which included up to twelve incoming members to a single node.
On site, not only were the existing cores used for permanent support of the fixed roof steelwork, they were also used as temporary support for the four tower cranes used during construction, for which they were temporarily post-tensioned.
The use of structural steelwork, with offsite fabrication and rapid site assembly, allowed the site construction to take place in the windows between Championships. Further, the spans required for the roof trusses and the stiffness needed to control the movements at the critical interfaces between the fixed and moving parts, precluded any form of construction other than steel.
Overall, the structure has been designed to be in keeping with the existing structure, whilst including up-to-date technology. This has allowed the architectural aspects of the existing site to be maintained.
Taplow Riverside Footbridge
This elegant bridge comprises twin steel arches, triangular in section, with the deck suspended via inclined steel plate hangers. The result is a distinctive, slender structure providing a valuable link between communities and fitting in sensitively with its environment. The steelwork is beautifully detailed, and trial assembly helped ensure trouble-free installation, using barges on the river, despite challenging conditions.
S H Structures Ltd
Land & Water
Taplow Riverside Footbridge is the latest crossing over the River Thames. Opened in November 2018 by the Prime Minister, Theresa May, this 40m arch bridge crosses the river at a site with very limited construction access and in an area of natural beauty.
Since Victorian times, Ray Mill Island, located in the River Thames between the town of Maidenhead and the village of Taplow has been a popular attraction for both locals and tourists. However, access to the island has always been restricted by local geography. The redevelopment of the derelict 48-acre Taplow Paper Mill site by Berkeley Homes offered the opportunity to connect these communities and allow the riverside to be once again open to all.
An initial proposal for a truss-type footbridge was criticised by local residents, who felt the design was ‘industrial’ and out of character with the area. This prompted Berkeley Homes to appoint Knight Architects and COWI to develop a slender and elegant design to suit the natural beauty of the site and enhance user experience whilst still being cost-effective.
The steel structure was designed with structural efficiency in mind but allowing a clear architectural identity to be developed comprising three key features; the arch, deck and flat plate hangers.
The slender arches drew inspiration from the nearby Maidenhead Railway Bridge by Brunel, famously employing the ‘flattest brick arches in the world’. The arches are triangular in cross section and lean outwards to produce a dramatic visual effect, opening up the views from the bridge of the river and surrounding landscape. The arch geometry was devised to avoid double curvature to facilitate fabrication from steel plate, and the triangular shape results in a much greater transparency, emphasising the slenderness of the design. The white colour of the arch was chosen to stand out from the natural backdrop, whilst adding to the slender appearance and giving an attractive profile along the river.
The arches support a slender composite steel-concrete deck formed by a steel tray comprising the edge beams and bottom plate, which was filled with in-situ concrete after the bridge was installed. Transverse stiffeners are revealed below and extend outwards to form the hangers. This composite construction results in improved structural behaviour, particularly from the point of view of dynamic response and acoustics, but also facilitated easy construction. The bridge dynamic response is clearly perceptible but within acceptable limits as defined by the Eurocode.
The flat plate hangers, with expressed pinned joints at their ends, provide lateral stability to the arch, enabling greater slenderness than if wire strands had been used. To maintain the desired transparency achieved by using thin plates, high strength steel was used for the shortest hangers which experience out-of-plane moments created by relative longitudinal movement between the deck and the arch.
The parapets continue the theme with closely spaced thin steel uprights welded to the deck supporting the timber handrail, maximising transparency when viewed from upstream or downstream. The dark grey of the deck and parapet contrasts with the white arch to accentuate the visual slenderness of the bridge.
The site for the bridge presented numerous challenges. Access was impeded to the west by a 1.5 tonne capacity timber bridge, to the east by a boggy marshland and the Berkeley Homes construction site and to the north & south by several protected trees on the riverbank. The only viable access route left was the river.
Offsite fabrication enabled a high quality of finish to be achieved and allowed for a trial assembly, ensuring a more efficient on-site build. The structure was transported in three parts by road to an assembly yard a short distance downstream of the bridge site. The bridge was built up on temporary works enabling a high quality of workmanship and dimensional control to be achieved before the entire steel structure was then lifted onto a pontoon, floated upriver and installed using hydraulic jacks in a single well-planned operation.
The ends of the arches have a simple end plate detail, with a knuckle arrangement which allowed rotation during concreting of the deck to reduce bending effects and minimise plate thicknesses. This was then cast into the abutment on completion to act as a fixed end for live load effects, avoiding the use of a bearing at these points for a maintenance-free solution which is particularly crucial as they are below flood level.
The pinned connections of the hangers to the arches employ stainless steel details, separated from the carbon steel by nylon separators.
The slender, sculptural form of the Taplow Riverside Footbridge has already become a popular local landmark. The use of steel was crucial in allowing the team to achieve a high-quality structure which adds to the landscape of this unique location. The final result far exceeded the client’s expectations, providing an iconic addition to the river area
Coal Drops Yard, London
The kissing roof that links the two original Coal Drop buildings led to a solution of three steel bowstring frames all working together, shaped to reflect the ribbon roof. New exposed steel at deck level is extremely well integrated and carefully detailed to be in keeping with the original structure, strengthening and extending it to suit its new purpose.
King’s Cross Central Limited Partnership
The King’s Cross development is a visionary urban regeneration masterplan in the heart of London. Coal Drops Yard was to form a focal point within this development, a vibrant retail destination that boasted public squares, bars, restaurants, and cafés.
The development is centred around the East and West Coal Drops. These long brick buildings were originally built in the 1850s and used to distribute coal around London, which had arrived from the north of England by train. The buildings were separated by a cobbled yard space, each building consisting of a 13m-wide, 3-floored, masonry structure with repeating cross-walls at 6m. This footprint did not lend itself to the large retail units desired of the scheme. Furthermore, the East Coal Drop was protected as a Grade II listed building, the buildings had suffered fire damage, and had evidentially been left for many years, so it was clear that significant intervention was required.
These constraints led the team to reach a design solution whereby the two buildings became linked at roof level by creating a new floor that ‘floated’ over the central yard space below.
While English Heritage and Camden Council gave their permission to link the roofs via a bridge-like structure, they requested to connect the roof in a way which maintained the idea that the East and West Coal Drops were two separate entities.
This request led to the ground-breaking architectural design of the ‘kissing point’ in which the inner roofs of the East and West Coal Drops stretched toward one another and delicately touched high above the central yard. The merging of the roofs was such that visually they appeared to connect at an indefinitely small point while the floor below hung from a roof that was seemingly devoid of any significant structure.
The structural solution to the kissing point concept presented several challenges. The roof support solution had to allow the complex roof geometry to be formed whilst being supported on buildings set 33m apart. The weight of the new roof and the floating floor below had to be carried without exposing the existing heritage structure to any additional loading or deflection. The magnitude of movements experienced by the delicate architectural façade, which spanned 7m vertically between the new floor and roof, had to be limited.
As this required both high stiffness and strength in combination with limited space for new structural elements, clearly steel was the only structural material that could deliver a cost-efficient solution.
The steel structure consists of three ‘bows’ (or tied arches) that rest upon one other. The central bow spans the shortest distance between buildings and hides a steel V-shaped member within the small cladding envelope that is available at the kissing point. Hidden below the floating floor a tie carries a 20MN tension to resolve this bow. The ribbon-shaped roofs are formed by the 2nd and 3rd bows. These consist of a ribbon shaped truss complete with three chords and bracing and are supported at the northern and southern end of the Coal Drop buildings and centrally on the first bow. Their horizontal forces are resolved with a tie hidden at floor level on the inner edge of each building.
The suspended floor is hung from the leading edge of the ribbon trusses, with a series of steel hangers that connect to composite beams of the new floor. The ties and floor diaphragm resolve horizontal forces allowing new steel columns and foundations within the existing building footprint to support the vertical load of the new structure.
The roof steelwork was fabricated to a pre-set geometry to compensate for the predicted deformations under dead loads. The aim was to ensure that the new suspended floor would settle to a flat and level position at the point of completion. This required points of the roof steelwork to be fabricated up to 90mm away from their final positions.
The majority of roof steelwork was broken into large fabricated sections which were transported to site then sub-assembled before being lifted into position. The process allowed for minimum site assembly, temporary works, and working at height activities.
The roof was erected on temporary trestles with a de-propping facility incorporated at each temporary support location, which allowed the roof to be erected and bolted together in its pre-set condition, and once it was acting as a single unit, to be depropped in a controlled manner.
The project contained considerable risk given the complexity of the site and the circumstances arising from the heritage structures. However, Coal Drops Yard is not only a success story of architecture and engineering. Forming a key locale within the King’s Cross redevelopment this project has significantly contributed to the revitalisation of this once abandoned and derelict corner of London. This legacy will surely form a catalyst for change in the surrounding areas.
Tombola HQ, Sunderland
This project exemplifies how, with a dedicated client and a top-class team, structural steel can produce cost-effective yet beautiful results that are much loved by its users. Through simple yet sophisticated design and rigorous attention to detail, this wonderful headquarters building exhibits exceptional quality and value. It is a clear Award winner.
s h e d
Brims Construction Ltd
Online gaming giant Tombola’s new headquarters building is an important element of their business plan for future growth and international expansion. Designed to attract and retain the best talent from the region and further afield, the building is key to their desire to double the size of their tech team by 2020. By providing a stimulating, attractive working environment, it will help resist the lure of larger centres like London and California, keep jobs in the region and boost the local economy. Beyond that, the building also raises the bar and sets a new standard for the 21st Century workplace environment for other employers to match by investing in higher quality working environments.
The new £7m building at Wylam Wharf in Sunderland stands next to the company’s existing headquarters, which is a former bonded warehouse dating from the late 1700s, within a conservation area, on the banks of the River Wear.
The 2,485m2 modern addition creates a riverside campus feel that incorporates both the old and new buildings as well as a new riverside courtyard created between the two. The scale of the new building, its multiple pitch roof and brick construction reflects the warehouses which characterise this section of the riverside.
Modern and technologically advanced open-plan offices for approximately 160 staff occupy the first and second floors, and a three-storey high atrium floods natural light throughout the glazed building. The ground floor includes a reception area, a bistro and gym for employees with bleacher style stairs leading to the modern openplan upper floors. The stunning 15m-high atrium with wide bleacher steps and 15m² video wall opposite, delivers a real ‘wow’ factor for all visitors and provides an amazing open space for meetings. The building also boasts a diverse range of informal training / presentation suites with the latest AV / video conferencing technology.
From the outset, Tombola wanted to create a working environment attractive to young professionals, an address with a historic industrial location and a campus hub for the activity that an ever-growing and busy tech firm generates daily. The resulting building provides an environment that encourages harmony with a workplace culture of informality and flexibility. Agile workspaces within the open-plan and airy environment, supported by strategically located private meeting rooms, enable closer collaboration with other teams with the removal of the physical barriers that existed in the old building. The new space facilitates more idea sharing, more communication, and more organic interactions.
Tombola HQ is a unique structural solution combining all three main design disciplines in one solution, architecture, M&E and structure. The exposed steel frame has been aesthetically detailed to a high standard rarely seen on commercial projects, with the additional integration of heating and cooling being cast in and monolithic with the exposed concrete slabs. Great care was taken through significant collaboration performed to ensure that all the elements worked seamlessly together. The steel frame is braced but also has moment frames, set-back columns on the northern edge to create the dramatic building overhang and cantilevers to form the full-height glazed eastern façade.
The office floorplates are clear span with exposed concrete soffits providing radiant heating and cooling. To allow the floors to appear to float and the fenestration to span fully to the soffits, all supporting beams are detailed as Rectangular Hollow Sections (RHS) with projecting plates welded to the bottom flange to carry the 250mm thick precast concrete floor slabs. The main columns are structural hollow sections too and this keeps their size to a minimum and continues the sleek lines of the building from the horizontal to the vertical. The RHS edge floor beams also support the brickwork façade thus providing a solution that is very efficient in terms of member numbers.
The roof appears to float which is made possible by utilising a moment frame. All the UC section rafters are within a shallow construction zone, with purlins within the depth of the UCs, and no horizontal ties. The structure-free rooflights provide unobstructed views of the dramatic skies.
The central atrium again is structure-free and provides an open-plan flexible-use area for the staff for collaboration and sharing collective inspiration. The AV and fire alarm systems are hidden within the hollow section structure, using them as containment systems to keep the sleek and uninterrupted finish.
Overall the building could not have been delivered in its amazing form without using a steel-framed superstructure. It has created a stylish and laid-back working environment that is completely in harmony with a culture of modern informality and flexibility. It’s the ideal workplace for smart software developers, wanting to build long-term careers and a great working environment for collaboration and innovation. The subtleties have to be seen to be appreciated with this truly spectacular setting granted a calibre of building it deserved.
Tottenham Hotspur Football Club, New Stadium
The new stadium is not just for football but provides amulti-function entertainment facility. The complex new building was constructed on the confined urban site of Spurs’ old stadium thus providing continuity for fans and community. The steelwork, which has been finished to a very high standard, plays an integral part in the form and architectural expression of the new building.
BuroHappold Engineering schlaich bergermann partner
Tottenham Hotspur Football Club
The brief was for a 60,000-seat stadium on a site north of the existing stadium, with a tight atmospheric bowl and a dedicated single tier home end for general admittance fans only. The addition of a sliding pitch was to allow for non-football event days and provided a design constraint that led to some of the most exciting architecture, engineering and fabrication on the project.
The 4-year construction period included the erection of five key steelwork features, representing a combination of elegant steelwork design and pure industrial engineering and fabrication, namely; east stand Y columns and transfer structure, south stand tree columns, south stand transfer structures, north stand cantilever structure and west stand atrium structure.
Supporting Level 3 of the east stand, the Y columns were one of the first major pieces of structural steelwork to be erected at the new stadium. They provide an atrium at the entrance to the east stand and reduce the number of columns coming to ground level by collecting a column on each branch. They also allow the façade to be cut back into the building producing a dramatic overhang.
Above the Y columns, an additional level of transfer structure is provided in order to create column-free space at Level 3. Formed using sloping steel columns to reduce the span and improve the dynamic performance of the stand, the transfer truss is two stories deep and spans 30m. The sloping columns are fabricated from plate to form tapering box sections.
The south stand tree structures were created to provide an elegant method of transferring the 17,000-seat south stand over the sliding pitch below. The culmination of elegant architectural design, robust structural engineering and careful fabrication, the trees are the main feature of the south stand. Using the same aesthetic logic as the east stand Y columns, 8 individual branches meet at a complex fabricated knuckle joint, before separating again as they meet a concrete plinth at concourse level.
Beneath the south stand there are a series of mega transfer trusses, spanning in three sections across the sliding pitch. These trusses have been coordinated and integrated with the architecture such that the concourses, toilets, concessions and vomitoria are all as uninterrupted as possible. The south stand was critical path to erecting the cable net roof, so temporary bracing was added to allow the back of bowl to be completed. The stand was erected from both pitch side and outside the stadium to reduce construction time.
The north stand cantilevers 10m over the tier below. This is formed using box section rakers, which are propped using plan trusses. The load is delivered into the reinforced concrete cores using prestressed high-strength bars. Significant dynamic analysis was carried out to justify the performance of the stand.
The west stand is supported on a series of slender steel box section columns, which are 21m tall. These columns create a spectacular space below. There are also several propped cantilever balconies and escalator platforms, which are hung from the structure above.
In addition to stadium operations, there is an entire level of offices within the stadium. The efficiency of the architectural and services design to accommodate significant office space into the structure was key and the steel frame provided the flexibility of column positioning to make these spaces viable.
The long span nature of many areas of the new stadium led to the use of steel and the shapes and forms created are both elegant and robust. Further, the construction programme was such that steel provided a significantly reduced erection period; some areas were even changed from concrete to steel construction at a late stage.
Minimising site durations was key in the design and fabrication. Where possible, bolted connections were used. In visible areas, hand holes and recessed cover plates allowed internal bolted connections to be used, reducing the need for site welding and shortening site durations.
The loads in some of the structure are significant and connection design was incredibly complex. The combination of heavy sections and high loads meant that splice connections had to be carefully located. Additional constraints such as the distance between the sliding pitch surface and the splice connections meant that even the orientation of the bolts was considered.
The complexity of some of the structure required special jigs to be created and the use of surveying techniques to ensure accuracy in fabrication.
The stadium has been designed as an icon, a new benchmark in stadium design, and as a catalyst for urban regeneration of the surrounding area. The steel structure plays a key role in delivering this unique aesthetic. The stadium is the first in the world to have a sliding pitch that splits in to the three parts. This allows the stadium to be adapted for multi-use, including concerts and NFL, increasing its utilisation.