Design Awards: 2017

The Leadenhall Building, London

© Paul Carstairs/Arup

Architect
Rogers Stirk Harbour + Partners

Structural Engineer
Ove Arup & Partners Ltd

Steelwork Contractor
Severfield

Main Contractor
Laing O’Rourke

Client
C C Land

The Leadenhall Building is a 224m high steel-framed commercial office tower in the City of London. In order to meet the client’s aspiration for a landmark tower on this sensitive site, the architects proposed a wedge-shaped building. This produced the highest office floors in the City, while minimising the impact on a cherished view of St Paul’s Cathedral.

The use of steel is fundamental to the value of this building. It is visibly integrated into the architecture to an extent that is highly unusual for a skyscraper, creating a powerful tectonic quality which enables people to appreciate and take delight in the way that the building is constructed.

Panoramic lifts were placed on the vertical north elevation so they could serve all the office levels. As a result there is no central core, and stability is provided by the perimeter braced steel ‘megaframe’ placed outside of the building envelope.

This steel design allows the floors to be exceptionally open, with views in every direction and spans of up to 16m, so that there are only up to six internal columns within floorplates of up to 43m x 48m, making them very flexible and attractive to tenants.

At the bottom of the building, floors are cut away and hang from the levels above, creating a vast open space, the ‘galleria’, which connects and relates directly to the surrounding public realm, regenerating the local environment and creating new pedestrian routes.

The architects wanted the building to express its engineering systems wherever possible. This significant challenge demanded a holistic and creative approach, with the engineers and architects working closely together from the outset. The most striking example of this is in the ‘megaframe’. Alternative bracing arrangements were proposed, studied and then optimised, leading to an arrangement that is both structurally efficient and architecturally coherent. Vertical columns are provided where they are most needed, on the east, west and north faces, and a diagrid structure on the more lightly- loaded south face. Connections are made through a family of separate fabricated node pieces. This ensures that the complex geometrical relationships between members are always resolved within welded joints and the site connections remain simple and standardised.

Within the ‘galleria’, floor beams are exposed, enhancing the character of the space. At level 5 these project beyond the ‘megaframe’ to form a canopy over Leadenhall Street. The levels below are suspended via hangers whose bespoke end connections provide a seamless transition between the rods and the supporting steel beams.

The ‘megaframe’ columns and braces around the ‘galleria’ are unrestrained over a height of 28m. Standard ‘megaframe’ sections are therefore subtly adapted, with tapering webs and additional stiffening plates, to significantly increase their buckling resistance without undermining the node connection principles or aesthetic proportions.

Steelwork is corrosion protected and fire protected where required. In external areas, epoxy intumescent coatings are employed for durability. Cast intumescent caps were placed over the ends of the ‘megaframe’ fasteners to preserve the ‘nuts and bolts’ aesthetic.

The most complex steelwork details were developed in workshops based around Arup’s Tekla BIM model. The model fed directly into the procurement process where it was used to explore the construction methodology and co-ordinate the temporary works. It also fed directly into the steel fabrication models, driving automated shop processes.

80% of the building was constructed offsite, reducing waste and improving quality, safety and programme.

All wet concrete was eliminated above level 5 by replacing conventional composite floor slabs with an innovative precast concrete panel system. The panels have pockets which enable dowels to be installed into the neighbouring units via cast-in couplers to provide diaphragm action. These dowels pass through circular openings in shear tabs pre-welded to the tops of the steel beams, to provide the required shear connection.

The primary steel system within the north core was built as a series of storey-high tables, with the services and concrete floor slabs pre-attached to them, minimising the number of crane lifts required.

The building was predicted to move sideways to the north during construction. An innovative approach was deployed to counter this, known as ‘active alignment’. The structure was initially erected straight and movements regularly monitored. At a later point, adjustments were made to the ‘megaframe’ diagonals which pulled the building back sideways, reversing the gravity sway. This allowed the ‘megaframe’ nodes to be fabricated with a simple orthogonal geometry and improved the overall accuracy of construction.

The Leadenhall Building provides the public with a unique and dramatic new space at ground level, offers tenants some of the most desirable office spaces in the City, and forms a sensitive and elegant addition to London’s skyline.

Judges’ Comment

This project had a committed client, architectural and engineering excellence, fabrication precision and construction ingenuity and innovation. They all combined to make a project whose achievements are even greater than the sum of the parts.

Structural steel is rigorously controlled to generate an architecture that is clear and legible throughout the building. Like most ground-breaking projects there were lessons to be learned, but the client and the team persevered to achieve final success.

This world-class project is an exemplar for large commercial buildings.

T-Pylon


Architect
Bystrup

Structural Engineer
Bystrup

Main Contractor
Balfour Beatty Power Networks

Client
Nationalgrid UK

The T-Pylon is a structure of only few parts that can be erected quickly and requires virtually no maintenance. It is designed to carry 2 x 400kV, but can be modified to alternative specifications, and is the result of a design competition in 2011 to find a 21st Century power pylon design for Nationalgrid UK. The challenge was to find an alternative to conventional lattice towers that minimised the visual impact on the landscape, whilst being cost-effective and functionally superior.

The use of steel for the T-Pylon has allowed for unique geometries. Contrary to conventional lattice tower designs, the arms of the T-Pylon are slightly raised, which gives the pylon a more optimistic and positive appearance. The few parts making up the pylon have been welded together and subsequently painted white. The tower design is shorter and leaner than traditional lattice towers resulting in improved aesthetics and reduced environmental impact. The use of a monopile foundation also minimises the overall cost, installation time and environmental impact of the T-Pylon.

The alternative design using steel has made it possible to obtain the aesthetic and functional goal, which is to minimise the visual impact on the surrounding landscape, while also providing an economic and durable solution.

The steel structure is designed in accordance with Eurocode 3 and fabricated in accordance with BS EN 1090-2 to Execution Class 3. The structural steel specification for the flanges, monopole and transition piece is for S355J2 to BS EN 10025-2 for thicknesses up to and including 50mm, and either S355NL to BS EN 10025-3 or S355ML to BS EN 10025-4 for thicknesses over 50mm. The steel plate also has to be accompanied by a Type 3.1 specific inspection certificate according to BS EN 10204.

A radical innovation is the re-assessment of the conductor/cable arrangement. The prismatic configuration of the cables allows a reduction in the pylon’s height of more than 30%. The footprint of the power lines, as well as the electro-magnetic field (EMF) radiation, is thus reduced.

The most remarkable characteristic of the T-Pylon design is that all conductors are carried by a single attachment point. Traditionally, such a structure would have three separate arms – each carrying an individual conductor.

This unique attachment point was studied closely to ensure its robustness and resistance to fatigue. Complex analysis and physical loading tests were carried out to simulate climatic conditions such as extreme winds and ice loads. Investigations were made into the dynamic performance of the structure under simulated vibrations.

The pylon is made from S355 steel plates that are curved and welded to form cylindrical sections. The shaft is fabricated in either one or two pieces according to the length needed, the requirements for hot dip galvanizing, and transport limitations. The steel plate thickness used for the shaft is optimised according to the design load cases and varies from 22mm at ground level to 14mm at the top.

At the top of the shaft a cast node connects the shaft to the two arms. The node is cast in one piece to ensure the optimal load transfer from the arms to the shaft. The result is a highly effective and smooth node that transfers the shape and forces from the arms to the shaft. The node is connected to the arms and shaft by non-visible internal bolts.

Dynamic external wind loads experienced on the pylon arms result in a bending moment at the pylon foundation. However, the cast node must withstand the transfer of internal stress from compression and tension at the node due to the pylon arm distributed load case. The cast node is designed to withstand both the magnitude and the dynamic behaviour of the load case.

At the end of the arms another node connects the insulator configuration to the arm in an aesthetically pleasing way. Again, the node is connected to the arms by non- visible internal bolts.

For the UK market the pylon is hot-dip galvanized and painted light grey. This duplex coating system gives the pylon an expected lifespan of at least 80 years. For other markets the pylon can be produced in stainless steel or weathering steel.

The design of the shaft is similar to the design of towers for wind turbines. Consequently, it was possible for the steelwork contractor to use the experience from wind turbine towers to produce the shaft using automated processes in controlled factory conditions. Maximising the offsite fabrication simplified on-site operations and reduced the number of operatives required for the installation process.

The new designs have significantly reduced maintenance requirements compared to traditional lattice towers. The durable coating system and lack of edges and bolted connections increases the future maintenance intervals and makes re- painting the towers much faster. Also, no anti-climbing devices are needed for the monopole shaft, which would otherwise require frequent replacement.

Judges’ Comment

The T-Pylon represents a generational step change in power transmission hardware. Analytical design from first principles included re-examination of arrangements for insulation and maintenance.

The result is a family of compact pylons which can be deployed in sensitive landscapes, with prefabrication enabling consistent finish, smaller land take and speedy erection. This is a steelwork design classic.

LSQ London

Architect
make

Structural Engineer
Waterman Structures

Steelwork Contractor
Bourne Steel Ltd

Main Contractor
Multiplex Construction Europe Ltd

Client
Linseed Assets Ltd

LSQ London is a Grade II listed building refurbishment, which required over 2,000 tonnes of structural steel and metal decking to be installed within what is thought to be Europe’s largest retained façade. Situated in London’s busiest tourist area, this project required meticulous planning and organisation to ensure the most efficient use of time and craneage without bringing the surrounding streets to a standstill.

The completed building was erected to the required tolerance of 15mm over the full height, making this project challenging for the engineering and delivery teams, but was completed on time, to budget and to the client’s satisfaction.

The existing building envelope is partially retained with new upper storeys of commercial floor space being provided. The design delivers two basements, two floors of retail space and seven floors of high quality office space with a new entrance on Whitcomb Street. Upper floors are enclosed by a new curved mansard roof. On the lower floors, new retail space has created active frontages at street level with new, clearly defined entrances.

The contemporary roof design is supported by a structural steel-framed central core and new perimeter stanchions, with complete column-free office space and spans of up to 12m providing very efficient floor space to appropriate market standards.

A new two-storey basement was created by the installation of a secant piled wall inside the retained façade profile. Shallow floor construction was used for the B1 and ground floor slabs to maximise headroom in below ground spaces, whilst minimising excavation depths.

The project was designed using Revit 3D modelling techniques to capture the integration and interfaces of both architecture and building services. This assisted the design and construction activities, but also provides full integrated models for future use.

The design of the building naturally leant itself to using steel for the primary structural elements. The design of the new steel structure introduced a new central core, and enabled clear, open-plan floorplates improving the office spaces within the building. One of the key aspects of the façade retention scheme was the alignment of new floors with existing window openings. This was assisted by integrating the suspended services within the structural downstand beam zone, such that the depth of floor zone against the façade was minimised. The use of a steel frame offered the flexibility needed to suit the various interfaces that occur with the existing façade.

The steel-framed façade dates from the 1920s and 1930s, however some areas were added during the 1960s. The steel columns are all encased in Portland stone and consequently in good condition. However, steelwork originating from various decades required extensive laboratory tests to determine its make-up and suitability prior to making the welded connections for 250 new façade retention brackets. This demonstrates the adaptability of steel-framed structures both old and new.

The new fifth floor is clad with Portland stone to integrate with the retained façade below. This floor level’s steelwork is topped with a ring beam that goes around the entire perimeter of the building.

The ring beam is formed from jumbo box sections measuring 650mm × 450mm with a 25mm thickness. The sections were brought to site in 3.5m long sections each weighing three tonnes. The box section ring beam performs two functions, one is to support the columns for the feature roof as these are not aligned with the main columns for the rest of the building, and the second function is for the stone cladding panels for the sixth floor as they are hung from the beam.

The steel feature roof slopes outwards from the two centrally positioned cores and is formed with a cranked steel frame, which in turn supports a lightweight aluminium frame and glazing. This new and elegant curved mansard roof encloses the building and offers a modern interpretation of the traditional mansard style where arch geometry sits atop a classical base. This respectful, contemporary addition to the building composition reduces the existing top-heavy visual mass of the building and, with the curved design, also seeks to ensure the building blends in seamlessly with the surrounding iconic buildings of Leicester Square.

Judges’ Comment

The use of structural steel for the new internal structure, including cores, enabled new clear-span floorplates to be achieved, whilst respecting the existing listed façade. It minimised disruption during construction in London’s busiest tourist area.

With its graceful three-storey ‘top-knot’, the building has a new lease of life as a striking yet respectful landmark in the West End.

This project showcases the role steelwork can play in the extension and re-purposing of historic buildings.

HGV Egress Ramp, Selfridges, London

© Kevin Sansbury 2015

Architect
Gensler

Structural Engineer
Expedition Engineering

Steelwork Contractor
William Hare

Main Contractors
Blue Sky Building and SRM JV

Client
Selfridges

The Duke Street phase of the redevelopment project included forming a new staff entrance into the building below Edwards Mews and realignment of the HGV entrance ramp to the loading bays. However, the primary feature of the first phase of works was the insertion of a new 50m long 165 tonne steel-framed bridge structure, through the existing store, to improve HGV egress from the basement loading bays. This new structure is a braced steel tube linking the loading bay within the basement to Duke Street.

The structural works were designed to keep the loading bay active throughout the works, while staff access was maintained and retail operations continued within 1m to 2m of the structural works.

Design and execution of the structural interventions was made more complex by the limited existing building information, and numerous historical alterations that were discovered during the build, requiring modifications to be made to the new construction as it progressed on-site.

To maximise retail space for the client, the preferred routing of the egress ramp was tight to the perimeter of the building. This routing allowed the ramp to be partially supported on the existing steel structure, but necessitated the partial removal of three existing columns that then had to be re-supported on bespoke steel transfer girders integrated into the new ramp structure.

The routing of the ramp meant it would span over an occupied three-storey basement. To minimise disruption to these basement spaces, and to minimise the need for new foundations, support was taken from the existing 1920s steel structure along the northern edge of the ramp. On the southern edge of the ramp vertical support was limited to two new columns between which a new steel truss would span. The new columns were threaded down through the existing building and supported on new hand dug pad foundations.

Reuse of the existing 1920s steel frame on the northern edge of the ramp provided an economic solution. The existing steelwork comprised of built-up riveted sections, which geometrically added complexity to the connections, formed to the existing structure. However, the existing steel proved to be of a weldable nature and so site welded connections were adopted.

Where the new ramp is supported on existing steel columns, these were checked for a change in loading and restraint condition due to the removal of the ground floor beams. Some of the columns required strengthening but, as this was governed by buckling capacity, the columns could be strengthened relatively simply via the addition of welded plates to the existing sections to increase their stiffness. The size of the strengthening plates could be easily tailored to suit constraints on site and manual installation.

To allow the ramp to connect between road level and the loading bay within the basement a large slot was cut into the existing ground floor slab. The stability of the building is likely to be provided by a combination of frame action and some contribution from the masonry infilled building cores. The unquantifiable nature of the system meant the diaphragm action of the ground floor had to be maintained. This was achieved in the temporary condition via a temporary propping arrangement, and in the permanent condition by making connections between the existing ground floor frame and new ramp. The new ramp structure was then designed to transfer any diaphragm loads back to the existing retaining wall, with steel members being tuned to provide an appropriate stiffness.

The two transfer structures used to re- support the columns above the ramp were deemed to experience vertical deflections exceeding acceptable limits for an occupied building. However, to negate the existing structure above experiencing these movements, an erection approach utilising jacking was adopted. This allowed the load from the existing column sections to be transferred into the new transfer structures in advance of the lower column sections being removed. The jacks were used to push the transfer structures down, realising the anticipated deflections before connections were made to the existing columns.

A key challenge in the construction of the new steel ramp structure was the fact that it was to be constructed within a live existing building. As the structure was to support HGV vehicles the steel forming the structure was of a scale that, although lighter than other forms of construction, could not be manhandled. The contractor team therefore developed a series of temporary works that spread the load of a spider crane across the existing suspended basement floor. The crane could then be safely driven into the space via the existing loading bay entrance without back propping through the levels below.

The creation of the new egress ramp was a highly complex piece of engineering design and construction successfully delivered by close collaboration between the whole team.

Judges’ Comment

The creation of this new egress ramp within an existing steel structure was highly complex, yet successful. A key challenge for the engineering design and construction was that the work was to be carried out in a live and busy existing building, with ongoing high-end retail operations being immediately adjacent to the work zone.

The outstanding success of this complex project was achieved through very close collaboration between the whole design and construction team.

Oriam, Heriot-Watt University, Edinburgh

© Reiach and Hall Architects

Architect
Reiach and Hall Architects

Structural Engineer
Engenuiti

Steelwork Contractor
J & D Pierce (Contracts) Ltd

Main Contractor
Bowmer & Kirkland

Client
Heriot-Watt University

Oriam, Scotland’s new Sports Performance Centre, comprises a full size indoor 3G synthetic pitch for football and rugby with spectator seating for 500 people, a nine- court sports hall, a 100-station fitness suite, as well as a high performance wing that includes areas for hydrotherapy, strength and conditioning, rehabilitation, offices and a classroom.

Oriam presented truly fantastic opportunities to be creative. With long spans and a simple but elegant diagram, the cross section forms the principal structural concept. Steel arches at 7m centres span over the football hall and sports hall from buttresses on each side onto a central street of piers.

The arch profile for the football hall roof offers a high rise : span ratio and considerable curvature, giving rise to a highly efficient structure with a comparatively low overall weight. The arch is a naturally efficient form allowing the structure to work primarily as axially loaded, with relatively small bending moments generated.

Tensioned PVC fabric was chosen to clad the football hall roof as it offered the necessary light transmission properties so as to limit the need for artificial lighting of the pitch space, whilst managing heat gain. It was also preferable in the structural design, given that the fabric is lightweight and forgiving to structural movements and deflections. The diagonal arrangement of arched secondary CHS members ensured that the fabric shape could be prevented from flattening under heavy imposed loading, whilst also creating interest to the roof form itself.

The sports hall roof comprises steel arches on a 7m grid, with straight secondary steel members spanning between the arches, and curved tertiary steel members spanning between secondary beams to provide intermediate support for the roof cladding. In this case, the original trapezoidal section was re-engineered to work as a standard UB section, further increasing the material and prefabrication savings.

Central piers support the ends of the football hall and sports hall arches, which converge at a single point behind the listed wall in an area known as the Street. Initially these piers were conceived as reinforced concrete elements. However, the overall programme advantages of bringing this element within the steel package were explored and, following this review, the steelwork option was selected giving both programme and cost advantages.

The roof structure acts as an umbrella over the public fitness area and high performance wing, which are both constructed as conventional steel-framed structures and accommodate the high performance spaces and the public fitness suite, café and accommodation. In the public area the tight limitations on available floor depths meant that cellular floor beams were needed to span the full width of the structure without intermediate columns, leaving the gym and café spaces completely column-free. The same was needed in the high performance wing in order to maximise the column grid spacing and minimise the disruption to the floor layouts.

The roof arch is formed from three curves meeting at tangents and, whilst this is stable once vertical, it has little structural strength in its minor axis. This meant that building the trusses flat on the ground and then lifting them vertically would require extensive temporary works, which with 13 to lift would have substantially increased the build costs. Building the trusses vertically on the ground was ruled out due to the height of any temporary frames which would have been required for temporary stability during assembly.

The solution was to utilise the permanent design for the temporary works. Simple stubs were designed to transfer the load from truss to truss with chord ties, and then match all these stubs at each truss so truss components could be connected directly to the previous truss; this allowed a full truss to be built in the air. The challenge was then how to erect the first truss! As this was at a gable the slender gable posts, which were themselves trussed, could be propped first and then roof truss segments landed on top and joined together to form one complete arched truss.

All the steel structural elements were very precisely fabricated to tight tolerances before delivery to site, which enabled a rapid waste-free assembly and a comparatively quiet construction process. This was important as the existing Centre and Academy buildings needed to remain open during the construction work. Erection procedures were planned in detail using 3D models.

The steel-framed structures and regular column grid arrangement for the office, café and elite sports areas are all adaptable for future changes of use.

The structural steel was efficiently engineered for fire resistance with the structural elements supporting floors required to achieve a 60 minutes’ fire rating. For exposed elements this was achieved through the use of high visual quality intumescent paint. For those which are not exposed, intumescent paint with a basic finish was used.

The project team has delivered a world-class facility that also provides extensive access for the local community.

Judges’ Comment

Two parallel vaulted forms spring from a central spine; the larger one covers a football pitch, whilst the smaller covers a sports hall. The elegant lightweight steel trusses resulted from a collaborative effort by the designers and contractor, with the construction methodology informing the roof structure and supports from which it springs.

Striking and effective steelwork.

The Curve, Slough

© David Butler

Architects
BBLUR architecture and CZWG architects

Structural Engineer
Peter Brett Associates

Steelwork Contractor
Caunton Engineering Ltd

Main Contractor
Morgan Sindall

Client
Slough Borough Council

The Curve initiative is a major development comprising a mixed-use, vibrant community facility with multi-functional spaces and wider cultural offerings, based around the arts with opportunities for performance and exhibitions.

The 90m long x 15m high building’s form, a curved ‘tube,’ features fully glazed entry façades, and opens onto two new public squares created at each end of the building.

A heavily serviced building with a single two-storey enclosed plant area presented particular challenges in incorporating horizontal distribution routes within the building. The composite steel frame allowed floor depths to be kept within a stringent floor zone, whilst allowing for the services’ distribution.

The composite steel solution allowed for full flexibility in the design of an irregular column grid, and provided minimum depth cantilever façade support sections. The aesthetic of the circular hollow section columns has been retained and expressed throughout. The double height performance space required column removal, for which composite universal column sections were able to achieve the spans in the shallowest depth possible. The constrained site utilised a single mobile crane to perform all lifting operations in a carefully planned three phase construction sequence, allowing free site areas open to other trades.

Detailed 3D modelling allowed efficiencies to be gained in specifying a constant bend radius for the façade members, and limiting the supporting tubular transfer beam to three discrete bend radii. This 45m curved CHS beam was then spliced using carefully detailed non-visible connections. Curved edges to the internal atrium required cantilever decking sections to arrive at site with the bend radii pre-cut. Staircases, both front and back-of-house, were formed offsite in steel and installed quickly and prop-free to open up the site to the follow-on trades. Detailed 3D model coordination allowed for accurate placement of pre- applied cladding fixings and secondary support steelwork.

A shop applied painted system provided the corrosion resistance to the members. Fire resistance throughout was provided by intumescent painting of the main structural members up to the 120 minutes’ period required in some areas. The design methodology to use bolted connections reduced the risk of compromising steel coatings that can occur when site welding is required.

Judges’ Comment

Part of the implementation of the masterplan for the regeneration of Slough Town Centre, The Curve provides popular and accessible community facilities. Its striking curved form arose from its proximity to a church and probably could only have been achieved by an integrated team using coordinated BIM design, analysis, fabrication and erection.

Elegant and effective steelwork meets unusual demands.

West Croydon Bus Station

© Alex Upton

Architect
Transport for London

Structural Engineer
Price & Myers

Steelwork Contractor
B&W Engineering Services Ltd

Main Contractor
Quinn London Ltd

Client
Transport for London

West Croydon Bus Station has been transformed from an unsightly, uninviting and poorly functioning 1980s ‘shed’ into a customer-friendly landmark that supports the regeneration of Croydon. The old building has been replaced by an open concourse under an elegant weathering steel canopy. The canopy wraps around and connects two small buildings – a retail unit and a bus operations building.

The choice of materials was carefully considered to reduce whole life costs. The primary material of the project is weathering steel, and is used throughout the canopy structure in combination with translucent Kalwall panels, creating a structure requiring minimal ongoing maintenance.

The weathering steel canopy wraps around the two brick clad buildings, tying the station architecture together on the linear site. The brick and weathering steel were chosen to complement the surroundings. The style and natural flow of the station achieves this while also providing an interesting play of light, shadow and texture. Timber seating and planters are fully integrated into the steel canopy structure, along with customer information and lighting to minimise visual clutter.

The canopy is based around a repeating module where canopy and supporting columns are linked by a curved haunch. This haunch is perforated with variable sized holes to both create interesting visual effects and demonstrate the changing stress intensity across the haunch.

Guttering, downpipes and lighting have all been integrated into the structure to ensure services are not visible nor impact on the final impression of the structure.

The buildings are highly sustainable and environmentally friendly – with solar panels, air source heat pumps, LED lighting and building materials that maximise the buildings’ performance. A building management system ensures energy efficiency and reduces light pollution.

The opaque canopy provides natural lighting and manages glare and heat transfer. Night lighting on the canopy creates an attractive and safe environment.

The whole life value of all aspects of the design was assessed to inform design decisions, such that:

  • The weathering steel of the canopy reduces maintenance and future carbon impact.
  • The design modelled different climate scenarios and was carefully detailed to minimise waste during the build.
  • Prefabricated insulated timber panels reduced on-site labour and the energy required to mechanically condition the buildings throughout their lifetime.

Anti-social behaviour was a significant issue at the old bus station. The new improved open layout, architectural lighting and soft landscaping tackle this issue and create a safer, more socially sustainable, environment.

Judges’ Comment

High quality design and careful selection of materials, with low maintenance a major objective, are evident in this project. The lightweight canopy is framed in weathering steel, carefully prepared and detailed, to provide visual interest.

This is a facility which has transformed passenger experience and provided a significant contribution to the environment.

Central Square, Leeds


Architect
DLA Design

Structural Engineer
WSP

Steelwork Contractor
Elland Steel Structures Ltd

Main Contractor
Wates Construction

Client
M&G Real Estate

Central Square is a 20,400m2 development, providing 18,700m2 of Grade ‘A’ offices with 1,700m2 of retail, leisure and health/fitness.

The scheme provides office accommodation on 10 floor levels and is said to have the largest floorplates available in the city. These are arranged so that they can be subdivided, providing the occupier with both flexible and highly efficient floor space. The development offers an outdoor Sky Garden on the level nine, providing entertainment opportunities and views across the city. In addition, a stunning seven-storey fully glazed atrium houses Central Square’s Winter Garden, where the created contemporary square offers a mixed-use destination for members of the public throughout the day and evening to enjoy the high quality public realm, retail and leisure facilities on offer.

Sitting above a two-level concrete basement, the steel frame forms a U- shaped structure with the central void occupied by the fully glazed Winter Garden created by the glazing sloping down from the underside of level eight within this central void.

In total five 27m long tubular ‘vertical’ bowstring trusses, which were delivered to site in two pieces, form this indoor zone. These trusses, carrying heavy dead load from the attached glazing, are pin connected at the base to architecturally exposed fabricated base assemblies.

The heaviest steel assembly on the project was the 43 tonne, storey-high, Vierendeel truss that supports level eight’s balcony that overlooks the Winter Garden. The Vierendeel truss, comprising heavy UC members, was brought to site in individual sections that were then assembled on the ground before being lifted into place by a 300 tonne capacity mobile crane.

The majority of the steelwork was erected using the site’s tower cranes, with two erection teams being employed that divided the structure in half and erected the frame three levels at time, incorporating placing and subsequently fixing of the metal deck flooring. Each of the U-shaped structure’s tips contains one of the building’s three cores, and this provided each erection team with an ideal and stable starting point. From these outer concrete cores each of the teams worked its way to the centre of the structure, meeting up at the third centrally positioned core.

Above the lower levels of retail and leisure, the offices begin at level two and extend upwards to level 12. Levels two up to seven are identical, with a central portion positioned inside the Winter Garden. The reception for the offices is positioned at level one and accessed via a feature escalator, with the upper levels being accessed by a number of lifts including two glass-clad wall climber lifts.

Judges’ Comment

Central Square is a landmark office and leisure complex within two minutes of Leeds City Station. In this BREEAM ‘Outstanding’ development, the floors are supported on long-span beams enabling 25,000ft2 floorplates, the largest in the city.

The ground and first floors are accessed through a large atrium ‘winter garden’, forming a new and exciting part of the public realm in the area.

STIHL Treetop Walkway, Westonbirt, the National Arboretum

© Paul Box/Forestry Commission

Architect
Glenn Howells Architects

Structural Engineer
BuroHappold Engineering

Steelwork Contractor
S H Structures Ltd

Main Contractor
Speller Metcalfe

Client
Forestry Commission, Westonbirt, the National Arboretum

The STIHL Treetop Walkway is the longest structure of its kind in the UK. Reaching heights of up to 13m, the walkway transports visitors effortlessly on a journey through the park’s Silk Wood.

The structural solution that developed is a hybrid timber and steel structure. The basic structure is a simple arrangement of two perimeter beams supporting the 1.8m wide deck and the balustrades. These beams are formed from curved galvanized steel RHS sections which, with the CHS cross beam, create a laterally stiff structure to transfer loads back to the supports. The use of galvanized steel provided the required stiffness and allowed a shallower deck profile to be created with something that was durable and maintenance-free.

Building the walkway using small assemblies and single elements enabled the size of the construction equipment to be reduced and allowed the use of bolted connections throughout, meaning that future dismantling would be straightforward.

Various support solutions were considered due to the unique constraints of the site. The chosen solution was to use shallow reinforced concrete pads supporting pairs of inclined timber legs at 10.5m centres. The inclined columns provided a more natural feel and allowed the base positions to be easily moved in plan to avoid areas of heavy root coverage.

The client was keen to accentuate the treetop experience by having a degree of movement, and the use of a non-linear dynamic model of the structure enabled an acceptable behaviour to be achieved.

The walkway is punctuated by a number of independent platforms, such as the Crow’s Nest, which serve as educational areas. The ability to curve the hollow steel sections to a very tight radius enabled the architect’s requirements to be achieved. Whilst the main walkway is inherently stable, the cantilevered Crow’s Nest is somewhat more lively, providing visitors with the more dynamic experience requested by the client.

Four different capacity cranes were used during construction, including a mobile tower crane which provided the required reach over the trees. Mobile cranes of up to 350 tonne capacity were used in other locations where prepared bases were constructed in areas that would cause the minimum impact.

The construction followed a repetitive sequence with the pairs of columns being installed first. The columns were temporarily supported with guy lines fixed to concrete kentledge blocks. This methodology enabled the column heads to be positioned correctly whilst causing no damage to the trees and their roots.

Judges’ Comment

Owing much to the romantic tradition of great English landscapes this sinuous walkway carefully winds through the canopy of ancient working woodland, whilst avoiding the precious root zones. A ‘tuneable’ structural system addresses the varying dynamics and geometrical restraints.

The curvilinear route, which heightens the sense of drama and discovery, was facilitated by the use of steel. Apparent simplicity conceals sophistication in this project.

The Hurlingham Club Racquet Centre

© Metsä Wood

Architect
David Morley Architects

Structural Engineer
Price & Myers

Steelwork Contractor
Tubecon

Main Contractor
ISG

Client
The Hurlingham Club

The state-of-the-art Racquet Centre replaces an existing marquee structure and has modern functionality practical to its purpose. Designed to meet high standards of sustainability, it fits sympathetically into its rural surroundings. The Centre is home to four indoor and two outdoor tennis courts, four squash courts, a multi-use games area and changing facilities.

Steelwork was the natural and most cost- effective material, due to the long spans needed by the large 38m x 70m column- free space required by the tennis hall.

A series of tied-arched steel frames, tied with large diameter bespoke tension bars, at 16.5m centres form the primary roof structure. These purpose-engineered welded box sections were paired together to also act as Vierendeel trusses to help distribute horizontal forces back to vertical bracing systems. Further efficiencies of the arch were gained by portalising the structure and providing raking ties at the roof ends, which help minimise the depth and weight of the box sections.

These arches support timber stress-skin panels which span between the arches. The structure is predominantly internal; however, where the structure becomes external, an offsite painted protection system with a life to first maintenance of 20 years has been used and, where necessary, with a decorative top coat. By using steel, the main elements could be fabricated offsite while the groundworks were completed. This prefabrication meant a fast erection process on-site and also assured a high quality finish – imperative as the majority of steelwork is left architecturally exposed.

A key consideration was how the timber panels would bear and tie to the steel frame without visible fixings, and what tolerances would be needed. Creating the primary arches from welded plates allowed the bottom plate to provide a bearing ledge for the timber panels, which were tied to the tops of the steel with steel straps and self-tapping screws to allow them to be completely concealed. Designed to be transportable, the trusses had to be fabricated in three sections with bolted splices connected on-site using a seating jig. Once connected, a tandem lift then raised the 40-45 tonne trusses into place, which cantilevered approximately 2m-3m.

With architecturally exposed steel, all processes required acute accuracy and highly skilled workmanship, from detailing and connection design through to the fabrication and erection process.

Judges’ Comment

A detailed, yet compact, building adds generous new indoor play areas, whilst meeting the requirement for a low profile at the edge of the Club grounds. The stringent dimensional and technical constraints were answered by well-coordinated structure, services, equipment and enclosure.

Steelwork again enables elegance and efficiency in modern sports facilities.