Design Awards: 2017: Award

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.