Structural Steel Design Awards
Mary Elmes Bridge, Cork City
- © Arup
- © Henry O’Brien
Cork City Council
In September 2016, a design competition was held for a pedestrian bridge crossing the River Lee, located in the heart of Cork city. The requirements were for a single span, sympathetic with the local city architecture while providing a minimum walking width of 4.5m, unimpeded views of the river while having minimal impact on the existing flooding regime.
The initial concept developed by the winning design team focused on a structure with a central spine beam, aiming to conceal the structural depth within the railing height. With a 66m span between quays, it was identified early in the design phase that a significant depth was required; a 1.8m-deep beam at midspan was suggested with the beam going to 2.2m-deep at the supports.
Transitioning the central spine beam from below to above the deck along the span introduced a small arch effect which, along with fully integral abutments, resulted in increased stiffness in bending, thus increasing the slender appearance of the bridge. To further increase the structural efficiency, the pedestrian walkway is integrated into the structural system with the position of the walkway favourable relative to the position of the neutral axis of the main spine beam at both midspan and at supports.
Combining the shallow slender arch with transparent mesh parapets delivers an understated but visually appealing design with uninterrupted views of the river and cityscape and embedded benches on either side of the central beam has resulted in the bridge becoming part of the urban realm. The widening at midspan creates a natural meeting point.
Extensive modelling was used to optimise the geometry of the bridge and deliver the most efficient shape to minimise the amount of steel required. The 3D models of this lightweight structure were also used for virtual design reviews to ensure full alignment and coordination throughout the project.
The bridge was fabricated offsite in nine sections before assemblage at a shipyard downriver from its eventual home. In May 2019, the completed structure was transported up the River Lee on a custom-made barge. It was then lifted into position by cranes located on each quay during an overnight road closure in a tandem lift of 160 tonnes.
Honouring Mary Elmes, known as Ireland’s Oskar Schindler, this transformative bridge link is part of Cork’s drive to become a more accessible, sustainable city and is a sympathetic companion to the surrounding quays, buildings and urban realm.
The elegant and deceptively simple design of this bridge has turned a new pedestrian and cycle city centre river crossing into something of a destination in its own right. In addition to the celebrated appearance, particularly at night, the judges were impressed by the daring success of the barge delivery and overnight installation of the bridge.
The Post Building, London
Allford Hall Monaghan Morris
Brockton Capital LLP and Oxford Properties Group
A former 1960’s Royal Mail Sorting Office, The Post Building has been redeveloped into a modern, mixed-use development with considerable amounts of the original steel structure being retained. In total, the redevelopment has created 44,000m2 of floor space, with eight floors of offices and seven floors of adjacent residential above two floors and a basement containing a variety of public uses including shops, cafés, galleries, and a GP surgery.
After a large-scale demolition programme, a horseshoe-shaped section of the original steel frame was left containing the ground, first and second floor. Keeping some of the original steel frame fitted into the overall design aesthetic, which has exposed steel beams and columns creating a modern ‘white collar factory’ office building.
Retaining a large steel frame required the use of more than 200 tonnes of temporary steel propping and bracing, as the frame’s original stability system had been demolished. The stability system was completely remodelled to remove the existing cores from the key corner areas and create a new steel core in the central part of the site.
The site’s basement and raft foundations have both been reused and a lighter steel core helped avoid the need for new piles. It was also deemed to be in keeping with the desired overall design aesthetic of exposed steelwork throughout the building.
The original grid pattern for The Post Building’s ground floors was 12m × 20m to suit post office vehicle movements and required a series of deep transfer beams, which concentrated the original building loads into widely-spaced points on the raft foundation. As these long spans were no longer necessary, new columns were added to reduce the spans and spread the increased overall building mass more evenly on the existing foundations.
The now redundant transfer beams have been slimmed down from 2.0m-deep to 600mm-deep members to allow mezzanine floors to be inserted and maximize the available headroom within the existing floor-to-floor heights.
An entirely new steel frame was erected around the retained portion completing the lower three floors and filling up the entire site’s footprint. Sizeable transfer structures were included at level five to allow the upper floors to have a long span suitable for modern office spaces. The floor-to-ceiling height changes again on level eight, to include a mezzanine, and the roof profile steps back to minimise visual intrusion.
The Post Building achieved sustainability ratings of BREEAM ‘Excellent’ and LEED Gold.
This is a great example of a steel-framed building being adapted to give a new life for a different use. The existing steel frame was retained wherever possible to produce impressive and unusually generous commercial spaces. New steelwork was added to increase the floor area without overloading the existing foundations and the architecture is enhanced with careful detailing. Maximising the re-use of the existing structure resulted in a build with a much smaller carbon footprint.
Centre Building, London School of Economics
- © Mark Gorton, RSHP
- © Joas Souza
Rogers Stirk Harbour + Partners
Billington Structures Ltd
London School of Economics
Situated at the heart of London School of Economics’ (LSE) campus, the new Centre Building offers spectacular views across London’s skyline. Built in two sections, six-storey and 13-storey structures interlinked by an atrium, the Centre Building replaces four previous buildings that were demolished. With a gross internal floor area of 15,507 m2, the scheme also includes a new landscaped public square.
The overall superstructure system of steel beams and columns, concrete cores and precast concrete floor slabs facilitates simple and flexible floorplates which can easily be adapted to suit LSE’s current and future academic needs. The careful location of service cores and expressed ceiling services also gives the client flexibility to adapt spaces or enhance facilities with minimal impact on the existing building fabric.
The expression of the superstructure, the exposed steelwork internally and externally, seeks to give the building a distinct and contemporary appearance. Shallow floor construction maximises available space and comprises RHS or plated floor beams featuring bottom plates to support long span precast floor units, which sit within the depth of the beams.
Over 1,000 tonnes of decorative finish, fire-protected structural steel was used with many of the steel members having internal bolted connections, hidden from view and accessed via a pre-formed aperture in each box section beam. Flush plates, flush welds
and shadow gaps were present in many details, all of which had to be manufactured with a high degree of accuracy in order to achieve the correct fit-up.
The main steel frame of the superstructure was erected entirely by tower crane, apart from two girders, measuring 17m-long which needed a 400 tonne-capacity mobile crane. Full stability of the structure was only achieved once the entire frame was erected and all the precast flooring was installed. Until that point, a temporary bracing system was required, which was only removed once each level was fully complete. At either end of each block, exposed SHS bracings bookend the project and form another highly visible exposed steelwork element. This exoskeleton bracing, which sits approximately 300mm outside of the building envelope, provides stability along with two concrete cores.
For this complex and challenging project on a confined inner-city site, collaboration was key to the project’s success. The project achieved a BREEAM ‘Outstanding’ rating and a bespoke sustainability tool was developed to help reduce the building’s carbon footprint. The high-performance façade helps to reduce overheating and provides a naturally-ventilated working environment for inhabitants.
Carefully crafted, exposed steel frame building, worked into an extremely constrained university campus site. Close collaboration between the design team and steelwork contractor has produced a high-quality appearance to the steelwork with careful attention paid to the connection details and paint finish. Exposed external bracing, with expressed connection details, bookends the lean steel frame of each block.
Waterloo Station Roof Infill
- © Michael Cockerham
Bourne Group Ltd
Wessex Capacity Alliance
Waterloo Station has been transformed by a programme of works, rebuilding the former Waterloo International Terminal (WIT), allowing platforms 20-24 to be brought back into use with modern facilities, new track, signalling and a new layout.
As part of this programme, a roof infill structure was required to bridge the gap between the three-pin arch roof of the Grimshaw-designed WIT, and the trussed 1920s steel roofs forming the main station concourse. Given the contrasting architectural forms, the design concept was based on a simple glazed box, linking two contrasting structures, showcasing the curved International roof, rather than seeking a strong visual identity of its own.
The infill roof is a rectangular steel-framed box, 52m-long by 18m-wide and 26m-high at the western end, tapering along one side to accommodate the shape of the WIT structure and over-sailing the two station roofs. It is 21m-high at the eastern end and supported at either end by steel-framed and glazed gable walls. The eastern gable wall is supported by Waterloo’s 1840s-built masonry walls, but otherwise, the new structure is self-supporting, sitting closely over the two roofs without touching either existing structure.
The greatest challenge was the development of a suitable foundation system to support the new structure, as it sits directly above four London Underground lines. Consequently,
structural concepts focused on opportunities to re-use existing support structures. The entire roof structure including glazing weighs only 400 tonnes and the solution required two new tapered 508mm-diameter circular hollow section (CHS) columns to help support it in the middle.
Forming the main span of the roof is a 4.2m-deep 52m-long spine truss, weighing 27 tonnes. Brought to site in three sections, the longest element, which spans between the CHS columns, weighed 13.5 tonnes. The central spine truss supports eight pairs of gullwing trusses sitting perpendicular to the main structure, forming overhangs on either side. Steelwork for the project was erected by a 300 tonne-capacity mobile crane. Keeping the frame stable during erection was critical and the structure was erected on substantial temporary works.
Hidden connection details were developed allowing coordination of architecture, structure and services distribution. This allowed concealment of the building services providing a clean and unobstructed aesthetic. Utilising tapered circular columns meant the steelwork is less harsh on the eye, and importantly, located to not hinder the views in the station concourse, which was an important part of the architectural brief.
The major challenges for this infill roof included foundation conditions requiring the use of existing supports and restricted site access. The solution is a steel frame sympathetically designed to reflect the detailing of the existing structure, and ingeniously erected in a live station, facilitating a huge increase in station capacity.
One Bartholomew, Barts Square, London
One Bartholomew is the latest element of Bart’s Square to be completed, a new mixed-used quarter in Farringdon, London. The 19,920m2 Grade A office building offers 1,820m2 floorplates and is designed to benefit from high levels of natural light.
Architecturally, One Bartholomew is a simple but finely detailed form that marks the step change between the edge of the Smithfield conservation area and the larger developments of the City. It is distinctly modern in both form and materiality, with metal screens, and floor-to-ceiling glazing to reflect and embrace the scale of the modern City of London. The design integrates the building into the extensive public realm improvements which turn Bartholomew Close into a pedestrian-friendly environment, enlivened by a café and restaurant on the ground floor.
The building is highly adaptable to the changing requirements of its users with generous floor-to-ceiling heights and a highly-efficient and adaptable floorplate design. The fully-glazed, active, double-skin façade system is linked to the building’s management system via sensors which allow the façade to adjust the level of solar shading in response to changing external conditions.
In addition to meeting the latest environmental performance and occupier requirements, the scheme includes an ecological roof, combining both extensive and intensive green roofs, with PV panels incorporated into plant areas at roof level.
The environmental credentials of the building are demonstrated with a BREEAM ‘Excellent’ rating.
One Bartholomew has been designed with tenant-focused amenities in mind. One example is fitting the structure with world-leading digital capability, which has been awarded Wired Certified Platinum for its connectivity infrastructure.
The building’s aesthetic and ability to integrate services was important to the development of One Bartholomew meaning that a steel construction scheme was the best option. Steel also suited the long spans required for fit-out flexibility, and provided an efficient construction method for the busy city centre location. In total, 2,350 tonnes of steel were used across the 12 storeys, basement, roof and plant levels.
The steelwork gains its stability from a reinforced concrete core and the diaphragm action of the floor slabs. The ground floor features a 225mm-thick composite slab with transfer beams ranging from 600mm to 1200mm-deep and up to 15 tonnes in weight. The varying sizes and weights of the beams required were determined following a value engineering exercise using BIM modelling. A typical floor comprises a 140mm-thick composite slab spanning 3m between 610mm deep cellular plate girders.
The project showcases how steel can deliver a highly flexible long-span commercial building within an urban context. The nine-metre corner cantilever of the upper floors over the entrance enhances the presence of the building in the public realm. The progressive procurement approach meant that the steelwork contractors were appointed early, supporting the design team to maximise efficiency of design and fabrication.
The Standard Hotel, London
- © Timothy Soar
Heyne Tillett Steel
Crosstree Real Estate Partners LLP
A former Camden Council office building has been transformed into The Standard, London, a contemporary, boutique 266 key hotel with sustainability and low carbon at the heart of its conversion.
The initial brief was to analyse the development potential of the 1970s building and assist the client in preparing their bid to purchase the site. A full Revit model of the existing building was created from archive drawings to understand the risks and opportunities within the existing structure, and the potential benefits of retention rather than a rebuild. The conclusion reached was that the building could be retained, refurbished and enhanced as a hotel.
Extensive research into the existing structure was undertaken along with intrusive and extensive testing of the capacity of the structure, foundations and ground to reveal their spare capacity. The result was that the concrete frame and under-reamed piles would allow the conversion of the building and the addition of a three-story extension.
The design solution to support the new ninth to 11th floors was to provide new steel perimeter columns from the first-floor transfer slab. This was the simplest structure with a direct load path and reused the existing foundation capacity. Adding the three storeys, a 30% increase to the weight of the building, only required discrete strengthening to four existing columns.
The new floors comprise 150mm-thick composite slabs supported by, and acting compositely with, steel beams. In order to limit beam depths, UC sections were used as beams for most spans.
The steel beams are supported by steel columns, with sway frames above the eighth floor providing stability to the extension. Perimeter steel columns installed through the existing building from the first to eight floors continue through the additional floors to the 11th floor. These were threaded like needles through the existing waffle slabs to the first-floor transfer slab.
The use of steel enabled the new floors to be lightweight and shallow depth while adhering to tight hotel vibration criteria and a long span existing office column grid below.
The existing façade is constructed of highly durable load bearing precast concrete units which were restored and thermally improved, reducing capital costs and providing significant embodied carbon savings. The top three storeys of the building are clad in PVD coated stainless steel over aluminium framing with double glazed units, which are lightweight, durable and can be disassembled, making them flexible for future use.
Through forensic analysis of the existing building and highly intelligent design responses, this project showcases the role of structural steel in repurposing and enlarging this existing building, maximizing the retention of embodied carbon and creating a new landmark at the end of one of the capital’s principal arteries.