Architect Make’s monolithic new ‘groundscraper’ for UBS at Broadgate utilises a steel structure to accommodate four huge trading floors
5 Broadgate is a steel building through and through. Not only does the new City of London headquarters for UBS have a main structural frame of steel but, unusually, so do its basements and most of its cores. Moreover, it’s clad with 240 tonnes of stainless steel, one of the most extensive applications of such cladding in the UK.
At 65,000m2, the project is Make’s largest to date. The architect was appointed in 2010 after British Land decided to redevelop its sites at 4 and 6 Broadgate to provide the larger premises it needed to bring UBS’ 6000 staff together in one office for the first time. UBS preferred this option, which crucially provided four trading floors of 6000m2 each for up to 750 traders per floor, to a refurbishment of its old Broadgate premises or relocation to Canary Wharf.
A 120m by 60m ‘groundscraper’ building was designed to accommodate the extensive trading floors. Make describes its concept for the 13-storey building as resembling a perfectly machined, solid metal object akin to a giant engine block, which is cut into to allow light in and views out, particularly on the upper office storeys that aren’t constrained by the trading floors. After experimenting with other approaches, the architect decided on a single expression with no movement joints, allowing the trading floors on levels 2-5 and the more conventional office space to be read as one. Trading support is on level 6 with client and meeting levels on floors 7 and 8 and offices on floors 9 to 12.
‘We set out to achieve an expression of quality and timelessness, which UBS could relate to in their brand,’ says Make architect Matthew Bugg. ‘We wanted to unify the architectural expression as a single form to reflect the single office function.’
The need to accommodate such large trading floors led to the choice of steel as the framing structure. Space planning requirements for 750 traders per 6000m2 trading floor drove the design of the 13.5m by 12m structural grid and in particular the position of the four stability cores along each edge.
‘It was critical to maximise the inside space so the cores were pushed to the periphery,’ says Bugg, adding that the lack of a central core meant that concrete was quickly discounted as a framing option.
‘We would have needed to use post-tensioned concrete, which would’ve been very complicated and not cost-effective,’ he says.
The core arrangement meant that the cores on the north and south of the building provided the restraint while allowing the building structure to ‘breathe’ horizontally. Because the grid had to be maintained in the two-level basement zone, it was decided to use steel rather than the customary concrete.
The frame uses a primary beam arrangement of pairs of floor beams either side of supporting fabricated H section columns, laid out on a 12m by 13.5m structural grid. It creates 5m floor-to-floor dimensions, accommodates a chilled beam ceiling system and yields 3.5m floor-to-ceiling heights – rather more than the 2.8m of typical offices.
A number of transfer structures are incorporated, most significantly on the north-west corner (see bottom right) where the presence of a sewer beneath the footprint of the building prevented any foundations in that area.
Everything was detailed to take account of progressive collapse issues, which also influenced the use of steel rather than concrete for most of the cores.
Buro Happold was heavily involved with steelwork contractor Severfield for the connection design, defining and prescribing the position and spacing of the bolts and the thicknesses of the end plates.
‘We were very prescriptive on the steel to steel connections,’ says Buro Happold director Franck Robert, adding that this ensured the connections would be ductile enough and behave the way they wanted them to in the event of one of the unprotected columns being taken out accidentally. ‘We provided the client with a resilient building cost-effectively.’
Steel sizes were driven by stringent vibration considerations. In total Severfield fabricated, supplied and erected 13,000 tonnes of structural steelwork, which received 75 and 120 minute intumescent fire protection applied offsite. The steelwork included 270 flights of staircases and 90,000m2 of metal floor decking.
Make favoured a metal-clad building from the outset. Large-scale cladding in 6m by 1.5m panels and the lack of mullions and joints help to break down the mass of the building, according to architect James Goodfellow. Stainless steel was chosen over aluminium because it suited the aspiration for panels to be as flat as possible. These are 380mm deep and after a linear pattern had been pressed into the metal they were bead-blasted to give a less glossy finish.
The building is 65% clad, with windows positioned to maximise light but minimise solar gain.
A BREEAM ‘Excellent’ rating is anticipated for 5 Broadgate with 65% lower carbon emissions than the two buildings that preceded it on this site. The facade and structure are calculated as accounting for 57% of the construction carbon footprint and 28% of the whole life carbon footprint, while building operation accounts for 51% of the latter.
Fit-out by interior architect TP Bennetts is under way and UBS is expected to take occupation towards the end of the year.
Developer: British Land and GIC
Structural engineer: Buro Happold
Steelwork contractor: Severfield
Double primary beams
5 Broadgate is Make’s first steel-framed project with double primary beams. These are spaced 1m apart and span 12m. Secondary beams at 3m centres span 12.5m between these. The pairs of primary beams run either side of the supporting steel columns. They are connected to these via steel stubs that are factory-welded to the side of the columns. The primary beams are bolted to the stubs on site. The dual arrangement enables moment continuity over the column locations and so creates a more structurally efficient system, enabling the engineers to reduce the span of the secondary beam. This cost-neutral solution resulted in a 12% reduction in weight and a 9% reduction in carbon impact for the steel floor structure. Both the primary and secondary beams are typically 650mm deep, with 400mm diameter circular holes and 1500mm wide x 400mm deep slots through the web to allow for flexible integration of services. This system supports a 150mm thick composite floor slab.
A storey-height, 19.5m truss was installed at lower roof level to support the 6m x 9m hung north-west corner of the building. Installation was tricky because the 40 tonne truss had to be positioned in exactly the right place at the corner before the addition of the extra weight of the cladding. This precision was achieved by weighing down the facade with water to mimic the effect of the cladding weight, and draining off water as the cladding was installed.
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