Steel diagrid supports Paddington’s Brunel Building

In the valley of dinosaurs that is Paddington Basin, there’s a new predator in town. Standing out from the generic silver-clad, green-glassed offices, whose facades form the dead gorge through which the Grand Union Canal now diminutively angles its way, Derwent London’s Brunel Building seems more purposeful. An eye-catching diagonal steel exoskeleton and shock of orange, it appears on its haunches, ready to spring at Brunel’s 1854 Paddington station, just out of reach on the canal’s southern bank. 

Architect Fletcher Priest’s £116 million, 22,600m² building rises 16 storeys from the canalside. Its dramatic 9m high reception area has a huge rolling glass wall that slides to open out to the new 6m wide towpath – which, for the first time in 200 years, gives the public access from that side of Paddington Basin to Little Venice.

But it is for its 71m high exoskeleton and stealth bomber-like cladding that the building is most noted. After 11 years of development, either side of the recession, by the architect, engineer and fabricator, construction began in 2016. While the criss-cross pattern on the cladding’s aluminium spandrel panels references the configuration on Bristol’s Clifton Suspension Bridge, it is in the external structure that the Brunel engineering references are writ large.

Several considerations drove the design rationale, explains Fletcher Priest associate Chris Radley. Client Derwent London wanted maximum flexibility of office space; so once beyond the line of the central spine concrete core, it favoured a column free arrangement to the cladding line. Maximising the floor area on this high-value site, the architect pushed the building line to the edge of the demise to create an irregular, six-sided floor plate. Engineer Arup, meanwhile, mindful of the Bakerloo Line tunnels running beneath it, minimised piling with secant piling and capping beam bounding a raft slab. With the principal elevation facing full south, there was also a need to consider methodologies for solar shading. Aesthetic decisions also came into play; the architect felt the proximity of the building to the canalside Crossrail exit of Paddington station meant that the usual post and beam approach, as exhibited elsewhere on the Basin site, was heavy and confronting. It wanted a lighter-touch – a visual solution that drew the eye up and away from the ground. Initially dealt  with as isolated concerns, all were answered at once with the diagrid exoskeleton.

Not that this meant the project got simpler – there were knock-on implications for the design team. Architect and engineer agreed that any external structural rationale would continue inside, so beams running back to the concrete core would mirror the 6m spacing of the diagrid, creating unique cantilever conditions at the corners and necessitating serious M&E co-ordination further down the line. In its realised form, the 66m wide south facade diagrid travels west until the 9th floor, turning back east to the 16th. 

 

Two types of steel connection are evident on the diagrid; end plate and fin plate. End plate connections carry the lion’s share of the building’s load with fin plate connections transferring the diagrid’s bracing loads. Fletcher Priest wanted their expression to be clearly visible from the outside without looking oversized from within. Sitting directly in front of the cladding, the diagrid structure acts as solar shading for the 3.5m storey heights, delivering 20% shading on the south, despite the sizeable glass panels.

The cladding behind the structure had to deal with significant diagrid movement, says Arup’s Conor Hayes. This is partly the result of the building’s tendency to want to twist due to the asymmetrical nature of the plan and wind loads, but is also due to thermal expansion and contraction of the exposed steelwork. While the internal steelwork beams might be a steady 22˚C, they are connecting to a diagrid that, orientation and season depending, might be exposed to polarities of -10˚C to +77˚C. Movement concerns were exacerbated by the fact that the diagrid was connected by the internal beams to a concrete core that wasn’t moving at all.

As a result the temperature of the structure was monitored on installation, cladding fabricator Scheldebouw wanting exact dimensions on the positioning of the steel at any point, according to Arup’s usual debate, ‘perhaps more precise than they were necessarily accurate’. In all, the design team attended five workshops with the cladding and steelwork contractors to ascertain worst case scenarios and determine the movement tolerances at every junction. As a result, steelwork tolerances were resolved to +/-10mm, rather than the usual +/-25mm. Movement at some key points was facilitated with slotted bolt connections, fixed with elliptical bolt heads until the structure was complete, and then swapped for round headed sliding bolts to deal with structural movement. Strongback steel spandrel cladding with aluminium external wrapping and solar-controlled glazed panels were delivered to site in 6m sections with interlocking end pieces, and fixed in line with where the beams penetrated, to connect with the diagrid. 

To mitigate cold bridging, the interface of the external structure and internal beams was resolved with a composite GRP separator encapsulated in an insulated steel collar, painted in signature orange. Nine versions of these deal with every type of angled junction on the external diagrid. External steel elements are intumescently painted and 60-minute fire-rated, with internal steels at a 90-minute rating.
 

The asymmetric form of the building as well as programme demands had knock-on effects on the design of internal beams. The desire to maximise daylight at the facade side and allow for duct runs at the core resulted in their characteristic thinning at both ends but the story is more complex than this. With differing loads being transferred over the diagrid, internal beams too needed to account for the divergences. With Fletcher Priest insisting all beams should be the same overall depth, Arup was faced with having to model the structure and alter the web and flange thicknesses of every beam to account for various diagrid loadings. Achieving the narrowed beam form, not by the steelwork fabricator’s preferred bending method, which would have meant a different curve to each beam flange, but by cutting and welding, made for a difficult conversation with Severfields. Floor plate facet shifts show in a more  complex structural elevation where they occur, as additional structure extends out to pick up the additional beams required. It might seem like gestural formalism, but Arup assures that all the exoskeleton is justified structurally.

Logistics-wise, the diagrid structure was constructed in three storey quadrants around the concrete core. Once the steel had gone up, the internal pre-cambered beams, fitted with steel ledger plates on the web, were run back to the cores to stabilise it. As steelwork began on the next quadrant, slim 75mm, precast reinforced concrete planks were installed on the ledger plates and cast in-situ to the requisite 225mm. Steelwork, concrete and cladding construction thus spiralled around and up the core, allowing the three contractors to work concurrently.

 

Services co-ordination with this ever-moving structure was challenging. Even at pre-tender stages it required detailed 3D modelling of every floor to ensure any conflicts were ironed out before construction. The precast concrete soffits on their ledger plates provided a highly finished backdrop from which to hang ducts, lighting and conduit. The subcontractor projected a 1:1 model of the service runs directly on the concrete soffit to ensure pinpoint accuracy for the setting out, leading to ‘almost creepy’ services alignment now looking along the floor plate.

The quality of finish, internally and externally, works in the client’s favour. The building was 100% pre-let, partly due to the flexibility of the open plan and column-free floor plates. One tenant has allocated more space to its bar, café, gym – and barber’s shop – than office space. And a global hedge fund manager has leased the two top floors for a mere 23 staff; luxuriating in a spatial redundancy that counter points the hard work the building is doing everywhere else.