Squire and Partners’ hybrid concrete and steel frame for 7-15 Baker Street is the secret behind its elegant design, writes Felix Mara
Some buildings reveal their construction. Others conceal it. But Squire and Partners’ mixed development at 7-15 Baker Street, in London’s West End, completed last month, does both as part of an elaborate formal and tectonic game.
The formal game involves wrapping a screen-like arrangement of precast columns and spandrels around the office and retail section on the Baker Street side of the development. The width of the columns that form this screen is exactly equal to the depth of its spandrels - 350mm - and this screenframes glazing that steps in and out, sometimes flush with its outer face and sometimes set back.
Behind the screen of 380mm-thick precast units is a frame of stanchions at 4.5m centres that support deep primary steelwork beams spanning 17m.Because the graphic horizontal line of the shallow, white transoms is so prominent, the edges of the 980mm-deep floor zones are visually suppressed, assisted by stub beams at first floor level stanchion junctions, where the bending moments are at their lowest.
The 4.5m structural grid is deliberately lost in the pattern of vertical precast concrete screen elements, of which some are dummy columns, while others coincide with the stanchions. In some locations, stanchions are set back behind large areas of glass or, in the case of the central ground floor entrance, transferred out altogether. This avoids a columbarium effect generated by the 4.5m grid and the 3.68m floor-to-floor heights.
To an extent, the proportion of precast columns diminishes towards the top of the screen, where the structure carries less load and the elevation is visibly lighter, although the grid of stanchions still marches on behind. At ground and first floor levels, composite columns with stanchions embedded in concrete take the place of the solid precast units, helping to form a street-level colonnade. So the facade, though animated and asymmetrical, divides into three horizontal zones and, in certain places, reveals the logic and components of its structure.
This formal and tectonic logic, rather like a 3D game of chess, dovetails with commercial requirements for large, column-free, rectangular floor plates and, by a subtle twist, with the expectations of the planners.
‘We were encouraged by City of Westminster planning officers to develop a “modern and contemporary design”,’ says Squire and Partners project associate Stewart Kendall. But the planners also wanted what he calls a ‘Portland stone look’.
Having sold the planners the idea of a screen of uniform and slender components, Squire and Partners was able to argue that, if these were precast concrete, they could be slimmer and therefore closer to its design intent. The crunch point was the two-storey columns that pass through the ground floor arcade. These could not have been 350mm wide if they had been stanchions clad in Portland stone.
‘This detail was of particular complexity,’ says Kendall. ‘The preferred precast jointing rationale dictated this to be a double-height precast column, the cross-section size of which was then required to change above and below the beam connection to accommodate the conflicting requirements of maximising net area and transferring load.’
As in a pyramid of playing cards, the logic built into Squire and Partners’ facade has survived the project’s design and build procurement route, although a 100 per cent steel frame was discussed at ‘value engineering’ stage. ‘The contractor had bought into quite a lot of detail,’ says Kendall. But with so many steps needed in the EPDM membrane to accommodate the set-backs in the glazing, it can’t have been easy to construct the facade; nor, for that matter, to co-ordinate the structural steelwork with the precast concrete.
It is hard to discuss the facade design for 7-15 Baker Street in isolation from its structural frame. Its precast concrete units are, in a sense, structural although only its low-level composite columns are part of the primary frame. In a very loose sense, it could be described as Mannerist. It may be contrived, but this elegant solution is no more artificial than its Portland stone-clad neighbours.
Precast concrete column and beam units Evans Concrete Products bespoke mix including mica granules, with medium-grade acid etch finish.
ProtectGuard (by Guard Industries) applied to all visible surfaces to act as water and oil repellent, and to ease graffiti removal. Arbosil 1090 (by Adshead Ratcliffe & Co) silicone sealant to all joints.
Glazing units Metallbau Früh bespoke PPC aluminium thermally broken frames, with argon-filled, double-glazed units. Opening vents for smoke clearance designed to have identical frame size and profile for uniformity.
EPDM SIG Group XL400 synthetic rubber membrane.
Fire stopping to slab edge Siderise Insulation CW-FS60 cavity barrier.
Acoustic barrier to slab edge Siderise Insulation AB10 acoustic barrier quilt.
Semi-rigid insulation Kingspan Kooltherm K15.
Loose-fill insulation Rockwool RW5 slab.
Steelwork with 60min intumescent paint Severfield-Rowen.
Start on site December 2009
Duration of contract 16 months
Gross internal floor area Approx 12,900m2
Procurement Design and Build
Total cost Unavailable
Client AUB (Baker Street)
Development architect Squire and Partners
Structural engineer Scott Wilson with Taylor + Boyd
Quantity surveyor Rider Levett Bucknall
Main contractor McAleer & Rushe
Estimated annual CO2 emissions 27.7kg/m2
Overall area-weighted U-values 0.87W/m2/K
Average for walls 0.31W/m2/K
Average for windows 1.94W/m2/K
Average for ground floor 0.21W/m2/K
Airtightness 5m3/hr.m2 @50Pa
Summer thermal target for energy reduction 24°C
Proportion of floor area with daylight factor >2 per cent 100 per cent
Proportion of floor area with daylight factor >5 per cent 43.2 per cent
Background and wall-washing illuminance 150 lux
BREEAM rating Excellent