Reinforced concrete: Schmidt Hammer Lassen's campus for City of Westminster College
Schmidt Hammer Lassen’s campus for City of Westminster College displays the design potential of concrete frame construction
When project teams debate the relative merits of steel and concrete building frames, design quality is seldom the only criterion. Construction time and cost are usually paramount considerations. The new campus at Paddington Green for City of Westminster College by Danish practice Schmidt Hammer Lassen Architects, which is set to open in January 2011, is not exempt from these factors.
The college of further education has a strong vocational focus and includes workshops, labs, a sports hall, a theatre and a public café. The project is subject to strict cost control. It is destined to be remembered as ‘the one that got away’, the largest project to be awarded funding by the Learning and Skills Council (LSC). The vast majority of LSC-funded projects have since been shelved.
A reinforced concrete frame was chosen for the college, with steel sections embedded in concrete members. There are also steel columns and beams in the gymnasium and performance space. According to Mark Jeffs, senior architect at Schmidt Hammer Lassen, a predominantly concrete frame was chosen because of the design potential of reinforced concrete: its visual appearance, robustness, low maintenance and its provision of thermal mass for passive night-time cooling. Also, there is no need for separate fire protection, and it provides flat soffits because large spans and cantilevers are possible without the need for downstand beams.
The typical floor construction at the college comprises 300mm-deep reinforced concrete slabs that span between columns on a 7.8m2 grid. These large spans and shallow structural depths help provide a generous sense of scale, with high ceilings and large spaces, many of which are column-free. The cantilevers that are possible with concrete slabs enabled the architect to provide overlapping openings in the floorplates, which form the dynamic geometry of the atrium.
The slabs contain GGBS (granulated ground blast furnace slag), a recycled by-product of steel manufacturing. Buro Happold associate Jim Martin explains that ‘this has the effect of lightening the colour of the finished concrete and reducing the early thermal shrinkage in the movement-joint-free slabs’.
The strength of the concrete in the slabs was slightly lower than usual, reducing early thermal shrinkage, and the concrete finish was agreed with the subcontractor after inspecting test pours off-site, avoiding the ambiguity of a written specification.
The project benefited from the fact that there is little risk of sound penetration through in-situ concrete slabs, and roof upstand waterproofing details are easier to construct than fiddly frame and panel assemblies. In-situ concrete construction can provide fixing points for cladding, thus limiting requirements for secondary steelwork supports, which are often disputed areas of responsibility. At Westminster, concrete perimeter upstands limit deflections of slabs above
windows and provide fire stops to the raised-floor voids.
There is some variation in slab depths, for example where larger spans are required and in the cores, where the top of slab levels are higher to reduce the depth of finishes in areas that do not require access floors.
Raking facades are required on the north-west and south-east elevations to maintain rights of light and for solar shading, and Buro Happold proposed 600mm-diameter circular raking columns instead of transfer beams. The inclined columns on the north facade start at second floor level and rake into the building at an angle of 25°. This introduces large tension forces in the second-floor slab, which are transferred to the cores through tightly reinforced concrete nodes. The compression forces in the floorplates above are transferred to the cores. The tops of the raking columns had temporary horizontal supports during construction to prevent excessive cracking before the floor slabs reached sufficient strength.
Due to the raking columns, large horizontal loads act on the reinforced concrete pile groups. These are resisted by the basement slab and ground beams, which transfer loads from the columns to the pile caps. The construction of the 18° raking columns on the south-west facade is similar.
On the south-east side, the columns rake away from the building from the ground floor to the roof at an angle of 25°. The floorplates have tension forces from the columns that need to be transferred to the cores. In the foyer, where raking and vertical columns intersect, Buro Happold proposed steel H-sections, with in-situ concrete casings. The joints at the points where the columns cross use self-compacting concrete to ensure high-quality finishes. There are steps in the perimeter slabs to accommodate insulation and cladding to their external soffits. This would have been difficult to construct with a steel frame.
Name of project City of Westminster College, Paddington Green campus, London
Start on site August 2008
Expected completion date January 2011
Form of contract JCT design and build 2005 rev 1 2007
Cost £67.7 million
Client City of Westminster College
Architect Schmidt Hammer Lassen Architects
Structural and civil engineer Buro Happold
M&E consultant Buro Happold to end of stage E
Project manager and planning supervisor Knight Frank
Cost consultant and employer’s agent Stace
Main contractor McLaren Construction
Selection process Expression of interest via OJEU followed by a design competition in 2006
Structural frame Generally reinforced concrete with 300mm-deep slabs spanning up to 8m between columns
Structural frame to gymnasium and performance space 18m steel roof beams supported by steel and concrete columns and concrete beams
Lateral stability Cores with 250mm-thick reinforced concrete walls
Movement joints in floors Not required because cores are centrally located with maximum separation of 30m
Spandrels and external soffit cladding Reconstituted stone fixed to concrete upstands and slab soffits
Ceilings Generally fair-faced concrete
Windows Thermally broken composite windows
Acoustic control Sound-absorbing ceiling baffles