A steel sandwich construction near London’s Waterloo has a diverse filling, housing and teaching students at different stages of their education
Urbanest’s latest student development is a hybrid in more ways than one. Not only does the 20-storey building incorporate teaching and accommodation provision, community workspace and a health centre, it is constructed using a sandwich of framing solutions with steel structure at the upper and lower levels and concrete in the middle.
Designed by Allford Hall Monaghan Morris on a former office site alongside London’s Waterloo station, the project will provide accommodation for 1100 higher education students as well as a private sixth form college for 700, some of whom will live on site. The 6,000m2 college, which occupies the lower four levels, is due to complete this spring while the upper residential rooms will be ready for the autumn term.
‘AHMM is doing a lot of city sandwich architecture – buildings thrive on having lots of different uses,’ says Vasiles Polydorou, senior architect at AHMM.
The design has to ensure clear separation between the college and higher education provision, with separate entrances and lift access to both sorts of accommodation. HE students will enter into a double-height common room before travelling up to their rooms, and will have the use of a further common room plus terrace on the top floor. College provision is centered around a four-storey, top-lit atrium for assembly and refectory use. There are two rings of classrooms, the inner overlooking the atrium and the outer around the perimeter. A health suite containing a swimming pool and gym shares the basement with the college.
The building form was inspired by Eames’ turned Model A stool, designed for the Rockefeller Centre in 1960. In response, floor slabs at the new Urbanest building step in and out to give a turned form emphasised by horizontal ribbons of glazing and bands of rainscreen cladding, which provide reveals of around 500mm to shade the glazing. On the external elevations these are white, but for inner courtyard above the atrium they have more of what Polydorou calls a ‘tweed’ effect, with a weaving of darker colours at different depths. The concept, he says, was to make each a ‘room with a view’.
The hybrid structural solution was driven by height, weight, programming and servicing issues. Since the overall height of the building was fixed to avoid breaching views towards County Hall protected by the London Views Management Framework, the structural frame had to be as efficient as possible to enable the client to accommodate its desired amount of student rooms.
Initial plans for a wholly concrete structure were rethought and a steel frame was used instead on the lower four floors, set out radially from two cores and supported by 62 steel columns. This was in response to both the wider spans required for the college floors, and the need to accommodate services horizontally through the building structure when these vertical risers came down to college level from above. With slab depth crucial, the ability of the steel structure to accommodate this servicing was a major advantage.
Built in just 13 weeks by Bourne Steel, the radial steel lower structure had the additional advantage of speed, being 25% faster to build than a concrete frame. This swift start was particularly advantageous logistically for a site on the busy Westminster Bridge Road.
‘Typically, the college floors are a composite steel frame construction with concrete slab on profiled metal decking, while stability is provided by the cores and an additional stair core between the basement and third floor. Most of the steel beams are designed as downstand beams acting compositely with the slab above,’ says Mitesh Patel, project director at structural engineer Ramboll.
The most crucial element was the transfer deck that unites the two structural systems, necessitated by the removal of an entire row of concrete columns at this level. According to Polydorou, managing this transition between the steel of the college grid and the concrete structure of the student grid and dealing with the servicing was one of the biggest challenges on the project.
‘The third floor was designed as a transfer level to support the reinforced concrete columns which in turn support 15 concrete floors above. Steel allowed the flexibility of using shallower beam sections compared to concrete, and also allowed the integration of M&E services through openings within the beam webs,’ says Patel.
Bourne also supplied steel for the uppermost 18th floor and mezzanine housing duplexes, social and study space, used this time because of its lighter weight and ability to accommodate the structural gymnastics required for the sloping roofline to avoid breaching sightlines. This led to an increase in height from 2.4m to 4.5m as the roofline slopes up away from County Hall.
‘Steel was important to accommodating the 6º angle as well as being lightweight,’ says Polydorou. ‘It’s quite a dynamic structure.’
Lucky residents will benefit from splendid views of Westminster and across the capital. AHMM has provided five room types ranging from non-suite cluster flats to one-bed flats for college house parents. The faceted perimeter means these room sizes vary again depending on their location within the building. Floor to ceiling heights are 2.3m except for the rooftop duplexes where students will enjoy a 5.4m high living space with a terrace and a mezzanine bedroom.
A steel structure on the 18th floor podium slab provided a lightweight top storey that could accommodate the geometry imposed by height restrictions driven by the protected views. Steelwork contractor Bourne modelled the hot-rolled steel frame using Tekla software. To achieve the right solution, the fabrication process involved a large amount of setting out and a high proportion of low volume batch fabrication.
‘It was critical that we found a solution that matched what the architect wanted to see and gave the planners what they wanted,’ said Bourne’s divisional manager Iain Griffiths.
Steel decking forms the roof itself while spans between steel beams provide lateral restraint. The sloping profile of the roof and radial column grid presented extra complexity and resulted in an array of beams splayed in all three principle axes, according to the structural engineer. The splayed perimeter beams support a coping that projects 1m from the edge and also restrains the facade mullions. The perimeter beams were designed for torsional connections at the ends and also to resist the loads induced by abseilers during maintenance.
‘The roof is stabilised by a combination of the main concrete cores, vertical steel bracing elements along the wing tips and plan bracing within it to provide diaphragm action. The eastern wing of the roof consists of an inaccessible sedum roof and PV panels,’ says Mitesh Patel, project director at structural engineer Ramboll.
The roof structure was also a major logistical challenge, with 800 pieces of steel craned to the top of the building to form the frame.
THIRD FLOOR TRANSFER DECK
The third floor transfer deck is formed by 44 fabricated plate girders ranging from 1000mm to 1250mm deep and weighing up to 19 tonnes each. These transfer the load from the concrete upper structure to the steel columns below.
The longest single member spans 19.5m across the base of the atrium. The heaviest section – at 21 tonnes – is made of 700mm wide by 85mm thick flange plates with an 85mm thick web plate.
All transfer beams here are at the same level as the top of the slab and designed as non-composite, says Mitesh Patel, project director at structural engineer Ramboll: ‘Typically, the transfer beams cantilever out past the perimeter steel column below, picking up the full length of the concrete column above and supporting the cladding line on the building perimeter,’ he says.
Bourne Steel divisional manager Iain Griffiths reveals that the fabrication process was further complicated by the need to provide beam stiffening to the service penetration holes and the notched tapered end detail.
Main client Urbanest
Architect Allford Hall Monaghan Morris
Main contractor Balfour Beatty
Structural engineer Ramboll
Steelwork contractor Bourne Steel