An inventive structural solution has enabled Mulberry House School to expand upwards

Buildings.

Mulberry House School, an independent nursery and pre-prep school in north London, wanted to expand on its very restricted site, and close collaboration between the design and contracting disciplines led to an ingenious solution.

“The architecture was intended to express the human energies within, to create extra indoor and outdoor spaces that help develop concentration, exploration and imagination, and a place to freely run, jump and play in all weathers”, says design architect Doug Clelland. “It was also to achieve a very high level of environmental sustainability, to act as a beacon for the neighbourhood, and be enjoyable and a source of pride.”

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1960s building with water tower removed (ph: DC)

Doug Clelland – design architect
Our visits to Mulberry House School – originally a two-storey 1960s Camden Council day nursery built under borough architect Sydney Cook – were filled with the sounds and silences of learning, triggers that inspired the architecture.

Given the limited size of the existing front and rear playgrounds, classroom and outdoor space for an extra 80 to 100 pupils could not be achieved horizontally, so we needed to go upwards. That decision took on particular poignancy when health and safety concerns dictated that no serious construction work could take place during school hours. Refining that challenge, it meant we were limited to short holiday periods. Adding further spice was Tim Macfarlane’s judgement that the existing building’s foundations could not support any new loads.

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Parallel to the planning negotiations, and in order to share the programming challenge early on, nine contractors were interviewed, and Rooff emerged as the partner judged best able to manage the numerous offsite construction and onsite assembly packages.

The ‘villa on a corner’ emerged as a design concept, with each elevation responding to its urban context and orientation – a single divisible volume with a play roof, whose canopy was partly determined by rights of light to the adjacent building.

The plan, locked in place by the need to extend the existing internal stair vertically, evolved with a circulation gallery, a thick wall of ‘servant’ accommodation and, immediately adjacent, the ‘served’ learning and performance space. The overall composition – cylinder, glass prism and ‘cloud’ – seeks to balance the internalised nature of concentrated learning (hence the use of translucency) with framed focus on the best of the context (transparency).

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Attention then focused on additional layers of detail design, which included minimal numbers of columns impacting the playgrounds, resulting in sizeable cantilevers; a ‘cloud’ roof over the play terrace with soffit light ‘stars’; an oculus aligned with the sun at midday on 1st June (International Children’s Day); the beak, providing solar shading to the south facade, with a rainwater chain facing Shoot Up Hill; the top of the drainage stack clad like a pencil; the escape stair as a beanstalk’ rising to the cloud, its cladding representative of three ages; a mini-cloud over the reconfigured entrance, in a reverse geometry of the roof cloud; Lucie Sutton’s artwork etched onto two key windows; and, within the learning space, lights acting as suns.

Trying moments included the search for unexploded wartime bombs, the paucity of affordable onsite construction skills, and so on. Despite such frustrations, there were some laughs, and mention of Yona Friedman’s theoretical structures raised over Paris, Russian Constructivism, robust minimalism, a beacon of learning, and a neighbour’s observation that “a spaceship has landed”.

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Tim Macfarlane – structural engineer – GL&SS
The requirements for extra space above the existing building, and to avoid relocation, called for a structural solution facilitating as much offsite prefabrication as possible. The existing building couldn’t support the new accommodation, and groundworks can be time-consuming and disruptive, so it was decided at an early stage that a bridge structure, independent of the existing building, offered the most viable and least disruptive approach.

In order to limit the number of support elements, four 508mm-diameter circular hollow section columns act as vertical cantilevers, each supported on a piled base. The pile caps were fabricated from a steel beam grillage, each set on four 320mm-diameter, 19.5-metres long piles. The steel grillage allowed the piles and pile caps to be installed during the short Easter break.

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The new floor measures 20×10 metres, and the most efficient way to span between the columns was to use the external walls of the rectangular volume as storey-height trusses. Exposed within the classrooms, these were fabricated from 300×150 SHS bottom and top booms with 150×150 SHS diagonals. The shorter, 10-metre-wide end wall trusses sit on the steel columns, cantilevering out to the corners. These were transported to site in two parts, then joined and lifted to their final positions.

The floor and roof elements were prefabricated offsite as 10×3-metre cassettes using ply-covered cold-formed steel joists spanning onto 305x305mm UKC sections which in turn spanned the 10-metre width. A curved CHS 209mm-diameter framework supports an array of cold formed steel joists, supporting the playspace roof. Thin ply sheets were bent to the curvature and covered with a single-ply membrane.

The external stair and lift tower were fabricated from 100x100mm galvanised steel SHS sections with folded metal durbar plates for the treads. The core structure around the lift is a vertical cantilever off a concrete base, and stability for the platform lift was achieved by steel plate stiffeners. The outer structure is clad in curved perforated aluminium sheets.

Mike Popper – building services – p3r
The classrooms were to be extensively glazed and acoustically sealed, so a very high level of insulation and solar control was required in the curtain walling. The chosen system achieves a U-value of 1.2W/m2K (a 55 per cent improvement on Building Regulations), and a 30 per cent g-value (solar heat transmission reduction).

Heat recovery ventilation and heat-pump air conditioning provide fresh air and temperature control. Classrooms are served by individual air-handling equipment in the fire-rated ceiling void above the gallery and service and store rooms. The void acts as an plenum through which extracted air from the classrooms is partly recirculated through its ducted air-conditioning fan-coil unit; the remainder is vented from the south-east facade after passing through heat recovery units. Heated or cooled air is supplied to classrooms by a fan-coil unit, and fresh air at room temperature is supplied by the heat-recovery unit.

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The heat-pump air-conditioning system is a Daikin Heat Recovery VRF. The technology uses refrigerant as the medium to exchange heating or cooling to the indoor fan-coil units from an outdoor compressor. Heat is supplied (or rejected) by the system to ambient air. The system converts electrical energy for the compressor into heat at a carbon emission rate greater than a modern efficient gas boiler. It also benefits from heat recovery in spring and autumn, when the energy to cool one classroom can be used to heat another.

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Andris Berzins – executive architect –Andris Berzins & Associates
As executive architect our role was to absorb the brief and design intent to help develop proposals with the design architect, and to manage, integrate, disseminate and implement a diverse range of information and criteria relating to the overall concepts, while contributing our own attitudes to good design.

An unusual aspect of the project was the intensive land use – the seemingly impossible site was unlocked by building on top of the existing building with creative engineering, but this posed difficult parameters of a ‘site in the sky’.

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Rooftop play terrace

Landing a new building above a working school on just four points of support (neither at the corners, nor structurally dependent on the existing building), then executing the construction with the school in occupation by children other than for periods of heaviest lifting, with the critical playground interventions of piling, foundations and principal column erection taking place in the short breaks of a busy school timetable, drove all considerations of buildability and safety for children, staff, public and operatives.

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While not volumetrically modular, the construction employed a high degree of offsite manufacture, especially of structural components. The perimeter bridge trusses provide a primary framework within which span 3×10-metre prefabricated floor and roof ‘cassettes’. These were road-crane-hoisted to provide working platforms at two levels.

Because of the need to neither bear upon, lean upon or structurally touch the existing school, both vertical and horizontal movement had to be predicted. Various key junctions then had to be resolved, not least at the stairs, which by necessarily connect the old to the new.

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Reflected ceiling plan

Design development for the Mulberry project was not some abstract pre-emptive activity, but design that happened continuously throughout the project, from pre-site to on-site. Within the resources, skills and procurement procedures of the contemporary construction industry, the relative complexity of the project was a challenge and at times was excruciatingly painful for all. The project team –clients, consultants and constructors – shared that experience and met the challenges. Architects seldom contemplate that a building is as good as it could have been. That said, it’s a pretty good building.

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Cross section

Mark Horn – contractor – Rooff
The client brief focused on maintaining a safe and controlled environment to enable the school to function throughout the construction without the need to decant. Rooff was appointed at an unusually early stage so that the construction technology, methodology and logistics could be considered during the design process.

A two-stage appointment was adopted with initial procurement works carried out under a Pre-Construction Services Agreement, culminating in a JCT Intermediate Building Contract with Contractor’s Design 2011.

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Our implementation strategy sought ways to construct the building, as far as possible, without the need to occupy either of the playgrounds or the existing buildings. Essential works in these areas were carried out in advance in a series of phases during school holidays. The piled foundations, for example, were installed and playground surfaces made good during the Easter break.

The four large steel columns, high-level steel trusses and floor decks were then built during the summer holiday. These bursts of intense construction activity required a high degree of up-front logistics planning, and a design strategy that enabled as much prefabrication as possible.