Streatham & Clapham High School, London

Introduction

Streatham & Clapham High School, a successful independent day school for girls, has been transformed and upgraded by Cottrell and Vermeulen Architecture. A new main entrance and dining hall has given the 1930s building a fresh face, while a new rooftop pavilion houses the sixth-form centre, a place distinct from the main school where pupils can transition from sixth form to higher education. The new spaces - their structure, floors and walls – are of cross-laminated timber (CLT) and glulam; they were chosen for technical, cost and sustainability advantages, but, as the architect explains; ‘the most palpable effect for the users of the building is the beauty of its internal timber finish’.

The upgrades took place in two phases. The first, the new sixth form centre, is a new top floor of exposed glulam and CLT which follows the elongated plan form of the 1930s building below it. It contains book-ended common and study spaces connected by a spine of well-lit classrooms, offices, a science laboratory and service spaces. Extended and new stair towers connect the new sixth form floor with the rest of the school, with new plant areas, escape stair and a new passenger lift.

The newly completed second phase extends the ground floor to add a large new dining hall and kitchen, new entrance lobby and main entrance, reconfigured to act as a focal point to the new approach to the school.
The two phases are similar in concept, with exposed glulam columns which support CLT roof slabs with exposed ceiling soffits, and CLT walls with the timber exposed as the wall finish. Initially the decision to use CLT and glulam was taken as the off-site prefabrication would reduce onsite construction time and minimise disruption to a working school, especially in the case of the sixth form centre, built on top of a three storey, occupied school. But, as the design developed, other advantages were discovered, in particular the relative lightness of the construction which avoided additional foundation work while providing sufficient mass for acoustic separation and fire protection compared to a framed structure.

Unlike conventional construction, the structure was developed using three-dimensional modelling as part of the offsite fabrication. This allowed complex forms to be created for offsite fabrication, while the preparation and precision of the CLT components allowed a swift and problem-free erection.

The design of the CLT and glulam structure

Martin Waters, structural engineer and director of Engineers HRW, describes the principles of the structural design.
‘The building comprises a combination of glulam and CLT structures whose space-forming structural expression helps define the interior architecture of the project. The exposed flat soffits are reminiscent of in-situ concrete flat slabs and the architectural expression of exposed slab and column goes back to Corbusier’s Maison Dom-Ino house.
However, unlike a concrete slab, a CLT ‘slab’ is not a continuous two-way spanning system; rather it is constructed of wide discrete planks which have dominant stiffness in one direction. If the structural approach is to force a material (CLT) to conform to the rationale of a column-supported two-way flat slab system, it will inevitably be compromised. To make the floor span in two directions (and so across a joint in the long edge of the plank) requires a moment connection. This is not in keeping with robust construction and visually unobtrusive detailing.

Using flat slab terminology, CLT planks are ‘beam strips’ in one direction only. A reasonable grid (say 5 metres by 7 metres) can be achieved if every alternate CLT plank (the ‘beam strip’) is supported by columns and intermediate planks are supported by the adjacent beam strip.

The lack of continuity perpendicular to the span (in an idealised model) leads to instability if the beam strip is supported from a single point. For this project, three variations of column supports were designed to provide stability against this unbalanced case. Their design, shape and layout were tailored to suit each specific space.
The design evolved using simple elastic methods (mostly hand analysis) during the early stages. FE models by KLH’s structural engineer Price & Myers verified these scheme calculations.’

The sixth form centre roof top extension

A steel structure was placed over the existing roof to transfer loads from the new sixth form centre to the existing reinforced concrete columns below. The walls and roof structure of the new classrooms and study spaces are all of CLT and in most cases the CLT roof slab soffits and internal wall surfaces are exposed. Glazed timber-framed curtain walls bring natural light into the interior and are shaded by an overhanging roof to minimise solar gain and glare.

The new structure has two types of column supports. In the library, the column supports are formed of 1300 mm x 90 mm vertical slabs of CLT, wide enough to support the CLT roof slabs above and forming simple sub-divisions of the library space while retaining a light-filled and spacious environment.

In the sixth form common room the 100 x 200 mm spruce glulam columns are assembled in crossed pairs, minimising the impact on the floor area.
Plasterboard panels are suspended below the exposed CLT roof slabs to improve acoustic separation and to conceal services.

The ground floor dining hall extension

In the dining hall, which is nearly four metres high, a slightly different approach was taken, using three-point column supports instead of two-point supports. The structural purpose of the columns was to act as table-like supports to the CLT roof slabs to reduce the span. Reminiscent of a branching tree, each column support consists of three 240 x 220 mm glulam columns, bolted together as they rise vertically and branching out at just under two metres above ground level in a gentle radiused curve to support the CLT roof deck above. The three columns are bolted together at their bases to limit their intrusion into the dining room space. This geometry induced in-plane forces in the CLT roof deck which were simply transferred and resolved through the perimeter racking walls.

At the curved perimeter of the building the CLT roof slabs are supported by glulam beams with radiate from the main dining hall space and at the main entrance.

Sustainability

This phased project extends, re-uses and refurbishes the school’s existing building, reducing its overall embedded carbon footprint and avoiding unnecessary demolition.

The CLT and glulam subcontractor, KLH-UK, provided PEFC certified spruce sustainably sourced from Austria, together with a rigorous system of design and analysis to minimise wastage, both in terms of material and time through offsite construction. Sustainably sourced timber was the primary construction material, providing efficiency and low wastage. The use of CLT and offsite construction also minimised disruption to the school which had to be continuously occupied during construction.

A new planting scheme provides valuable green spaces including a green roof, and the introduction of new trees has been designed to give colour, texture, sensory stimulation and seasonal variation to the space, while enhancing the surrounding biodiversity.

Awards

Winner, RIBA Regional Award, 2018
Shortlisted, Wood Awards, 2018, Education and Public Sector
Winner, AJ Retrofit Awards, School Project of the Year
Shortlisted, Structural Timber Awards, Education Project of the Year
Commendation, New London Awards (NLA Education category

Completion date:

March 2018

Location:

London

Architect:

Cottrell and Vermeulen
Architecture

Structural Engineer:

Engineers HRW

Main contractor:

Rooff Ltd

Timber supplier and installer:

KLH-UK

Timber elements:

structure, walls, roof

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