Woodland Trust Headquarters, Grantham, Lincolnshire

Introduction

The Woodland Trust is a charity devoted to the conservation of the UK’s woodland. Its new headquarters in Grantham, by Feilden Clegg Bradley Studios, aims to communicate the mission and values of the Trust and, in practical terms, to accommodate 200 workstations in open plan office space. As the headquarters of a charity, the building also had to demonstrate value for money and frugal use of funds.

 

Building description

In plan the building is like an ascending spiral with flattened edges which rises seamlessly, enclosing a woodland garden at its centre. The highest portion is a three storey, 15 metre wide block orientated north-south which houses the open plan office space. From this a wing of service and cellular space descends along the western boundary, terminating in a single-storey bike store, the final enclosure. The woodland garden is an amenity space for staff and the arrival space for visitors, the intention being that a ‘woodland experience’ should mark the entrance to the headquarters. From the garden, visitors enter a reception space and the staff breakout and refectory. All floors are connected to this space by a rooflit atrium and the main staircase. The tapered wing contains meeting rooms, a training room and service facilities serving the Trust’s operational needs.

 

Timber construction

The building is constructed of cross-laminated timber panels. The design team prepared a detailed matrix comparing a timber structure to that of steel and concrete structures, which demonstrated that as well as the environmental benefits of timber (reduced embodied energy compared to steel or concrete), there were also advantages of reduced time on site, neat detailing, and lighter foundations. Once the decision was taken to use cross-laminated timber for the main structure, it soon became logical to use it elsewhere in the building, including the lift shaft, stairs and balustrades.

 

Heating, cooling and thermal mass

The team wanted to take advantage of the lower running costs and environmental impact of a naturally ventilated building coupled with night-time cooling. But the latter requires large areas of exposed thermal mass, such as exposed concrete ceiling soffits; these perform particularly well by absorbing rising heat at high level and their large surface area provides good radiant cooling to people below. But they require a strong building frame to support them, ie of steel or concrete.

The design team considered this paradox; a design brief that alluded to timber, but an environmental strategy that appeared to require concrete. It was the starting point for research that produced an innovative solution: the walls, floor and roof of the building would be of cross-laminated timber panels, and concrete panels would then be fixed to the ceiling soffits. The concrete panels act structurally as a composite with the timber panels and provide the required thermal mass. The timber panel construction forms a box-like structure with floors spanning from one side to the other. There is no moment connection between the floors and the walls. Shear walls resist the lateral action of wind.

The concrete soffits were designed as an array of rectangular panels which became known as ‘concrete radiators’. The panels are ribbed to achieve a depth of 250mm, the structural depth required to act as a composite with the cross-laminated timber, and are shear fixed to the underside of the timber. Each panel is 2m wide and sits within a 2.4m wide cross-laminated timber panel.

In developing this solution an analysis of carbon dioxide generated by the structure was undertaken and an assessment made of savings generated compared to a conventional concrete solution. By using more timber and less concrete, the building will save over 400 tonnes of carbon dioxide from entering the atmosphere when compared with an entirely concrete structure, and the timber fabric of the building will sequester over 500 tonnes of carbon dioxide. The sum of this is equivalent to approximately 20 years of carbon dioxide emissions from the occupation of the building.

 

Environmental design

The environmental design of the building evolved from the extensive combined experience of the team in designing low energy workplaces, including lessons learned from post occupancy studies on previous buildings.

The building form is designed to allow straightforward self-shading and to maximise northlight. The relatively shallow 15 metre wide plan depth of the main office floorplate and the 4 metre floor-to-floor heights allow an exemplary level of daylight, maintaining an average daylight factor of approximately 4%. This is improved by the verticality and even distribution of the glazing and the addition of light shelves to the south façade.

The ventilation strategy for the building relies on natural ventilation through opening windows at high and low level. The majority of the high level windows are controlled by the BMS, with the low level windows mainly manually controlled.

The building achieves an airtightness of 2.4m3/m2/hr at 50Pa, which is less than 25% of the current level required by Building Regulations. The main airtightness line on the building is the cross laminated timber structure. All external vertical joints are taped. All interfaces with the cross laminated structure are sealed with EPDM flashings. The project has achieved an ‘Excellent’ rating for BREEAM Offices 2006.  At the design stage, the team assessed the potential for the generation of renewable energy and concluded that the most beneficial energy investment would be to consider the energy use of the server room, and to incorporate efficient blade servers and free cooling techniques. The IT strategy for the building is to have all computing power in centralised servers with low power terminals in the office space rather than PCs. This has the advantage that the heat gains are concentrated in one place and can be removed efficiently by cooling units installed as close as possible to the servers, between the server racks.

The 11.5 degree south-facing roof of the building is designed to allow the potential for future installation of solar water heating and photo-voltaics. Low water-use sanitary fittings are installed in all of the toilets.

 

External design

The elevations are inspired by the sense of space and verticality of woodlands. The windows to the north and south elevation are three-storey vertical strips, maximising the opportunity for daylight and high level vents and interspersed with vertical strips of timber cladding. This comprises 150mm wide vertical boards of untreated Scottish larch.

Walls and roof are insulated with a mulched natural wood-fibre product which is screw fixed to the cross laminated timber. The construction of external walls and roof is designed to be breathable, with the cross laminated timber as the vapour check layer. The sheer depth and density of the material negates the need for any additional vapour membrane. The construction works on the basis that moisture will naturally travel from a high density material (vapour tight) to low density materials (vapour permeable) and the building fabric has lower density materials to the outside (sarking board) promoting a natural migration of vapour from the inside to the outside.

 

Interior design

The cross laminated timber is exposed on perimeter walls, soffits, tea points and the main staircase. The timber is coated with a translucent three-coat lacquer to meet surface spread of flame requirements. This whitening effect allows the quality and grain of the timber to be perceived, without the yellowing (which would otherwise develop over time) of the natural spruce finish.

A slatted timber screen separates the main entrance, reception and visitor waiting areas from the offices. The atria above, which link other parts of the buildings, are lined in ash slats, which extend the concept of verticality in the timber-clad elevations. Bespoke furniture in the main entrance is fabricated from solid ash; the reception desk is constructed from a single ash tree and is reminiscent of the stacking of drying timber.

The main staircase is constructed entirely from cross-laminated timber and is self-supported by three timber panels spanning from ground to second floor. Its risers and treads are braced between these panels so that no additional structure and steelwork is required. The panels provide an element of containment and solidity but are limited to the risers, allowing landing and half landings to remain largely open and maintain views and connections between stair and office. Timber slats support the balustrades and continue the language of verticality. A timber stringer winds to the second floor as a continuous ribbon, becoming the balustrade at the top.

 

Procurement/Costs

The original brief demanded a highly innovative and sustainable building within a market rate budget. A benchmark exercise was undertaken to provide the Woodland Trust with confidence that the use of the cross laminated timber and concrete panels for natural ventilation was not inflating the overall cost of the project. This reviewed a number of different building shapes, along with the use of a more traditional construction of concrete and/or steel versus the cross-laminated timber. The results justified the shape of the building and the natural ventilation solution, showing that the cost per square foot was acceptable, and was only marginally higher than a concrete frame building of this type; £147/ft2 as opposed to £144/ft2, at the time of tender.

The building is innovative as it uses ‘standard’ materials in an inventive way to reach an affordable solution to achieving thermal mass in a new office building. Rather than expending budget on innovative technologies and materials, the solution sought to use standard materials in a new way. It has demonstrated that sustainable solutions can be truly affordable.

Completion Date:

October 2010

Year Published:

September 2011

Building Type:

Office

Location:

Grantham, Lincolnshire

Architect:

Feilden Clegg Bradley Studios

Services Engineer and BREEAM Assessor:

Max Fordham

Structural Engineer:

Atelier One

Project Manager:

Buro Four

Main Contractor:

Bowmer & Kirkland Building Services

Joinery Sub Contractor:

B&K Timber Structures

Timber Structure Supplier:

KLH Austria

Timber Cladding Supplier:

Russwood

Timber Element(s):

Structure, insulation, external and internal cladding, furniture

Timber Specie(s):

Scottish larch, spruce, ash

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