Brockholes, Brockholes Wetland and Woodland Nature Reserve, Lancashire

Introduction

The new visitor and education centre at Brockholes Wetland and Woodland Nature Reserve near Preston is set on a floating ‘island’ on a lake. The design, by Adam Khan Architects, won an RIBA competition and was inspired by the now-destroyed magical landscape of the Marsh Arabs in Southern Iraq, recorded by the anthropologist Wilfred Thesiger in the 1950s. Thesiger photographed the floating islands, houses and platforms, all built out of the indigenous material of the landscape – reeds.

 

Building description

Brockholes is a 67-hectare nature reserve, flanked by an escarpment of ancient mixed woodland and containing a large lake, formed from the excavated pits of what was once a gravel quarry. The river Ribble runs alongside the lake and regular flooding is a problem - the river, it has been estimated, could rise up to four metres. The obvious solution would be to build the visitor centre above flood level, but to Khan, this would be to deny visitors the important experience of being next to the water and among the reeds. So he has set the visitor and education centre buildings on a 2.25 metre deep pontoon, constructed of concrete cast around large polystyrene void formers, to create a solid yet buoyant raft foundation. The pontoon is kept anchored in place by four piles embedded in the lake bed and is reached by means of articulating bridges, which allow the pontoon to rise in flood conditions: it is a concept which could potentially be applied to other buildings constructed on flood plains.

The centre itself is made up of a cluster of single-storey timber-framed and oak clad buildings; they range in scale from large-scale bustling public areas - a café, a hall for conferences and a shop - to quieter, small-scale more reflective areas for learning about and viewing the surrounding wildlife habitats. The pontoon not only avoids floods, it allows both internal and external spaces to be set closer to the water’s edge than would ever be possible for a land-rooted building, allowing unique views across the water; from the café windows it is possible to see ducks gliding alongside.

The buildings are grouped as separate elements on the pontoon but to Khan, the spaces between are just as important; their inspiration comes from the piazza - the classic urban space in an Italian city - and similarly each external space on the island has a different character – a market square, a social space and an activity space.

 

Construction

Timber was the obvious and most appropriate choice of material for the project, not only for its visual qualities but also for its lightness of weight and sustainability.

All the buildings differ in height and size but have a consistent horizontal eaves line; it is relatively low and designed to correspond to the height of the reeds which will eventually surround the centre. The roofs, which rise up to 10 metres in height, are steeply pitched with hipped ends and clad with oak shakes. The walls are clad with vertical oak boards and are glazed with full-height double-glazed fixed units and sliding doors.

The basic structure - both walls and roof - of each building consists of a series of glulam portal frames covered with a skin of structural insulated panels (SIP). The glulam frame gives stability; the structural insulated panels (SIPs) provide racking resistance to the structure while also giving a high level of insulation and air tightness.

The glulam columns are vertical but at the top of each column the glulam rafters splay out in a V-shape, creating a geometric pattern that belies the complexity of the connection needed to achieve it. Building stability is achieved by stiff connections at the apexes and eaves, where large forces are transferred from the rafters to the columns. Rather than using finger splices to connect the timbers, as can be done in simpler multi-timber joints, the two rafters are connected to the column head using a steel flitch plate, allowing the 8.5 degree splay and 30 degree tilt in each rafter to be achieved. The back of the flitch plate is also connected to a steel ring-beam that distributes the high horizontal forces to the sides of the building. The three-piece connection detail is mirrored at the top of the roof where the two rafters meet a single cross-piece.

The glulam elements were sustainably sourced in Austria, where they were precision engineered to reduce on-site time and eliminate wastage.

The SIPs consist of a sandwich of OSB and an inner expanded foam core; they cover the glulam frame and provide stiffness to the structure using stress-skin theory. The panels act together in the roof plane to form a large 'beam' where the glulam rafters prevent out-of-plane deflection of the roof while the SIPs resist the in-plane racking forces. Although this theory cannot be applied for the long-term dead loads of the building due to creep in the insulation layer in the panels, it works effectively to resist the shorter-term high wind forces. Glulam V-bracing has also been used between the columns to provide stability to the buildings below the eaves level.

The roofs are clad in oak shakes - rough tiles formed from tree stumps that would otherwise have been discarded and which will weather in time to a silver. Internally, recycled newspaper insulation was sprayed on to the underside of the SIPs, providing an excellent low-cost and sustainable acoustic dampening in public areas.

 

Sustainability

The project aimed for zero-carbon both in use and production, using materials of low embodied energy, high levels of thermal insulation and building airtightness, natural ventilation, and off-site prefabrication as well as on-site energy generation and waste treatment. The concrete for the pontoon used cement incorporating 31per cent recycled fly ash – waste products from industry - and the foam is BRE-rated A (although this may seem a surprising rating for a petro-chemical product to achieve, the truth is that polystyrene is 98% air). The sourcing and durability of the materials used, potential for recycling, and the distance to site were all considered within the design. The highly-engineered timber design of the project, together with the reduction of on-site material wastage, site time and embedded carbon cost, contributed to achieving a BREEAM ‘Outstanding’ rating at design stage; the building is currently going through post-construction assessment but is on course to be one of the first buildings in the country to achieve this rating on completion.

Completion Date:

May 2011

Year Published:

April 2012

Building Type:

Visitor centre

Location:

Brockholes Wetland and Woodland Nature Reserve, near Preston

Architect:

Adam Khan Architects

Structural Engineer:

Price and Myers

Services Engineer:

Max Fordham

Project Manager:

Bovis Lend Lease & WDA

Main Contractor(s):

Balfour Beatty Construction and Mansell Construction Services

Timber Supplier:

B & K Structures

Timber Element(s):

Frame, roof, wall and cladding

Timber Specie(s):

European whitewood, British oak shakes, larch battens, OSB structural insulated panels (SIPs)

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