Abbey Wood Railway Station, Abbey Wood, Bexley, South London

Introduction

Walking into the new railway station at Abbey Wood, the immediate impression is of the elegant timber roof which arches over the interior, imparting a warm, natural quality to this large public space – more than 25 metres by 25 metres. It is designed to allow large numbers of travellers to move easily and quickly and the direction of travel is clear; ticket office, toilets and admin spaces are aligned into side spaces while directly ahead a full-height glass façade gives a clear view of the destination – the tracks and the platforms. These are at the lower ground level and reached by lifts and staircases which descend at each side of the glass façade.

Designed by Fereday Pollard Architects as lead architects in a multi-disciplinary team, Abbey Wood station is a unique and complex structure which reflects its new importance in the railway network. Until recently a small suburban railway station on the Southeastern Line to north Kent, Abbey Wood has now become the terminus of the new Elizabeth Line, the south east spur of the Crossrail project. The new platforms serve the two new Elizabeth Line tracks which terminate here, plus existing Southeastern/GTR tracks which continue through into Kent. When the Elizabeth Line opens, the plan is for trains to run every five minutes with journey times to central London, Paddington, Heathrow and Reading reduced by half. This prospect has already attracted new development to the area which until recently lacked regeneration potential; the nearby Thamesmead Estate, for instance, is now earmarked for a £1 billion refurbishment.

The station building sits directly over four railway tracks with its entrance at the upper level, facing a generous pedestrian concourse. External staircases and lifts lead down to local shops and streets at ground level. On the far side, the concourse is connected directly to an existing elevated flyover, the Harrow Manorway, with bus stops at each side. The public spaces around the lower level of the station have been improved and upgraded to prioritise pedestrians and cyclists and to reinforce the existing ‘village’ character.

The timber roof of the station arches in a gentle curve, created by a structure of curved glulam beams supporting a curved cross-laminated timber (CLT) deck. Glulam was chosen for the roof structure as, the architect explains, ‘it was the most appropriate material for the station’s distinctive architectural form, conceived to express the flow and direction of passenger movement, helping to provide intuitive way-finding and a sense of arrival at a destination. We specified larch for the glulam for its visual character and durability, a species that would bring a sustainable natural element appropriate to this residential location.’

The timber roof

The roof consists of four single-curved primary support glulam beams, each 900mm x 180mm and 45 metres long which run from entrance to rear, supported by cruciform steel columns clad with larch. The four beams are not spaced equidistantly across the roof but set in pairs, spaced 4.45 metres apart at each side of the ticket hall and directly above the two staircases which lead down to the platforms and tracks. At this point the beams curve downwards, expressing the direction of travel and acting as dramatic but effective shelters to the staircases; they are supported at platform level by pairs of canted steel columns.

A series of single and double-curved 610mm deep glulam beams run between the four primary beams. They extend at each side of the building, projecting beyond the walls in a gently tapering soffit, clad with zinc sheet to match the roof.

Tertiary glulam support members are fixed between them, forming a support grid for an exposed layer of load-bearing cross-laminated (CLT) panels. This in turn supports an insulated zinc roof with standing seams.

The beams are connected to each other at the upper edges by concealed purpose-made brackets.

The glulam beams were prefabricated to their exact curvature using advanced CNC machinery in the highly automated facility of the fabricator, Wiehag, in Altheim, Austria.

All exposed glulam members are of ‘visual quality’ larch, with spruce used over the cellular side accommodation where the beams are not visible. Likewise, the lowest layers of the CLT three-layer panels above the beams are of ‘visual quality’ larch with upper layers of spruce. Acoustic panels of larch slats are located within the glulam grid.

The structural implications of the roof shape, its deep overhangs and its position on the supporting structure were investigated by Tony Gee and Partners, as Martin Allnutt, structural engineer and associate director, explains: ‘A computational fluid dynamics study of the roof wind load was carried out as the structure is not a regular shape and hence is not covered by the relevant design codes in determining pressure coefficients and zones of application. Model wind tunnel testing was suggested but was considered to be uneconomic in view of the small potential member cost savings compared to assuming potentially slightly conservative pressure coefficients by incorporating a relatively small dynamic augmentation factor.

The roof is supported by a structural steel frame of beams, columns and tie rods which also provide horizontal and vertical bracing around the north and south perimeters and plan bracing of the glulam roof. The centre section of the steel frame is supported on a precast concrete concourse structure spanning over the railway tracks. The northern and southern areas of the frame are supported on the concourse slab, constructed using in-situ concrete as it is separated from the running lines by substantial impact shear walls at track level, thus allowing the use of falsework.’

The installation of the timber roof and its steel support structure – on a compact site directly over live tracks – was complex. The roof was erected in several stages; the central section was erected first and required significant temporary bracing as the structure was only stable when complete. Some elements, such as the glulam roof structures over the staircases to the platforms, were prefabricated in larger sections so that they could be lifted over the track and installed during a weekend ‘possession window’, when no trains were running.

The external timber cladding

The external walls of the station are curved in plan and flanked by broad public staircases which lead to the upper level concourse. To accommodate these curves and slopes while maintaining visual qualities of clarity and simplicity, the façade is clad with chamfered brickwork at lower levels and with a timber rainscreen at upper levels. The aim, as the architect explains, ‘was to form a lighter timber fin façade screen above the more solid brick clad base of the station. The concept was to unify and modulate these facades which form the formal civic faces of the building to the adjacent neighbourhoods. The cladding configuration was intended to express movement and reinforce a sense of place.’

After considerable research, thermally modified fraké (Terminilia superba) was chosen for the fins, a sustainable hardwood which is light, easy to work, straight and durable. The rainscreen consists of vertical 40mm fins of fraké set 75mm apart and fixed to a steel sub-frame. The fins also act as a visual screen to upper walls and high-level glazing in the ticket hall. Above the staircases the fins are cut at their bases to follow the diagonal slope of staircase and balustrade below.

Awards

Offsite Timber Construction Awards 2017 – Best Use of Timber
Structural Timber Awards 2018 – Contractor of the Year

Completion Date:

October 2017 (Crossrail not yet open)

Building Type:

Railway station

Location:

Abbey Wood, Bexley, South London

Architect:

Fereday Pollard Architects

Structural Engineer:

Tony Gee and Partners LLP

Main Contractor:

Balfour Beatty Rail Ltd

Timber Supplier:

Wiehag GmbH

Timber Elements:

Roof structure and lining, external wall cladding

Timber Species:

PEFC-certified larch and Austrian spruce, FSC-certified fraké (Terminalia superba)

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