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Great Glasshouse by Foster + Partners

Foster + Partners’ Great Glasshouse on the rolling hills near Camarthen is the largest single-span greenhouse in the world and is a breath of fresh air for modern architecture in Wales

The Great Glasshouse, located well off the beaten track in south-west Wales, must be Foster and Partners’ least talked-about public building, and one of the lesser-known beneficiaries of the Millennium Commission. The centrepiece of the new National Botanic Garden of Wales, it was finally opened to the public at the end of May - only shortly before the very much more high-profile Millennium Bridge in London, now the subject of some embarrassment. In contrast, the glasshouse seems to have been received with quiet satisfaction in Wales, despite considerable local opposition to the project at the outset. Not only does it represent a key resource in the new institution’s research programme, headed by director Professor Charles Stirton (formerly deputy director at Kew), but it also represents a significant new ‘visitor attraction’ in this part of the country with the enormous virtue of offering shelter from the wet and misty Welsh weather. It is hoped and expected that it will give a welcome boost to the local farming and holiday cottage economy.

 

Original opposition was based around two issues: the increase in traffic to the surrounding area, delivered via a new exit to the A48/M4 from Swansea; and the scale and character of the intervention in the landscape. From afar, the armadillo-shaped glasshouse is distinctly visible, but not immediately recognisable. The reflective quality of the surface means it is easily mistaken for a large patch of agricultural polythene sheeting; while on a sunny day, the glare of light off the glass suggests some form of communications installation. It certainly does not blend into the landscape of gently rolling, small-scale green fields, woodland, rivers, and the odd craggy castle, despite the architect’s suggestion that the elliptical, rounded form was inspired by the local topology.

 

On the other hand, the glasshouse represents the focal point of an ambitious and, it seems, successful initiative to restore, and bring into public use, hundreds of hectares of land on the decayed Middleton Estate. Redeveloped by one William Paxton at the end of the eighteenth century, with a mansion by S P Cockerell and an extensive series of lakes and ornamental water features inspired by the Mughal gardens, the estate had fallen into decay by the 1930s. The house was gutted by fire, the lakes drained, and the land bought by Carmarthen County Council, which divided it up into seven small farms. In the mid 1980s a painter and former exhibitions officer in the Greater London Council, William Wilkins, resolved to restore the lost gardens. He established a trust which persuaded the council to hand over the site on a 999-year lease, and the Millennium Commission to make a grant (in 1998) of £21. 7 million towards the overall project cost of £43. 6 million.

 

Considering Wales’s poor track record on commissioning new architecture, and its ambiguous attitude towards contemporary design, the realisation of the Foster glasshouse seems a remarkable, even mysterious achievement. One that seems to have gone almost unnoticed amid the Hadid Opera House fiasco, and the indecision over the Rogers Assembly Building.

 

This may be largely because the commission was awarded to Foster’s office on the basis of a competitive tender interview rather than design proposals. But it is also, perhaps, due to the obvious significance of the new institution in Welsh national life under devolution, its out-of-the-way location, and appeal to a wide cross-section of the public.

 

The distinctive shape and size of the structure grew directly out of the existing site conditions, notably a shallow oval platter of land that had been created in front of the old Cockerell house.

 

The site sits at the top of a gentle hill, slightly offaxis with the long approach path leading up from the car park and visitors’ entrance - a circular, timber ‘gatehouse’ also designed by Foster’s office, and described as ‘based on the shape of an ancient Welsh roundhouse’. From this approach, the glasshouse has surprisingly low visibility, and certainly does not dominate the terrain. The glass membrane seems merely to cap the top of the grassy mound out of which it rises.

 

The visual horizontality and simplicity of the structure establishes a distinct contrast with the characteristic loftiness and ornamentation of the historical, and more recently built, glasshouses known to the public. Foster’s Spencer de Grey describes it as the ‘antithesis’ of the Princess of Wales glasshouse at Kew, for example, and proffers a comparison with Foster’s Stansted Airport building instead. Both the glasshouse and Stansted were primarily designed to create a calm, neutral backdrop for their contents which interacts with the surrounding landscape, rather than to be attention-grabbing structures in their own right. But there is an undeniably spectacular element to both, particularly so the glasshouse, where much has been made of its status as ‘the largest single-span greenhouse in the world’.

 

Spencer de Grey thinks that ‘the fact that it is unobstructed is important’. Not only does it allow flexibility within the layout of the internal area, but it also generates a striking spatial effect from within. In addition, the uninterrupted curved surface of the glass envelope, supported on a tilted ringbeam, creates a good solar collector, orientated towards the south-west. On the north-east side, the building is less exposed, with an enclosed perimeter zone of exhibition space, services and circulation. Of the 4m x 1. 5m panes of glass, 150 are openable, creating an airflow through the building which not only ventilates it but also provides the necessary wind-force to encourage normal plant growth. Heating is generated from local timber in an adjacent boiler house and rainwater is collected off the surface of the glasshouse and stored for watering.

 

One of the implications of creating an unobstructed internal space was that the initial idea of zoning the glasshouse into distinct climatic areas was eventually abandoned. Professor Stirton preferred to focus on a single integrated climatic environment. The building is dedicated to Mediterranean plant species, taken from the five Mediterranean regions of the world, partly because this has become the most threatened of the natural habitats, but also because it is the environment which is naturally created under glass. It also, suggests de Grey, generates a pleasing contrast to the rich, lush character of the Welsh countryside, which a tropical display would not have done.

 

The planting is organised in terms of form, density and colour, rather than by species, which, as the designers for the internal landscaping point out, is ‘a strategy unused in other botanic gardens’ and establishes the innovative nature of the Great Glasshouse in terms of botanical interest and research. At the moment, due to the immaturity of the planting, it is difficult to appreciate, and the extensive excavation and re-shaping of the ground seems to be of more note. The result is an almost theatrical landscape of ravine, waterfall, terraces, and bridges, with a long ramp descending from the entrance to the lowest point by the water.

 

Landscape architect Gustafson Porter describes the strategy in terms of ‘multiple horizons and overlapping routes’ which ‘begin to stretch the scale of the space’ under the sky of the glass dome above.

 

The Great Glasshouse sits within an overall landscape strategy conceived by Hal Moggridge, which has created a coherent network of the various natural and manmade features and the buildings on the site - including an Aqualab on the Garden Lake designed by the Welsh School of Architecture. But much of the programme of works, including the restoration of the great double-walled garden, has yet to be carried out. The glasshouse is not necessarily the most important element of the new National Botanical Garden as an institution, but there seems little doubt that it will provide a strong visual symbol for the place that will be invaluable in the process of raising funds and support in the years to come.

 

STRUCTURE

 

Built on the crest of the hill on the site of the original Middleton Hall, the Great Glasshouse contains a temperate environment to exhibit plants from Mediterranean regions. The 110m by 55m oval ring raft foundation is constructed in a combination of in-situ and precast concrete bearing on the boulder clay and containing two storey plantrooms and escape tunnels.

 

The roof structure is an oval glazed dome, slightly tipped towards the south-west sitting directly on to the in-situ concrete ring beam which caps the substructure, see figure one.

 

After an extensive study into the various geometric forms that might be suitable to achieve the doubly-curved oval shape of the dome, a geometry based on a torus and the plane cuts of a torus was developed.

 

The torus is a tubular surface created by the rotation of a sphere, radius (r) about an axis (y) with a radius (R) in an x, y, z-co-ordinate system as shown in figure two.

 

The z-axis of the generating torus is tipped towards the south-west by approximately 7 degrees . The size of the glasshouse and the joint between the concrete ring beam and the steel glass structure are determined by plane cuts of the torus as shown in figure three The primary roof member takes the form of a 323m diameter, single, curved, tubular hollow section arch up to 58m long, spanning across the toroid at 4m centres. The arches follow the precise geometry of the toroid, with a constant radius of curvature in every member.

 

While a simple concept, the design of the arches required advanced non-linear buckling analysis to ensure that an adequate safety margin is maintained against global buckling. The tipping of the dome to one side also induces significant bending moments in the tubes and additional stiffness was gained by forming the T-section cladding supports and CHS into a composite section.

 

The secondary structure, which braces the arches against lateral buckling, is straight, 140mm diameter CHSs at 7. 5m centres. These steels run from the ring beam at either end of the roof, up towards the centre line, where an expansion joint is incorporated to prevent the build up of excessive stresses caused by operating in temperature extremes.

 

The steelwork was erected from either end with the arches arriving on site in several pieces. Once erected on trestles they were welded together on site. The end detail is a ball and socket connection machined from stainless steel, allowing the arches to drop into place regardless of the varying angle of incidence between the steel and the concrete ring beam. Under extreme wind loads the arches at the ends experience uplift forces which are resisted by an additional capping piece, bolted down to the socket/base plate.

 

The glazing system is aluminium and is free to move relative to the steelwork to accommodate differential expansion.

 

Costs

 

Costs based on final account

 

SUBSTRUCTURE

 

SUBSTRUCTURE £459. 33/m2 Substructures, concrete perimeter framework comprising precast slabs with in-situ concrete covering forming perimeter roof with waterproof membrane covering with seeded topsoil over. Basement tunnel to southern perimeter. Drainage

 

SUPERSTRUCTURE

 

FRAME £137. 76/m2 Structural steel frame comprising curved circular hollow sections (longest span 55m) with circular hollow sections secondary-bracing steelwork

ROOF £461. 54/m 24,500m2 of single-glazed laminated glass including secondary aluminium framing fall-arrest system and computer-controlled opening lights

PERIMETER GUTTER £29. 78/m2

EXTERNAL DOORS £11. 71/m2 Sliding doors and screens, non-framed electricallyoperated locks

INTERNAL BUILDING WORK £95. 05/m2 Blockwork partition walls to basement plant rooms. Glazed blockwork walls to WCs. Full-height metal faced doors and frames. Slate floor to concrete on screed, vinyl and ceramic floors to staff rooms in basement. Through-colour rendered wall finish to main concourse wall. Sanitary fittings to WCs. Stainless steel litter and recycling bins

INTERNAL PERIMETER CILL £6. 07/m2 Rendered internal cill at base of glazing system

HANDRAILS AND GRILLES £5. 52/m2 Handrails, grilles to form plenums to sloping sides of entrances. Grilles to perimeter heating trench, entrance grilles, entrance mats

 

SERVICES

 

MECHANICAL SERVICES £52. 15/m2 Perimeter heating system by conventionally- and biomass-powered boilers (located elsewhere) fed by underground insulated heating pipes. Localised extract fans with tempered air to bioverse areas. Hot and cold water services to WCs and mess area in basement. Waste, soil and vent pipework

ELECTRICAL SERVICES £61. 58/m2 Power, lighting, emergency lighting, fire alarms or basement, concourse, WCs and bioverse areas

CLIMATE CONTROL £15. 49/m2 Computer-controlled internal climate control, controlling temperature of heating installation and the number of, and opening of, lights in glazed roof

 

INTERNAL LANDSCAPING

 

CIVILS £65. 83/m2 Excavation, ravine with stone gabion walls, reinforced blockwork retaining walls, stone paths

STONE WALLS £65. 65/m2 Natural stone walls and features forming terraces, ‘wet’ walls, water seepage walls BRIDGES AND HANDRAILS £11. 05/m2 Two steel bridges with hardwood treads

MECHANICAL SERVICES TO WATER FEATURES £10. 74/m2 Water circulation pipework and outlets to waterfall and water seepage wall

ELECTRICAL SERVICES £12. 41/m2 Path lighting and emergency lighting. Public sound system

PLANTS AND PLANTING (BY CLIENT) £23. 90/m2 Special imported and mixed soils. Imported plants, shrubs and trees. Special gravels

 

PRELIMINARIES AND INSURANCES

 

PRELIMINARIES, OVERHEADS AND PROFIT £124. 54/m2 Apportionment of construction manager’s direct preliminaries, CM staff costs, fee

 

Cost summary

 

SUBSTRUCTURE 459. 33 27. 84

 

SUPERSTRUCTURE

 

Frame 137. 76 8. 35

Roof 461. 54 27. 97

Perimeter gutter 29. 78 1. 80

External doors 11. 71 0. 71

Internal building work 95. 05 5. 76

Internal perimeter cill 6. 07 0. 37

Handrail and grilles to entrance 5. 52 0. 33

 

Group element total 747. 43 45. 29

 

SERVICES

 

Mechanical services 52. 16 3. 16

Electrical services 61. 58 3. 73

Climate control 15. 49 0. 94

 

Group element total 129. 22 7. 83

 

INTERNAL LANDSCAPING

 

Civils 65. 83 3. 99

Stone walls 65. 65 3. 98

Bridges and handrails 11. 06 0. 67

Mechanical services to water features 10. 74 0. 65

Electrical services 12. 41 0. 75

Plants and planting (by client) 23. 90 1. 45

 

Group element total 189. 98 11. 49

 

PRELIMINARIES 124. 54 7. 55

 

TOTAL 1,525. 50 100. 00

 

Costs supplied by John Evans, Symonds Group

 

CREDITS

 

TENDER DATE Various, commencing May 1997, final September 1998

START ON SITE DATE July 1997

CONTRACT DURATION Six months, July 1997 - December 1998 (shell) Seven months, September 1998 - March 1999 (interior landscape)

GROSS EXTERNAL FLOOR AREA 5,439m2

AREA OF GLASS 4,500m2

FORM OF CONTRACT Construction Management

TOTAL COST £8,940,800

CLIENT National Botanic Gardens of Wales

ARCHITECT Foster and Partners: Norman Foster, Spencer de Grey, Max Neal, Nigel Curry, Oliver Wong, Jason McColl

LANDSCAPE ARCHITECT Gustafson Porter

LANDSCAPE ARCHITECT (GARDENS) Colvin and Moggridge

QUANTITY SURVEYOR Symonds Group

STRUCTURAL ENGINEER Anthony Hunt Associates

MECHANICAL AND ELECTRICAL ENGINEER Max Fordham and partners

CONSTRUCTION AND PROJECT MANAGER Schal

CONTRACTORS AND SUBCONTRACTORS excavation Jones Brothers; concrete structure Byrne Brothers; steelwork Watson Steel; roof Glazing Metallbau Fruh; general builders’ work Cowlin Construction; entrance doors T mechanical services FP Hurley electrical services Hyder Electrical; site wide BMS Satchwell Control Systems; environmental control climate controls Sewage Treatment Watershed Systems; external metalwork Dafen Fabrications; external works Alun Griffiths; landscape Masonry Vetter UK; internal metalwork RJP Engineering

SUPPLIERS AND SUBCONTRACTORS precast concrete Histon Concrete Products; laminated glass Bruder Eckelt; roof vent actuators Beikirch Industrieelektronik; slate McAlpine render Webber and Broutin UK; metal doors Accent Hasen; timber doors Jutlandia Doors, ironmongery Allgood Hardware; glazed bricks Forticrete; WC cubicle doors Detlef Muller; WC accessories FC Frost; slate paving Ffestiniog Welsh Slate; entrance matting Jaymart Rubber and Plastics; metal gratings Lichgitter UK; common blockwork Tarmac Render Weber and Broutin

 

WEBLINKS

 

National Botanic Garden of Wales www.gardenofwales.org.uk

Foster and Partners www.fosterandpartners.com

Anthony Hunt Associates: www.anthonyhuntassociates.co.uk

Symonds www.symonds-group.com

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