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Special report: steel

The complex geometry of a millennium greenhouse relied on the design team's cohesion as much as the steel fixings

I am pleased to say that my original draft, provisionally entitled, 'Another millennium scheme goes belly up', has been put on hold. In fact, in a remarkable turn around, worthy of a B-movie cliff-hanger, the latest in a long line of terminally ill millennium projects has lived to fight another day. After a hair-tearing, threeyear process of stop-start design and construction, the scheme is about to restart on the final leg to completion (though, given the turbulent history of this project, I don't want to speak too soon).

Project Suzy, a scheme earmarked for the ironically-titled Nature's World in Middlesbrough, is an ubiquitous millennium ecostructure and hydroponicum (a word unknown at the start of the project, but since made famous by Charlie Dimmock) - that is, a glorified greenhouse where plants are grown without soil. This ecostructure is impressively translucent, with a transparent roof and glazed curtain walling supported off a complex, geometric, light steel frame.

The scheme is billed as a visitor attraction, although Henry Aitchison, the client's agent who has manfully succeeded in keeping the scheme alive, suggests that it must distance itself from relying on visitor numbers as the primary source of revenue to avoid the problems encountered by similar Earth Centre-style projects. As with many millennium schemes where the recipient of the grant - the client - was a small-scale, ad hoc enterprise, the rigours of high finance proved to be too onerous. In May, with only four weeks to go on the scheme, the matched funding ran out (without the client's agent or the design team's knowledge).

Unfortunately, the scheme could not be allowed to continue in the knowledge that there were inadequate funds to pay the contractor. The contract was suspended. However, because of the remarkable rapport built up through adversity, the contractor agreed to continue until the building was effectively watertight. At the time of going to press additional funds have been pledged and it is hoped that the scheme will now be reconvened and completed by the end of the year.

The deceptively complex design, by Kevin Drayton of architect One17AD of Huddersfield, comprises a concre te perimeter retaining structure - to facilitate earth sheltering; banking up the as-dug material around 60 per cent of the circumference - and a steel-frame support structure carrying translucent inflatable roofing. The building is set into the earth to a depth of 12m or so, and the earth acts as a heat sink, which is factored into the engineering calculations. The south-facing frontage has 8m full-height glazing, with opening lights to facilitate breezes for the plants and ventilation for the public. A small lake butting up to the glazing (to encourage cooling airflows through the building, an integral part of the M&E design) has been put on hold.

On arrival, the visitor will pass through a maze, designed to optimise the internal ground floor area, and then through into a 'future world', examining forthcoming sustainable technologies.

This area is a windowless internal space leading to a public holding bay which bursts out through a waterfall and fronds into a fully-daylit double-height space; the building has tremendous potential for drama.

Notwithstanding the stabilising effect of the concrete mezzanine floor, the steelwork has to fulfil the function of internal bracing as well as acting as support for the lightweight roof and carrying the services at high level. The steelwork circular hollow section columns spring from the concrete mezzanine and branch out at high level to support rafters and beams at faceted roof 'ridge' level. The loading forces are carried below the mezzanine on concrete columns which have been built into a non-load-bearing block wall. Ove Arup and One17AD designed the overall system and worked closely with steelwork specialist Allotts Steelwork to engineer the complicated 3D geometry. The aim, says Mark Little of Ove Arup & Partners, 'is to provide a lightweight, elegant and aesthetically pleasing roof structure'.

The humidity within the hydroponicum will be considerable and the designers anticipate temperate climatic conditions at ground level and 'rainforest' conditions on the mezzanine. The steelwork structure has been designed to deal with the exigencies of both. The corrosion protection system involves blast cleaning to Sa2.5 of BS7079: Part 1 and coating (see details in box). No fire protection was required to any of the steel sections.

The complexity of the geometry was accentuated by the need to co-ordinate with concrete tolerances and air pillow roof panelling practicalities. The main contractor, Robert R Roberts of Leeds, carried out exemplary work to make it all possible. Concrete tolerances of 20mm were achieved on a rising drum, 300mm thick and 14m above ground at the highest point. Steelwork, fabricated offsite, was cut and sized from a computer-generated model of the scheme - prior to on-site checks being possible - and remarkably, everything fell into place on the day.

Roof structure

The roof structure creates a 3D form; not just rising to the ridge from front to back, but also from rafter to rafter. Therefore, the fixing angle for the air pillow roof panels alters between each rafter and between each cross brace. To accommodate this subtle shift in planar geometry, T-section fillets have been welded to the top face of the circular hollow section roof members with the top flange positioned at slightly varying degrees to take up the tolerances.

Similarly, at the point where the roof meets the concrete drum connection, a mild steel fixing angle, secured by postdrilled bolts, has to change its angle every 1.5m to accommodate the shift in the plane of the roof covering panels. These mild steel angles and T-sections were carefully pre-engineered and welded prior to the roofing panels arriving on site. Tolerances of only 5mm were acceptable to the roofing supplier. The aluminium roof panel frames were fixed with fasteners through the mild steel support angles and the ends ground off to minimise visual intrusion.

If, as it is believed, this scheme finally makes it to completion, it will be a triumph of construction professionalism over financial adversity.

STEELWORK COATING TO ECOSTRUCTURE

Primer,75 microns of shop-applied zinc rich epoxy (maximum 250 VOC/g/litre).

Intermediate coat,100 microns of shop-applied barrier epoxy M10, (maximum 250 VOC/g/litre).

Finish coat, site-applied,50 microns acrylic/urethane, (maximum 420 VOC/g/litre).

Zinc rich primers comply with BS4652. All coating materials have been supplied from a single source manufacturer and applied in accordance with their instructions. The quoted thicknesses are minimum dry film thicknesses. All materials comply with the maximum Volatile Organic Content given in the current Secretary of State Process Guidance PG 6/23

BOOKS

Steel Construction Manual.Schulitz, Sobek and Habermann, Birkhauser Publishers, ISBN 3-7643-6181-6. An indispensable reference work on building with steel. Includes details on principles of construction and an extensive bibliography of relevant European standards.

CIRIA Report R 174:1997 New Paint Systems for the Protection of Construction Steelwork. Reviews pertinent environmental, health and safety legislation. Includes standard material specifications for a range of environments commonly encountered in general construction with cost information.CIRIA Report R 93 Painting Steelwork.

Describes the principles of metallic corrosion, paint types and constituents and ways of preparing substrates.

Steel Construction Institute Publication 102: Connections between Steel and Other Materials: 1996 . Examines the design and construction issues in connections between steel and concrete/masonry.

Steelwork Corrosion Protection Guide: Building Interiors. Zinc Development Association. Exterior environments guide also available.

STANDARDS

BS 7079:1990.Preparation of steel substrates before application of paints and related products.

BS 5950 Part 1:1990. Guidance for the design of structural steel work with hotrolled sections, flats, plates and hollow sections in buildings and allied structures.

BS 5950 Part 2:1992. Guidance on specification for materials, fabrication and erection for hot-rolled sections.

BS 5950 Part 3:1990. Code of practice for the design of simple and continuous composite beams.

BS 5950 Part 7:1992. Specification for materials, fabrication and erection, using cold-formed steel.

ARTICLES

'Steel in the frame', AJ 03.07.97. Round up of developments in fire and environmental performance, construction systems and panels.

'Steel marches on', AJ 23.01.97 Developments in structural steel, including parallel flange sections.

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