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Bi-Steel is growing beyond its earlier security-sector applications into the construction mainstream, particularly with Corefast, a prefabricated building-core technology for lift shafts, stairwells and shear walls. The first project to finish on site is a lift shaft for Dundrum cinema, south of Dublin Bi-Steel is a simple idea. A Bi-Steel panel comprises two steel sheets separated by a grid of steel bars. In that, it is somewhat like a virendeel space frame. But then it becomes a composite construction, when several panels are connected as a structure and the void between their face plates is filled with structural grade concrete. More like externally reinforced concrete.

A typical panel could comprise two 8mm steel sheets separated by a regular grid of 200mm rods at 300mm centres.

Panels can be up to 2m wide by 18m long.

PANEL PROPERTIES Within the composite performance of the concrete-filled steel units, the differing parts have predominant roles. Steel face plates resist in-plane and bending forces. The concrete core resists compressive and shear forces. In tying the plates together, the connecting rods provide shear reinforcement to the concrete core; they carry longitudinal shear between face plates and core and they resist face plate buckling.

While panels have significant stiffness and strength, they are more ductile than reinforced concrete, which has made them particularly attractive for blast-resistant structures.

The steel component is created by friction-welding bars to plates, providing a high level of accuracy. The face plates can be pre-bent for creating curved panels, down to a radius of 1,500mm.

Fire resistance can be up to two hours when Bi-Steel is used simply as a compartment wall. Used structurally, walls achieve over 30 minutes' fire resistance and can usually be fire engineered to one hour while still unprotected. Two-hour resistance can be achieved with a 1mm intumescent coat. Often panels will be clad anyway, which could be with, say, 20mm of mineral wool insulation faced with a 6mm fire board.

The core's relative wall slenderness compared to reinforced concrete can add a little to each floor of a building's usable area.

CONSTRUCTION PROCESS Bi-Steel, a patented Corus technology, has been around for some years. But while many of the security applications so far have made engineered-uses of basic panels, what we have now in Corefast is a ready-to-use construction technology, which arrives on site as a modular kit.

How that kit arises will depend on the job. Corus' factory for Bi-Steel is at Scunthorpe in Lincolnshire. It can do the whole lot - making up panels into core modules, attaching plates and angles to connect to the rest of the structure, intumescent coating, etc. Or, as in the case study of Dundrum cinema, flat panels were made by Corus, then delivered to a local fabricator to add fittings and make panels into the three-dimensional core modules.

How much prefabrication is done is also a matter of choice - in two of the three examples overpage, which are the first three Corefast projects, they have gone for large modules.

The third is largely panels for on-site jointing.

Steel panels can be side-connected where widths need to exceed 2m, and corners made to create 'L', 'C' or rectangular-plan modules, which are more stable to handle on site than flat panels.

For ease of transportation, modules may be kept below the maximum of 18m length, typically 13m to 14m.

To date, projects have been for medium-rise buildings.

However, there is one of 18 storeys now in production and 100 storeys is felt to be achievable.

On-site assembly is, not surprisingly, a lot faster than slip-forming in concrete; an average of a storey per day should be readily achieved.

There is a variety of foundation options. Those explored so far include setting starter bars to connect with the concrete fill, setting the feet of panels in pockets in the foundation, casting them in, or using holding-down bolts, or a combination.

Once the lower modules are set, the upper ones can be fixed by crane, steered by erectors in a cherry picker. Removable screw adjusters on top of the erected modules allow accurate alignment - particularly important for lift-shaft applications.

Connections can be bolted or welded. Typically, the engineer's instinct is for the faster, simpler process of welding, but site managers often prefer to avoid welding on site generally.

Modules are normally filled from the top with pumped concrete, at a rate of up to 2m in height per hour. Lower modules are filled before upper ones are fixed in place. The top of the concrete surface is simply scabbled before the next pour. Continuity mesh reinforcement is not needed as continuity is provided by the steel face plates. In mixed-use developments, it is often only the lower retail floors that have the cores on the critical path, so the uppermost modules of the cores can be filled at a later convenient point. They can still be tied into the structural frame as it rises.

To date, concrete vibrators have been available on site but have not been needed.

In a well ordered world, all openings for doors, windows and services are precut in the factory and lined with steel or timber on site for the pour. In the real world, variations on site can be achieved with diamond-core drilling.

If the core has a stair, that can be fixed as the core modules go up, providing immediate site access.

Bi-Steel cores are high strength and fast, which may be more important at the low level of a building than higher up.

There, it may be more economical to complete the core in more conventional steel framing, as for one of the cores of the Manchester offices described overpage.

What follows are brief descriptions of the first three Corefast projects, one completed, the other two on site.

Contact www. bi-steel. com, email bi-steel@corusgroup. com, tel 01344 751670 DUNDRUM CINEMA The lift shaft rises six storeys, connecting basement car parks to the cinemas. The main reasons for using Corefast were speed and stiffness. The latter was important because the shaft becomes a feature lift shaft with two glass lifts in the cinema areas. There it is open fronted in layout, an 'E' in plan.

The core is 24m high and 2.4m x 5.5m on plan, made as two sets of three-storey modules. Each three-storey rise was made with two modules, the heaviest weighing 17 tonnes. On the floors where the lift shaft is open-fronted, modules are either 'C' or 'L' in plan. The six-storey core was completed in five working days.

The foundation solution was to slot the feet of the lower modules into a 1.3m foundation pocket among the pile caps, bolt down and shim. Shear studs factory-fixed on the face plates transfer forces to the substructure now the core foot is concreted in.

The site manager accepted welding of joints on site.

Checks on vertical alignment showed the shaft to be within 5mm of vertical over the 24m height.

The core 'kit' included fin plates and angles for connection to the frame and the brackets for attaching the guide rails on site.

Fire requirements were for two hours' protection. The architect wanted the interior painted, so an intumescent coat was applied.

The lift core was installed in May 2004, except for filling the upper modules, which was left until September 2004 when the steel frame and composite floors reached top-of-shaft level. The cinema opened to the public in September 2005.

POLLOCKSHAWS ROAD HOUSING, GLASGOW Westpoint Homes' largest project to date comprises 335 oneand two-bed apartments in six blocks. The current Phase One involves a six-storey and a seven-storey block. The lower storeys of the blocks are steel frame with timber infill panels. The upper four floors are timber framed, a method of building Westpoint frequently uses. Speed and accuracy were reasons given for using Corefast. QS Evan McKinnon estimates the cores were erected about five times as fast as they would have been had reinforced concrete been used.

In this project, Corus both prefabricated the cores and also acted as subcontractor in erecting and concrete-filling them. Corus expects that the future pattern will be for steel fabricators to erect the cores and for the main contractor to then concrete-fill.

The single-lift cores were trucked to site as two boxshaped modules per shaft; the lower four storeys as one module, the other module two or three storeys. Floor-to-floor height is 2.85m. The largest module weighed around 13 tonnes. There were also lift pit modules, delivered to the groundworks' subcontractor, which were cast into the substructure. In this case joints were made on site by welding One-hour fire resistance was specified, so the cores do not require any additional fire protection.

The four bilding cores of Phase One were installed in November 2005 over a period of seven working days.

FORTY SPRINGARDENS OFFICES, MANCHESTER Corefast is being installed during January 2006 for these citycentre offices. There are two basement floors and nine floors above.

There is a central four-lift core and an additional combined core with firefighting lift and stair. Both cores will run as Corefast from basement to level six, then in traditional braced steel framing for the top three levels.

Corefast was chosen in preference to conventional concrete core or traditional steel frame. It is a tight urban site and it was crucial to get out of the ground fast.

The shafts sit on pile caps, Corefast coping with a stepped foundation locally. It also copes with a cranked core plan rather than a straight line up of the four lifts. The fire requirement is set at 90 minutes.

For this scheme there are 90 Bi-Steel panels. As we write, corners were being made up by bolting at the factory. Otherwise, the cores are to be delivered to site as single panels, a choice mainly driven by the steel erector.

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