Core, what a success
Dundrum town centre, 6.4km south of Dublin, is a retail and leisure complex comprising 80,800m 2 of retail space, a 12-screen multiplex cinema, 15 restaurants, a pub, 3,400 car parking spaces and 16,700m 2 of office accommodation. Due for completion in spring 2005, the development is being constructed in two phases. At the time of installation of the lift core, approximately 70 per cent of phase one had been completed.
Lift cores are always troublesome, necessitating special programme requirements to avoid delays to followon trades and to minimise the effect on work in surrounding areas. Reducing the time needed to install a lift thus has tremendous benefits for any construction project. This case study examines the Corefast system used at Dumdrum.
Corefast comprises a modular, rapiderection structural core of fabricated modular Bi-Steel panels (two parallel faceplates jointed together) joined in situ with the Surefast bolting system. This combines the benefits of strength and stiffness, low cost and the quality and speed of off-site manufacture.
Critical path Structural engineer for the Dundrum development T J O'Connor & Associates (TJOC) approached Corus Bi-Steel in July 2003. A number of issues had been identified with the lift core for the cinema complex:
l The core was on the critical path for the site works and would take more than eight weeks to construct from conventional reinforced concrete. The Bi-Steel team reviewed the design criteria and proposed a solution that could be erected on site in under two weeks.
l The core forms a key architectural feature of the cinema atrium and so the architect laid down strict requirements for the slenderness of the core walls, which called for composite stiffness.
l The Bi-Steel solution simplified the connections between the frame and the core by providing a steel-to-steel interface, and facilitating the attachment of fin-plates, seating brackets and floor-support angles - off site, in a quality-controlled environment.
l Construction of the core using prefabricated modules minimised the site-storage area and the requirement for temporary works, easing operations on a busy and congested site.
l Following a positive response to TJOC's original enquiries, the Corefast scheme was developed further through detailed discussions between Corus, TJOC, steelwork contractor Fisher Engineering and main contractor Sisk.
l Bi-Steel was confirmed as the material to be used for the lift core on 1 December 2003. Design issues aside, the resulting solution overcame onerous structural requirements with a total on-site installation time of 40 hours over a two-week period.
Structural arrangement The Bi-Steel core houses two lifts, and is six storeys (24m) high, with plan dimensions of approximately 5.5 x 2.4m. The lift shaft extends to the sixth floor, connecting the basement car parks to the cinema complex and retail spaces. To assist the transportation of materials and to utilise existing on-site crane capacity, the core was designed as four similarsized modules. Each module was approximately three storeys in height, with two modules forming the lower part of the core, and a horizontal joint at about mid-height.
The largest module (module 3) was 12.4m high, 3.3m wide and 2.4m deep. This module weighed just under 17 tonnes when fabricated. Bi-Steel panels sized 8/200/8 @ 320 were used to form the core: that is, a panel constructed from two 8mm-thick steel sheets, 200mm apart, connected by bars at 320mm centres. The overall thickness of the core wall was therefore 216mm.
The core was designed by Corus in accordance with BS5950-1:2000.
TJOC provided dead, imposed and notional loadings using output from the RAM structural analysis software package. Close cooperation between Corus and TJOC ensured efficient exchange of their respective design data. A critical design requirement was the ability of the core to limit frame deflections.
The inherent stiffness of the steelconcrete composite core meant that a 'non sway' mode analysis was achieved, significantly simplifying the frame design. This onerous structural requirement was attained while complying with the architectural requirements for narrow wall thickness. A wall thickness of 216mm could not have been achieved using just reinforced concrete.
The main foundation connection was provided by embedding the lower modules 1.3m into the concrete base slab. Shear studs on the face plates transfer the forces to the foundations via the reinforced slab. The vertical connection between the two modules consists of RHS sections with shear studs. Surefast bolts were incorporated to clamp the two modules together prior to concreting and form an effective seal. In situ welded plates were used to form the horizontal joint at mid-height.
Fin plates and seating plates prewelded to the lift shaft provide a straightforward connection for the incoming beams. The metal decking sits on pre-welded angles and the horizontal forces are transferred from the slabs to the core by rebar tags. The local effects of loading from beams and floor slabs were assessed, and if necessary the position of the internal bar connectors was adjusted to provide additional stiffening to the Bi-Steel face plates. Brackets to support the lift guide rails were prefitted to maximise off?site assembly and to reduce the lift installation times.
The four-week design period began on 12 January this year.
Fire protection A key design criterion was the ability of the lift core to maintain its strength and structural integrity during a fire, and the fire officer specified a twohour fire rating. The open back face of the shaft meant that a fire had to be considered acting on both sides of the core wall. The situation was made more complex by an architectural requirement that the internal face of the core should have an exposed painted steel face. Intumescent paint was therefore the only option for fire protection in this area. Fire-resistant boarding was, however, an option for the stone-clad external faces.
A series of tests was undertaken at Warrington Fire Research in January to assess the thermal properties of the structural panel protected by trial thicknesses of intumecent paint.
These properties were used to undertake a detailed elevated temperature analysis of the core in order to assess the structural behaviour under fire loads. This analysis, which was subject to independent approval by the Steel Construction Institute, validated the two-hour fire rating for the core protected by 1,000 microns of intumescent paint on the internal face and standard fireboard on the external face.
Module production The module production was carried out in two phases:l Panel production and installation of Surefast bolts; and l fabrication of modules from Bi-Steel panels, and attachment of secondary steelwork.
The panel production and the installation of Surefast bolts was undertaken at Corus' works in Scunthorpe, England. Panel production began on 11 February and the first batch of completed panels was dispatched to Fisher Engineering on 25 February. Remaining panels were dispatched on 4 and 5 March.
Fabrication began at Fisher Engineering's works, Ballinamallard, Northern Ireland, on 29 February. Fabrication of the four modules, including a trial erection of modules 1 and 2, was undertaken between 29 February and the delivery of the first module to site on 4 May. Fabrication proceeded without incident, the trial erection was also successful and provided the necessary reassurance in advance of delivery of the world's first modular composite lift core to site.
Chris Beattie is the senior structural engineer at Corus. For more information phone 01344 751670 or email bi-steel@corusgroup. com