To reflect the architectural internal planning, elevational treatments and organic sinuous form of the internal leading slab edge, a structural grid of 8.4m x 6.0m was derived using reinforced concrete as the construction material. Concrete allowed the team to form the sculptured cantilevering edge as a fair-face finish and proved the most cost-effective for the typical spans as a 300mm thick slab.
The cantilevers were structured as inverted beams on grid lines within the raised floor zone. Service routes were integrated into the structure by locating the main risers in slab cut-outs to the more regular southern elevation with nominal penetrations through the upstand cantilevers to facilitate electrical route distribution. The contractor used large pre- fabricated forms for spans that were craned into position and this affected the decision to utilise a concrete roof slab that also accepted flexibility of location of the roof plant voids.
The building form lends itself to two separate structural blocks separated by a movement joint located within the stepped-back core area. The stability of each block is provided by reinforced concrete shear walls, located in core areas.
The reinforced concrete slabs are supported generally by in-situ columns and walls; however, precast units were introduced to enable the double 'V' columns to be formed and also the feature stairs. The 'V' columns were delivered to site as singular complete elements and then cast in place with the slab. To facilitate construction of the precast stair and enable tolerance interface with fixings, it was broken down into two primary structural elements of the main support spine with individual tread units bolted into position, each tread having accurately cast-in fixing details for cast-in locating the hand rails, integral light and glazed tread detail.
The highly expressive doubly curved form of the glazed north elevation facade was supported by virendeel steelwork trusses off which cast support arms were cantilevered. The trusses were formed as tubular top and bottom chords with steel plates between acting as web elements. The depth of the truss varied to provide stiffness and structural capacity to resolve the forces derived across the span and as the truss returned on itself at the lower support point. The whole system was braced by longitudinal ties run back to diagonally braced frames in the final structural bays. The taller vertical end bays were supported by vertical glass fin beams.
The most cost-effective substructure solution with the 3m average depth to stiff clay was ground enhancement by Vibroflotation technique with the introduction of replacement stone columns.