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The existing podium structure is a concrete slab supported on a combination of concrete columns and walls, which subdivide the shops below. These shops needed to remain open during construction and therefore the intention was to build the new structure off the existing podium slab and avoid intrusive strengthening works.

The slab had been designed for gym loading and could therefore accommodate the live loads. The limiting factor was the ultimate bearing capacity of the existing shallow foundations. This meant that the weight of the roofing and superstructure was critical, and the load distribution needed to be broadly the same as at present in order to minimise the risk of differential settlement. Ultimately this dictated the setting out of the building, leading to a lightweight roof supported on a steel frame, with columns located immediately above the existing columns and walls below.

The combination of transparent facades and open-plan spaces did not permit bracing, so stability is provided by all columns and beams acting as a moment frame in both directions. The sway movement under wind load that could be tolerated by the facade system is the governing design criterion; deflections at the top of columns were limited to 8mm, including the two-storey columns at the front.

The frame is constructed of standard 300 x 200mm and 300 x 100mm steel RHSs. The challenge was to provide a connection between beam and column that provided full moment continuity but was not visually intrusive. The final solution has welded connections at the columns, with an end-plate bolted splice connection in the beams that transfer a lower moment. This provides an almost invisible connection, with the bolts tightened through cut-outs in the top flange.

The distinctive slender timber joist and ply ceiling is used structurally to restrain the steel laterally and acts as a diaphragm so that lateral loads are distributed evenly over the building footprint.

Joist depth was chosen to match the 300mm-deep steel. Joists are connected to welded cleats on the sides of the RHSs so as to leave the top flange free for fixing the ply.

The extension to the front of the podium is also a steel frame, which acts compositely with the lightweight concrete slab-on-steel profiled decking so as to minimise the structural depth and maximise headroom below.

Site investigation revealed that the founding level was several metres below ground level and fell away from the podium. Consequently, to prevent undermining the existing pad foundations and to minimise excavation, mini-piles are used to support the steel frame at the front.

The extension to the ground-floor slab here, which incorporates lift and escalator pits, spans between the pile caps.

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