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The existing building consists of traditional filler joints, floor slabs supported on steel beams with cast-iron columns internally and loadbearing masonry walls externally. Floors generally span 4.0m with columns at 4.0m or 8.0m centres. A survey of the building and load testing of the floors established the extent of remedial/strengthening works required. Proposed alterations included the replacement of the main spine wall by rigid steel frames which incorporate circular hollow section columns to mimic the existing iron columns. The complexity of the connections, and the need to infill the hollow sections with reinforced concrete to provide the required fire resistance, dictated the extensive use of site welding in the fabrication process. Three floors were removed from the main tower to allow for a helical stair leading to the new viewing platform. Floor slabs were replaced with rigid steel ring beams fixed internally to the walls of the tower to maintain its rigidity.

The new seven-storey structure is generally of steel-framed construction. Piled foundations were adopted in a strip adjacent to the existing building to minimise any differential settlement between the new and existing structures and to support the heavier column loads in this area. Screw displacement piles were used to avoid excessive vibration and minimise the ground disturbance adjacent to the existing foundations. A raft foundation was considered appropriate in the area to the rear of the new structure away from the existing building - in view of the presence of an existing deep raft footing whose removal would have compromised adjacent structures, and the smaller loadings in this section of the building.

The new building is structurally independent of the existing building. Overall stability is provided by braced frames in the main cores, and rigid frames and concrete shear walls in the open areas. Composite action between the steel floor beams and the profiled metal-decked in situ floor slab was achieved using through-deck welded shear studs. The desire for an elegant building with large areas of exposed structure dictated the use of circular and rectangular hollow sections in open areas, and required close liaison with the steelwork fabricator to achieve appropriate connection details. The main columns in the public areas are large circular hollow sections infilled with reinforced concrete to provide fire resistance.

A building to the south of the main building was demolished to ground level. The replacement structure comprises a steel-framed building with timber floors and lightweight cladding. Foundations are bottom-driven steel tube mini piles which were used due to the restricted access to this area. The steel frame straddles two existing accesses to an adjacent underground carpark and a ground level service yard - access to these areas needed to be maintained during construction.

Steve Harris

Thorburn Colquhoun

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