The existing 15m tall structure is a State Heritage listed former warehouse (of “exceptional heritage significance”), built in 1878 and historically significant for its direct association with the large South Australian merchant firm of G & R Wills & Co. The three-storey building is of masonry construction with classical Italianate stucco detailing which is particularly decorative for a commercial building. The North Terrace façade retains a high level of integrity, however the interior is generally dilapidated with its significance limited to the undivided space on all three floors supported by cast iron and timber columns.
The project’s development approval incorporates specific heritage restoration works to the existing 15 metre tall building facade including window and door frames, face brickwork and corbelled detail. It also requires salvage, careful removal, storage, and retention for later re-use of many internal features including structural timber elements, ornamental ceiling finishes, tongue in groove timber ceiling and wall panelling, floor boards, cast iron columns, sliding and hinged doors and hardware, and timber windows.
The protection of the existing building’s facade during demolition, civil remediation and construction presented a significant challenge. The project’s early works phase sees the only path for material movement to and from the site through the existing 3m wide building opening affronting North Terrace. A detailed structural support system and construction methodology has been designed for the retention and protection of the heritage fabric during the works. Vibration monitoring and temporary structural support fixings have been employed to minimise physical damage to original fabric and facilitate repair on removal.
The tall, slender nature of the building and undersized footprint necessitated an innovative, value engineered raft slab perched upon a series of under-reamed bored piers, with the historically significant northern façade protected by retention piles and strip footings. Sunken into the raft are the lift overrun pit and main switchboard room supporting the building’s power supply. 135 tonnes of reinforcing steel has been lifted into the building’s basement for the construction of the 340 m³ raft slab.
The superstructure consists of a central insitu lift and stair core (up to 300mm thick walls, 80MPa), loadbearing precast facade panels, round and rectangular concrete columns and conventionally reinforced concrete slabs.
The tower design gives the building an interesting appearance through the use of vertical aluminium blades and a dual facade design consisting of sandstone coloured precast concrete sections, separating into three vertical elements by the dark metallic finish on Levels 12, 23 and 34. Facade changes to the State Heritage Plan have been kept to a minimum to ensure that the original heritage fabric of the building was preserved.
The project is scheduled for completion in 2022.