The combined turbine hall and boiler house is a huge volume, 160m long, 54m wide with a height from basement to roof of 44m. For the superstructure, a steel structure was selected, supported by a raft foundation. This addressed many of the constraints posed by building within the power station envelope - Arup's job was to effectively nest 276,000m2 of new, highly serviced floors within the original enclosure. The use of a raft foundation obviated the need for piling, which would have been difficult to install through the obstructions provided by the original mass foundations. It would also have been difficult to resolve piling with the turbine hall columns and the perimeter mass retaining walls which, of course, need to continue to function. The use of a steel frame complements a raft foundation as loads due to structure self-weight are reduced. It also allows the frame to be constructed within, and tight to, the retained power station envelope.
When Bankside was first identified as a possible site for the Tate Gallery it was disused, leaking, in need of fabric repair, and contained all the plant from its time as an operational power station. The asphalt roofing had broken down and flashings over brick piers were missing, so that water seepage into and through the fabric had been severe. These factors influenced the decision to replace the roof slabs.
To reduce the rate of deterioration of the structure some temporary measures were taken including installing a new waterproof membrane over the roof, temporary flashings over the copings to fetter brickwork, and repairs to the storm water system.
In the turbine hall, roof steelwork remains in place in the finished building and it worked as a stability diaphragm during construction. In the boiler house, however, the roof steelwork had to be removed to make way for the new building which is higher than the old.
The boiler house frame had to take into account:
movement, particularly relative movement between the retained envelope and the new frame;
the need to minimise intrusion of the vertical structure into the usable spaces;
provision of high load capacity for the gallery floors.
To meet the sensitive requirements of the gallery spaces, a low-velocity displacement air-conditioning system was adopted for these floors. Supply air is provided via service cores and distributed at high level on each of the floors. The air is then delivered to a low-level supply plenum beneath the floor via ducts located within the art display walls. The challenge was therefore to provide a gallery floor arrangement that minimised the structure, services and ceiling zone and provided:
the required high load carrying capacity;
a sufficiently airtight plenum with outlet locations adequate for the air supply;
a fire barrier.
The solution was to construct a second concrete slab 460mm above the main floor slab. The void provides the required plenum, and air is distributed in to the galleries via discreetly located cast-iron grilles set into the floor.
John Hirst, Faith Wainwright, Jim Shaw, Ove Arup & Partners