The very limited number of internal columns (only three on the lower floors and six above level 06) are strategically positioned to maximise the internal spans, and emphasize the internal spaces. Meanwhile, the continuous precast perimeter columns crank to form the ‘creases’ in the envelope of the building. Containing a core of structural steel at critical locations to provide slenderness, the precast perimeter columns are cruciform in profile, with ‘arms’ to support the precast cladding panels, glazing and brickwork.
Walls were designed to coordinate with the services design, ensuring that all cables were cast into the walls during construction and none were left exposed.
Floor plates are formed of long span in-situ concrete ribs, which support both the concrete floor plate and the precast concrete soffit planks. The services are located in the raised floor, dropping through carefully coordinated holes in the rib beams into the service slots below and cast-in cooling coils within the precast concrete soffit panels.
On level 10 an open terrace is located around the perimeter of the building. The “open” terrace, with hanging perimeter columns to the north elevation, is achieved by spanning deep steel beams from the main core and cantilevering from the central and perimeter columns where required.
In the Switch House, the existing steel frame was removed above level 03. A new steel frame now bridges the resulting void between the reinforced concrete tower structure and new steel columns adjacent to the existing Turbine Hall columns, providing 18m clear spans to the galleries. This is tied to the concrete frame and both structures are founded on piled foundations to ensure continuity through the two buildings. A storey-height truss was required at the level 02 west elevation to span over UK Power Network’s assets and long span transfer beams were installed to achieve the entrance openings.
At level 05 of the Switch House, the sloping north elevation of the tower meets the roof of the level 04 gallery. With no possibility of continuing the north elevation into the gallery space, a 1.8m deep spreader beam was installed. This distributes the loads from the perimeter structure into the roof beams which, in turn, transfer the load to the core and northern Switch House columns.
In order to distribute the loads effectively, whilst not allowing any deflection to occur, the spreader beam was supported on a series of 16 jacks. These were closely monitored during the construction of the north elevation, and jacking operations undertaken at strategic times to minimise impact on the construction above. Once the final brick was installed, the jacks were grouted with maximum differential movement between roof beams having been limited to the required 2mm.
To achieve the tolerances required for installation of the glazing and brickwork it was essential that the deflection of the structural frame to the perimeter of both the tower and Switch House buildings was limited. Although complicated by the geometry of the tower and the various types of transfer structures, deflection limits of 1 in 1000 were achieved.