The steel-frame structure and thermal envelope of the halls comply with the normal standards applied to power plant construction. Only the outer skin was adapted by the architect and constructed from silver-coloured coated steel structural liner trays. The recessed joints consist of black perforated sheet metal plates with a white refracting surface, which is illuminated from the sides at night by fluorescent tubes. The light-coloured frame with dark joints reverses its look at night and turns into a symbolic light sculpture of vertical bands of light.
Structural Concept »City Window«
The size and the two glazed main facades of the 60-m-high and 38-m-wide “City Window” ensure it is the most structurally challenging element of the power plant envelope. Buildings that were viewed by the operator as not essential for the functioning of the plant had to be detached from the boiler house and stack. For this reason, a 40-m-high opening was cut in the facade facing the power plant, which is connected to the boiler house by movement joints only. In addition to ten smoke extraction openings, the roof surface has a circular hole for the stack but makes no direct contact with it.
Stiffening
The inside of the rectangular cuboid is mostly empty, with no structural elements providing paths for transferring loads into the foundations. The two glazed main facades also contribute no stiffness to the structure. A northern stairwell core provides north-south stiffening alone, The cross-braced roof and an adjacent full-height steel-frame structure stiffen the main structure torsionally and in the east-west direction.
A 3D structural analysis using the response spectrum method showed that wind was the critical load for the stiffening system in both main axes, but all structural connections were designed to resist earthquake loads.
Vertical load transmission
All vertical loads are carried by the steel columns and the reinforced concrete core. The structural members above the opening in the facade facing the power plant transfer their loads through a truss into the flanking columns – with the building corner stiffened by a narrow trussed girder.
“Stator”
A solid wall in front of the glass facade takes up the direction of the power plant again and forms its architectural termination. This component is internally stiffened by cross-bracing. However, the facade columns are relatively flexible in the transverse direction and cannot transfer force onto the main building body. The “stator” therefore contributes hardly anything to the stiffening system, but influences the building’s dynamic structural behaviour.
Reinforced concrete core / in-built components
The stairwell core was slip-formed in reinforced concrete. The reinforcement gradually reduces with height. Pockets were left for the prefabricated staircases and landings to be subsequently lifted into place in the stairwell.
Connection fittings were precisely cast into the core during the slip-forming process to allow connection to the steel facade. This required particular care because the 25-cm-thick wall panels of the core towards the top have only a limited capacity to accommodate bending moments. After installation, braces and bolts are welded onto connection plates cast into the integrated building components to carry the loads from the beams.
The facade beams continue at a distance of approximately 60 cm from the narrow core sides and are each supported at two points on the wall. The beams do not transfer the compressive and tensile forces and horizontal forces directly but are bolted to a spreader beam. Each end of the spreader beam has steel plates to transfer compression forces and is held in place by a hook plate. The intermediate spreader beam minimises the lever arm in the transfer of the load into the concrete and provides the means of making adjustments during steelwork erection.
