The Eastern Tower at West India Quay at London’s Docklands is Britain’s tallest post tensioned concrete building. Planning restrictions limited the towers height, therefore, in order to maximise the number of storeys ( and tenants ), keeping the floor slab depth to a minimum was essential.
The original steel specification allowed for 30 storeys. An extra storey was gained by switching to a concrete structure and a further storey was made possible by incorporating a post tensioned system as opposed to using conventional reinforced concrete.
Post tensioned floors work on a similar principle to suspension bridges. Five steel tendons between 13 – 15mm diameter run through the structural concrete columns under tension to support the floor. The floor is , therefore, installed concurrently with the concrete frame.
Firstly, the framework for the concrete is built. Then the tendons, housed in ducts, are criss-crossed over the slab and the concrete is poured over. The following day, hydraulic jacks tension the tendons at the edges of the floor by 25% of the final loading. The tendons are fully loaded after 3 days and cement grout is poured in to the ducts in order to bond the tendons in to place and prevent corrosion.