Kier - Plymouth EFW

Project: Devonport EFW

Client: Kier Infrastructure and Overseas

Contract Value: £797,378

Contract Duration: 9 Months

Contract Overview: Design, supply and installation of 3 precast concrete stair cores. Design & supply of precast concrete bailing wall panels.

Waycon Precast Ltd were contracted to design, supply and install three large precast concrete stair towers for the new EFW facility at Devonport dockyard. Each of the 3 towers were significantly different and each one posed a different challenge.

The first tower P1UHD has a plan footprint of 7.3m x 3.5m with a total height of 41.01m. The core was to be erected to full height ahead of the main structure and therefore had to be capable of free standing. This was achieved by the use of post-tensioned tendons running the full height of the shaft. These tendons clamp the units together and allow the shaft to resist the over-turning forces from the wind.

The second tower P0UHD was the largest of the three towers with a plan footprint of 9.3m x 7.8m with a total height of 44.44m. The tower was split into 4 compartments; the main stair tower, a large goods lift shaft, a lift shaft lobby and a walkway through to the main structure.

The tower was to stand directly adjacent to the main bunker wall. The main stair tower was constructed in the same manner as P1UHD. The lift and lobby compartments were then constructed using flat wall panels tied between the stair tower and the bunker wall. Once the panels reached the top of the bunker, the panels were replaced with horse shoe shaped sections which were tied back to the stair tower.

The third tower P2UHD was the smallest of the three, with a plan footprint of 6.62m x 3.2m and a total height of 19.25m.

The tower was originally designed to be a free-standing gravity structure, however during the design process changes to the steelwork surrounding the tower meant that the shaft was to support lateral loading from the roof of the adjoining structure. As the foundations had already been cast, post-tensioning the shaft was not an option. The decision was taken to pin the shaft to the adjacent insitu wall, to allow the forces to be transferred across the tower into the main wall. This was achieved by using wall shoes to connect the top 3 boxes together to provide a much stiffer core. In addition the slabs at the top levels were cast into the box, rather than being separate sections, again stiffening the core. The core was then pinned using resin fixed dowels to allow the load transfer.

In addition to the supply and installation of the 3 stair towers, Waycon were also contracted to design and supply four precast bailing wall panels for a fire compartment within the main structure. The panels were to be connected to the insitu slab using wall shoes, then stitched together using a wire rope system. The panels were also stitched into an reinforced concrete beam cast insitu above the wall panels.

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