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Explore the use of post-tensioned slabs used to achieve greater flexibility, speed up construction and reduce CO2.
Strata SE1 is a 147-metre-high building which forms the focal point of the £1.5 billion, 170 acre regeneration of the Elephant and Castle area. The innovative design of the structure creates a dramatic landmark on the London skyline and is the first building in the world to have three integral wind turbines, each nine metres in diameter. Turbines are housed in the twenty metre section at the top of the tower.
This 36,600m 2 development comprises 408 apartments which have far-reaching views across the Capital. Post-tensioning specialist CCL was commissioned to undertake the design, supply and installation.
The objective for the design provided by CCL was to create a flat soffit within minimal floor to floor heights whilst at the same time maintaining an optimal slab thickness. A 200mm thick floor slab was achievable for typical residential floor spans - 3000mm structural floor-to-floor height; 2500mm floor-to-ceiling height to living rooms and bedroom.
The use of post-tensioning on this project made it possible to achieve long spans with difficult plan geometry, to maintain a structural depth of typically 200mm on spans of up to nine metres. This slab depth would have proved impossible using traditional reinforced concrete construction methods. At the same time the post-tensioned slabs provided deflection and crack control for these spans across the tower.
Also included in the design was a stage stressed transfer beam at level two Pavillion. This was used to control deflections as construction continued above, whilst minimising structural depth and headroom implications. Any requirement for downstand beams in the typical residential floor plate was removed which facilitated the routing of services and the soffit marking of tendons.
The floors themselves were of a curved design which created additional complications. These were overcome by the use of every size of anchorage from within the CCL XF range which was developed to exceed the requirements of BS EN 13391 and ETAG 013.
CCL was the first company within the UK to develop a range of post-tensioning anchors to these rigorous standards. The Strata project itself was the first in the UK which has these products installed. The use of the CCL XF flat slab system allowed the interconnection of smaller anchors with wider duct. The subsequent use of wider duct for the curved tendons around the edge of the building led to reduced friction when pushing the strand up to 30 metres in a multifaceted curve.
The construction of the tower began in July 2008 and the floors were completed in 49 weeks, which resulted in an impressive completion rate for the post-tensioned floors of just over one per week.
Use of post-tensioning produced a reduction of 50 – 75mm of concrete per floor when compared to traditional reinforced concrete construction. This permitted an overall saving of approximately 2000m3 of concrete within the superstructure alone (around 760 tonnes of embodied CO2). Further savings resulted in the substructure in the form of reduced depth and diameter of piles. The combined reduction in the concrete per floor was equivalent to approximately three metres of concrete, or an additional floor within the same building envelope when compared to traditional construction methods.
In terms of value for money, the post-tensioned floors produced savings of at least 15 per cent of the costs of the superstructure materials alone and further cost reductions would have been achieved because of the rapid construction schedule CCL was able to realise (just over one floor per week) and the use of climbing screens and a formwork hoist.
Minimal quantities of traditional reinforcement were required which in turn minimised the financial risk to the client over a long construction period in an uncertain market. Waste materials were kept to a minimum and all such items were recyclable.
Source of further information www.cclint.com
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