POST TENSION SLABS USED TO MAXIMIZE NUMBER OF UNITS – WARDIAN, CANARY WHARF, LONDON

Executive Summary

Interspan designed 200mm thick thisk with local 250mm edge thickenings. An equivalent reinforced concrete (RC) slab would have been 275mm thick. The application of post-tension resulted in 2 additional levels, which was essential in delivering the 800 units over a combined 105 levels within a height of 340m.  The client benefited from 16 additional units versus an RC solution by using post-tension.

Two iconic residential apartments Towers

The Wardian is a twin-tower residential structure appropriately located in the London borough of Tower Hamlets. It affords wonderful, unique views overlooking the South Dock.

One of the tallest post-tensioned residential developments in Europe, the towers, referred to as the East and the West, provide a total of over 800 residential units.

The East Tower is over 55 storeys and is 178m tall while the West tower is over 50 storeys and is 162m tall. The towers share a two-storey reinforced concrete podium, above the basement.

The project was delivered by O’Halloran & O’Brien Ltd for Ballymore Group Limited. Interspan (Europe) Limited executed the post-tensioning from May 2017 to April 2019. The first of 136 affordable homes at the scheme went on sale in the summer of 2020, with a penthouse however setting you back £2.5million.

Design Considerations for the Wardian Post Tension Structure

The post-tension floors needed to comply with a design life of 60 years, and for a fire rating of 2 hours. This limited the minimum slab depth to 200mm. Given the heights of the towers, the structure is considered as Class 3 for Robustness. A systematic risk assessment was required to comply with this classification to consider normal and abnormal hazards. In addition to this systematic approach, the horizontal and vertical tying requirements associated with a Class 2 structure also needed to be satisfied. For the slabs, a combination of post-tension and reinforcement was used to satisfy this requirement.

The lateral stability system of the structure relies on a combination of the central lift/ stair core in combination with cross-walls cantilevering from the foundation level. The post-tension slabs are required to transfer wind loadings from the cladding brackets to the cores by acting as a stiff diaphragm in the permanent case. In the temporary case, crane and hoist ties induced lateral loadings which were resisted by the design of post-tension ties along the defined load path to the core walls. This approach was adopted on both towers.

Detailed design of the Post-Tension slabs at Wardian

Extensive design coordination of external fixings for finishes and balconies had taken place prior to commencing the detailed design of the post-tension slabs. This detailed level of coordination was carried out on both the East and West Towers (both 38m x 18m). The application of thermally broken steel balconies was introduced to minimise slab edge loadings and resulting in reduced incremental and long-term deflections.

To save large quantities of plywood, which would have traditionally been utilised during construction, preformed steel edge shutter containing all the cast in fixings were used on this project. The anchors were attached to them utilising pre-drilled holes. The shutters once stripped were cleaned and reused.

During the design process, consideration was required to assess the impact of the cross walls on the applied slab stress. The stiffness of the walls about the major axis and its inherent impact on the axial shortening of the slabs during the stressing operation meant that some local areas contained tendons in one direction and reinforcement in the orthogonal direction.

Given that the central core and the geometry of each tower had no movement joint present in the structure. Careful consideration of shrinkage strain, elastic strain, creep strain and thermal contraction/ expansion was required during the modelling process particularly given the differential deflection constraint after cladding is 8mm between levels.

Our designers worked closely with the main client structural engineer (WSP) to ensure that project changes during both the design and construction phases were captured and dealt with promptly.

Execution Considerations for the Wardian Superstructure

The superstructure of the towers consists of varying post-tensioned slab thicknesses with significant transfer structures on the lower levels. The area of the slabs alone is an excess of 60,000m2

The design and construction programme on the project demanded the towers to be built simultaneously. With the substructure consisting of a piled raft foundation system, a simultaneous construction mitigated the risk of overloading and differential settlement during construction. The 100-week programme requiring 1 pour per week per tower ensured continuity in personnel, workflow and logistics.
As with other projects of this type, logistics was a major challenge. A lack of on-site space, delivery restrictions and the speed of construction of follow on trades meant that each delivery had to be well-timed to avoid bottlenecks.  Close cooperation with the Frame Contractors team lead to successful and positive ‘can-do’ attitudes all round.

Execution of the Post Tension slabs at Wardian

The relatively small site, located in the Isle of Dogs, is approximately 5,700m2. Considering the 1,700m2 podium, detailed planning of the logistics was required for the duration of the site works. Weekly coordination of deliveries was utilised covering all trades. Coordination of labour when deliveries arrived was also essential.

As previously mentioned, the construction programme required 1 pour per week per tower. Given the height of the structures, the project spanned over a 2-year period. This programme was adhered to throughout this duration of the project, with seasonal variances in temperature accounted for through changing concrete mixes. This ensured the concrete final stressing strength was achieved in line with the programme. Concrete cubes were kept on and crushed on site. Maintaining the cubes in the same environment as the slabs was essential to mitigating any potential delays associated with incorrect strength readings.

Utilising the coordinated steel formwork slab edges which were designed prior to the detailed design of the post-tension slabs – allowed for the speed advantages of pre-set slab edges. This solution allowed for efficient use of materials given they were reusable, as well as labour efficiency helping to maintain the site programme.

Value added to the client through adopting a Post-Tension slab solution

From a cost base, the post-tension slabs used were typically 200mm thick on the East Building and 225mm thick on the west building, the equivalent RC slab would be 275mm and 300mm respectively. This resulted in a saving of 7,000m3 of concrete and a circa 25% reduction in the load from the superstructure to the foundations. A post-tension solution also resulted in an effective saving in the reinforcement of 350 tonnes in slab loose reinforcement.

This saving in concrete thickness per level also resulted in 16 additional units based on the height of the structure. With the optimization exercise to adopt post-tension slabs, the 75mm saving has resulted in a reduction in the combined height of both towers of almost 8m equating to 2 levels. This was essential to delivering the 800 units over a combined 105 levels within a height of 340m.