A Flexible Approach to PT for Luxury London Residential Development

As the nature of trade and commerce in London has changed, so too has the capital’s cityscape, with many former dockside areas now becoming havens of luxury living.

One of the latest locations to see this kind of transformation is Wapping. Within easy reach of the City of London, the area has seen significant regeneration over the past 30 years and is now something of a residential property hotspot.

CCL is proud to have played a key role in this transformation. One of the recent projects we’ve been involved with is a residential development in a thriving new cultural quarter. The project will deliver 1,800 high quality new homes, offering residents a luxury spa and gym facilities and is set within 7.5 acres of landscaped open space.

Phase 1 is currently under construction and CCL has been responsible for design, supply and installation of the post tensioned slabs from first floor to level 25.

Benefits of PT

PT slabs were specified for the building in order to minimise the slab thickness, thereby enabling floor to ceiling heights to be maximised without increasing the overall height of the building. This is a common strategy in the luxury living sector, particularly for high-rise buildings where the incremental benefits of a thinner slab can even result in an additional storey.

Use of PT was also designed to enable large 10m column-free spans to achieve the architectural design intent of open-plan apartment layouts.

The PT slab specification has also provided cost, build time and environmental efficiencies on the project, with reduced concrete and steel rebar required, delivering a corresponding reduction in embedded carbon emissions.

Managing Deflections

The residential development has been designed with a single central core with apartments arranged around it in all directions. The footprint of the building increases slightly at level 7 before stepping in at level 8 and above on the south-facing elevation, creating a terraced effect as it rises to roof level.

The increased spans required by the design created challenges with managing deflections in the slab. It was clear from the early stages of the design phase that it would not be viable to manage deflections through moment transfer between the slabs and the blade columns that give the building its delicate, refined appearance. Similar challenges were evident with the connection of the slabs to the core walls. Instead of creating fixed connections, therefore, CCL had to find a way to accommodate the reduced fixity between these two structural elements, increasing the deflection challenges of the slab design.

To address this, CCL adopted almost pinned connections between the slabs and the core/edge supports using a coordinated layout of PT tendons to achieve the required level of rigidity to counter the deflections. Using CCL’s XF PT system, with 15.7mm strands, tendons with two to four strands each were laid out in a bespoke arrangement for each level to accommodate the deflections and deliver the performance required by the façade engineer, while maintaining a slab thickness of 275mm at every level.

Level 8 Challenges

While the design of the building resulted in slab design challenges at every level, it was level 8 that proved to be the most demanding.

Two of the columns on the southern elevation are inclined, first outwards, then changing direction at level 8, resulting in thrust force as the vertical load is diverted outwards. This had to be managed at all levels but was particularly challenging at level 8 where converging vertical and lateral forces required additional reinforcement to stabilise the structure and tie the columns back to the core.

Consequently, the level 8 slab has been designed with enhanced PT and traditional reinforcement as compared to the building’s other slabs. This enabled the design team to maintain the same 275mm thickness at level 8 as all for all the other internal slabs.

Developing a viable solution for the level 8 slab was a time critical requirement to avoid any project delays on the luxury development. Along with accurately calculating the additional reinforcement requirements, the CCL design team also had to work collaboratively with the design and construction team to agree a viable reinforcement layout that ties the columns back to the core while overcoming the narrowness of the architectural blade columns, which minimises the options for locating the PT and steel rebar

Due to the complexity of connection between the horizontal and vertical elements, CCL decided to model all the individual elements, and carefully considered exactly how each element would need to be assembled during construction. It quickly became apparent that, due to limitations of physical space, it would be impractical to construct the connection using standard hook rebar. Typically, reinforcement relies on long development lengths which can then affect construction workers’ ability to install multiple bars in various directions and layers. To overcome congestion issues within the slab, the decision was taken to use headed rebar. This type of rebar has a head at one end, which removes the need for a hook, because the anchorage is developed mechanically. This design choice meant congestion was reduced and detailing was simplified, resulting in faster, more efficient installation while avoiding issues with the tie steel or surrounding reinforcement.

Design Co-ordination

Co-ordination of the positioning of the slab reinforcement and anchors with other features of the building was also a key consideration for the slab design and a driving imperative behind the collaborative process.

The steel plate fixing for the balconies and the profile steel cladding fixings both needed to be cast into the slab. Space was also required for the glazing fixings. As a result, the slab perimeter was congested and meticulous co-ordination was required during the design process to avoid the need to manage clashes on site.

The CCL design team developed a 3D PT tendon layout that could be imported back into the contractor’s Revit model for clash detection purposes. We also worked closely with the main contractor and the subcontractors responsible for each package to devise a tendon and anchor layout that worked with their fixing requirements.

Again, it was essential that the design was developed with a degree of urgency and the CCL team took a flexible approach to facilitating this, offering several solutions so that the preferred approach could be adopted, avoiding any interruption to construction progress.

Positive & Proactive

This luxury residential development is an excellent example of the way in which PT delivers value engineering at the same time as offering flexibility to meet the specific requirements of a project’s architectural design. It also highlights CCL’s positive approach to working with delivery partners to resolve technical challenges throughout the design and construction phases in order to develop a best-fit solution within the required programme.