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Prestressed post-tensioned (PT) flooring systems

An innovative structural engineering solution for difficult sites, which enables developers to maximise their yield

Prestressed post-tensioned (PT) flooring systems are a proven technology in Australia, where they are routinely used on multi-storey developments. Yet, it’s rare to see PT used in New Zealand, despite the efficiencies it delivers to both developers and architects. The use of PT flooring systems can result in thinner floor systems and/or longer spans between supports in comparison to traditional reinforced concrete or precast concrete. This structural system can also reduce building weight, earthquake design forces and reinforcing steel, and shorten the building height. It’s particularly useful on tight sites, both in terms of land area and height boundaries - as it often enables developments that wouldn’t otherwise be feasible.

The benefits of PT transfer slabs for developers and architects

From a developer’s perspective, PT can make carparking under buildings possible by minimising, or even eliminating, the number of columns needed. PT requires less labour onsite compared to traditional reinforced concrete and can bring down the building height via thinner flooring systems, enabling developers to maximise yield. From an architect’s perspective, PT makes life easier. PT slabs give architects a simple structure to work with and longer spans allow planning flexibility with more open floor space. The PT concrete is inherently fire resistant so there’s no need to add fireproofing and a flat PT slab soffit makes it easier to coordinate services above ceilings. This technology minimises floor to floor heights, requiring fewer stairs and less cladding, resulting in overall cost savings.

A PT transfer slab in action

Structure Design is an early adopter in New Zealand of this proven technology. Working with established Australian and New Zealand PT contractors, we have now applied or are currently designing, PT for five developments including two apartment buildings, a retirement village, student accommodation, and a large house. It’s enabled these developments, some of which would not otherwise have been feasible, to go ahead. By way of example, a PT transfer slab has recently been constructed for one of the multi-storey apartment buildings. It is up to 750mm thick and permits upper level apartments to be suspended over lower level carparks on this tight site. Walls and columns are at different locations above and below the PT transfer slab to suit varying architectural layouts. This structural system has made an underground carpark possible and has enabled the architect to squeeze in an additional floor comprising four apartments, maximising the developer’s yield.

Want to know more?

If you’d like to find out more about this structural system or hear how it could benefit your project, please contact Peter Boardman on T: 09 302 0205 or E: prb@structruredesign.co.nz

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