Post-Tensioned Engineering
Thinner slabs, longer spans, fewer cracks
Post-tensioning is one of the most powerful tools in a structural engineer's toolkit. It takes a concrete slab and puts it into compression before it ever sees a load, using high-strength steel tendons that are stressed after the concrete has cured. The result is a slab that is thinner, spans further, cracks less, and deflects less than a conventionally reinforced equivalent. It is not new technology, but it remains one of the most effective ways to build efficient, high-performance concrete structures.
The principle is straightforward. Steel tendons (typically 12.7mm seven-wire strand) are placed inside ducts within the slab before the concrete is poured. Once the concrete reaches the required strength, the tendons are stressed from the slab edges using hydraulic jacks, applying a compressive force to the concrete. This pre-compression offsets the tensile stresses that would otherwise cause cracking under load, allowing the slab to carry more load with less material.
Our team designs post-tensioned systems for ground slabs on reactive soils, suspended floor plates in multi-storey buildings, transfer slabs carrying heavy point loads, and car park decks where crack control and waterproofing performance are critical. Every design is optimised for the specific loading, span, and construction requirements of the project.
Post-tensioning tendons laid out in a banded-distributed pattern before concrete placement on a commercial floor plate.
Why choose post-tensioning over conventional reinforcement
Conventional reinforced concrete works well for many applications, but it has limitations. When spans get longer, loads get heavier, or crack control becomes critical, post-tensioning offers advantages that conventional reinforcement simply cannot match:
Thinner Slab Profiles
A post-tensioned slab can be up to 30% thinner than a conventionally reinforced slab for the same span. On a multi-storey building, that reduction compounds across every level, reducing the overall building height, the facade area, the structural dead load on columns and foundations, and ultimately the total construction cost. A 50mm saving per floor over 10 storeys is half a metre of building height you do not have to pay for.
Longer Clear Spans
Post-tensioned flat slabs can comfortably span 10 to 12 metres between columns, and band-beam systems can reach even further. Fewer columns means more flexible floor layouts, easier coordination with services, and more usable space. In car parks, longer spans mean wider bays and better traffic flow. In offices, they mean open floor plates that can be configured without working around column grids.
Superior Crack Control
The pre-compression from the tendons keeps the concrete in compression under normal service loads, which means it does not crack. This is particularly valuable for slabs that need to be waterproof (car park decks, podium slabs, water-retaining structures) and for ground slabs on reactive soils where conventional reinforcement would allow fine cracking that permits moisture ingress.
Reduced Deflection
Post-tensioned slabs deflect less than conventional slabs because the pre-compression reduces the effective load on the section and the tendons can be profiled to apply an upward (balancing) force that counteracts the applied loads. Lower deflection means fewer problems with partitions, cladding connections, and services that are sensitive to floor movement.
"Post-tensioning does not just make the slab stronger. It makes it smarter, using less material to do more work."
IA Engineering Design Team
What goes into the engineering
Designing a post-tensioned slab requires a different approach to conventional reinforced concrete. The tendon layout, stressing sequence, and long-term losses all need to be considered alongside the standard structural analysis. Here are the key areas we address on every PT project:
Tendon layout and profiling
The tendons are not placed flat. They follow a parabolic profile through the slab depth, sitting low at mid-span (where the slab needs upward force) and high over the supports (where negative moments are greatest). The shape and spacing of the tendon profiles are designed to balance a target percentage of the applied loads, typically 60 to 80 percent of the dead load. We use banded-distributed layouts for flat slabs and concentrated band layouts for band-beam systems.
Prestress losses
Not all the force applied by the stressing jack reaches the far end of the tendon. Friction between the tendon and the duct, anchorage set (draw-in) at the stressing end, elastic shortening of the concrete, and long-term creep and shrinkage of the concrete all reduce the effective prestress over time. We calculate these losses precisely and account for them in the design so the slab has adequate compression at every stage of its life.
Punching shear and edge details
Flat post-tensioned slabs are susceptible to punching shear failure at column heads, just like conventional flat slabs. The tendons help by providing a vertical component of force at the column, but the punching shear check still needs careful attention. We also detail the anchorage zones at the slab edges where the stressing forces are concentrated, designing bursting reinforcement and edge trim to handle the high local stresses.
Where we use post-tensioning
Post-tensioning is not limited to one building type. It is used wherever the structural demands justify the additional construction complexity, and that covers a wide range of projects.
Commercial floor plates benefit from the longer spans and thinner profiles. Office buildings, retail centres, and mixed-use developments use PT flat slabs to maximise lettable area and minimise floor-to-floor heights. The reduced number of columns gives architects and tenants more flexibility in space planning.
Car parks and podium decks are natural candidates for post-tensioning because crack control is essential for waterproofing performance. A cracked car park deck leaks, and water carrying de-icing salts or contaminants will corrode the reinforcement below. PT slabs stay in compression and stay uncracked, significantly extending the service life of the structure.
Ground slabs on reactive soils use post-tensioning to provide the stiffness needed to resist soil heave and shrinkage without relying on deep stiffening beams. A PT ground slab can be thinner and flatter than a conventionally reinforced raft, reducing excavation and concrete volumes while providing superior crack resistance.
Transfer structures carry heavy point loads from columns above and redistribute them to columns or walls below on a different grid. Post-tensioning gives transfer slabs and beams the additional capacity they need to handle these concentrated loads without becoming excessively deep or heavy.
Standards & Compliance
Every PT design is certified to Australian Standards
All post-tensioned slab designs are prepared in accordance with the relevant Australian Standards and certified by our Registered Professional Engineers (RPE). Documentation includes tendon layouts, stressing schedules, and construction sequence requirements.
How we work with you
We start with the architectural plans and the structural brief, including span requirements, loading, and any constraints on slab depth. We assess whether post-tensioning is the right solution for the project, or whether conventional reinforcement or a hybrid approach would be more appropriate. Not every slab needs post-tensioning, and we will tell you if it does not.
Once the system is confirmed, we develop the full PT design: tendon layout and profiles, supplementary reinforcement, edge and anchorage details, stressing sequence, and construction stage checks. The output is a complete set of structural drawings and a stressing schedule that the PT contractor can work from directly.
We coordinate closely with the PT supplier and the builder throughout construction. Post-tensioned slabs have specific requirements for tendon placement tolerances, concrete strength at stressing, and the sequence of stressing operations. We review stressing records, check elongation measurements against design predictions, and carry out site inspections at critical stages to ensure everything is built as designed.