Post Tensioned Cables
The Invisible Muscle: Engineering the Weightless Slab
In modern residential architecture, we often chase the "impossible" cantilever—those thin, gravity-defying planes that define the work of John Lautner or Pierre Koenig. But to achieve that aesthetic of weightlessness, the structure has to work twice as hard.
This is where Post-Tensioned (PT) Cables come in. Unlike standard rebar, which is passive, PT cables are an active system. We aren't just reinforcing the concrete; we are putting it under immense compression to make it behave like a rigid, high-strength spring.
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| Concrete Deck w/ Electrical Boxes Installed |
The Anatomy of the Deck
The process begins with a meticulous layout. On this project, we used a board-form finish inlay on top of the plywood decking. This ensures that the underside of the slab—the "ceiling" for the floor below—retains the organic texture of wood grain once the forms are stripped.
Before the first cable is run, hundreds of electrical boxes and conduits must be precisely located. In a PT slab, you get one shot; once those cables are stressed, drilling into the concrete is a high-stakes gamble.
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| Post Tension Cable Rolls |
The Installation: A Steel Network
The cables arrive in massive rolls, greased inside plastic sheaths to allow for movement. We use encapsulated anchors at the edges to provide a watertight seal, preventing the "Achilles heel" of PT systems: cable corrosion.
The layout looks like a complex, woven basket. In this slab, the depth varies, meaning the cables must "drape"—rising high over the columns (where the tension is needed at the top) and drooping low in the center of the spans (where it’s needed at the bottom). It is a calculated "wave" of steel designed by the structural engineer to counteract the weight of the building itself.
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| Post Tensioning Anchors along the edge of the form |
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| Post Tensioned Installation |
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| Post Tensioning Layout |
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| Post Tensioning Wedges |
The Post Tensioning Wedges are installed together around the cable and inside the cone openings at the edge of the tennis court. If you look closely at the wedges you can see that they have teeth that allow the steel part to grip the cable. The teeth are angled in a way that the cable can easily be pulled past the wedges but when the cable is released the teeth cinch against the cable and the wedge shaped part is forced strongly against the embedded anchor. The excess cable now can be torched off and the post tensioned force is now fully engaged onto the slab. The force is so strong against the concrete slab that there is measurable shrinking in the slab as well as lift provided by the cables.
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| Post Tensioning Wedges Installed |
The cable at the edge of the slab is marked with spray paint and will be used for measuring the post tensioning when its been stressed. Each cable is calculated on the shop drawing to have a specific amount of pulled length that will be measured by an inspector for approval.
The Stressing: 33,000 lbs of Force
Once the concrete cures to a specific strength, the magic happens. We use a hydraulic stressing jack to pull each tendon.
When that jack engages, it pulls the cable with approximately 33,000 lbs of force. You can actually see the slab "lift" off the forms as the internal tension takes over. We measure the "elongation"—literally how much the steel stretched—to ensure the slab is performing exactly as modeled.
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| Stressing the Post Tensioned Cables |
The hydraulic stressing jack is inserted into the cones and sets the wedges as it grips down around the cable and pulls forcing the wedges forward into the anchor and the cable further out. The Jack is powered by a hydraulic pump and is engaged by the operator until it reaches proper stressing pressure. The jack then returns to its beginning position and the second operator moves to the next cable.
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| Inspector documenting proper stressing per each tendon. |
After the cables are stressed and inspected for proper elongation length and confirmed at approximately 5" per 60' cable strand. The tendon is then cut off close to the back of the wedge so a plastic encapsulating cap packed with grease can be fitted over the leftover tendon and exposed wedge. The cone pocket at the edge is filled with grout with corrosion inhibiter. Lastly a bituminous layer of waterproofing is painted over the entire edge to ensure protection.
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| Post Tensioned recessed cones filled with concrete grout |
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