Post tensioned concrete roof

A post tensioned roof is truly the icing on the cake to gain "Fortress" construction and unprecedented security for the occupants. A high strength steel cable, called a "tendon", encased in a lubricated sleeve, follows the path of tension, being stretched, in the concrete slab.  The tendon is then pulled tight with a hydraulic jack to a specified amount of tension (squeezing, compressing the concrete, to cancel out the tensile force.  A well executed pt slab is water tight because of there being no cracks.  People don't realize that tension rebar doesn't contribute anything until the concrete cracks.  The that point the tensile strength of concrete becomes zero and the entire tensile load is carried by the concrete.  It's an interesting image to understand.

I am a certified installer and inspector for post tensioning.  It's really no big deal.  Anchorage and proper placement of the tendons is everything, along with the timing.  There's less reinforcement to install, a thinner slab (less concrete), more savings, and a perfect cover (diaphragm) tieing all the walls together for a potent monolithic response to catastrophe..

Example:

Concrete: 4,000 psi, fy steel rebar 60,000 psi., tendon 270,000 psi

Slab 22' x 16' , beam supported all four sides

Can a 7" thick concrete wall withstand 400 psf of E5 tornado?

Yes, a 7" thick concrete wall can comfortably withstand ~400 psf (pounds per square foot) of distributed pressure. This equates to approximately 2.77 psi or 20 kPa. Concrete's high compressive strength allows it to easily hold this load, provided the wall is properly reinforced with steel and correctly loaded. 

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Key Factors for Success

• Orientation of the Load: If the 400 psf is an axial load (weight pushing straight down from above), a 7" poured concrete wall has immense load-bearing capacity and will easily support it. 

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• Lateral Pressure (e.g., Soil or Wind): If the 400 psf is a lateral load (such as soil pushing against the side of a basement wall), the 7" wall can still resist it, but it will rely entirely on the placement of rebar. Unreinforced concrete is weak in tension (bending), so steel reinforcement is required to handle the sideways pressure. 

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• Concrete PSI: Standard structural concrete typically has a compressive strength ranging from 3,000 to 4,000 psi, which is more than enough to handle this pressure. 

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For specific structural guidelines, consider reviewing the International Code Council Foundation Requirements or consulting a local structural engineer. 

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To help determine if this setup is safe for your project, let me know:

• Is the 400 psf acting as an axial load (pushing down) or a lateral load (pushing sideways)?

• What is the height and unsupported length of the wall?

• Is there steel rebar in the wall, and if so, what is its size and spacing?

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8 feet high walls tied into postensioned concrete slab roof

An 8-foot-high, 7-inch-thick concrete wall tied into a post-tensioned (PT) concrete slab roof can safely withstand 400 psf [1]. Tying the wall into the PT roof slab restricts movement at the top, significantly increasing the wall's strength against both vertical and sideways forces.

Scenario A: 400 psf Lateral Load (Sideways Pressure)

If the 400 psf is pushing against the side of the wall (like soil or heavy wind), the wall acts as a vertical beam supported at the top (by the PT slab) and the bottom (by the foundation).

• Bending Stress: An 8-foot span is relatively short, keeping bending forces low.

• Rebar Requirement: Standard steel rebar (such as #4 bars spaced every 8 to 12 inches) placed on the inside face of the wall will easily handle the tension caused by a 400 psf lateral load.

Scenario B: 400 psf Roof Load (Vertical Weight)

If the 400 psf represents the total design load of the post-tensioned roof slab pressing down on the wall:

• Axial Capacity: A 7-inch-thick solid concrete wall has a massive vertical crushing capacity (often exceeding 90,000 lbs per linear foot).

• Slenderness: An 8-foot height means the wall is stiff and highly resistant to buckling under vertical weight. Even with a large roof span transferring weight to the wall, 400 psf on the slab is well within safe limits.

Critical Engineering Details

• The PT Connection: Post-tensioned slabs store immense energy. The rebar dowels tying the wall to the PT slab must be properly detailed so the slab's horizontal shrinkage (shortening from post-tensioning) does not crack the top of your concrete wall.

• Concrete Mix: Ensure a minimum compressive strength of 3,000 to 4,000 psi.

To finalize your design safety, let me know:

• What is the span or tributary width of the post-tensioned roof slab?

• Are you building a basement/retaining wall, or is this an above-ground structure?

max span 22 feet above ground

Yes, an 8-foot-high, 7-inch-thick above-ground concrete wall can safely support a 22-foot span post-tensioned roof slab carrying a 400 psf design load. Mathematically, the wall's ultimate structural capacity is roughly seven times greater than the maximum gravity load this roof configuration will transfer to it.

1. Calculate Maximum Applied Vertical Load

To find the maximum load hitting the wall, we calculate the load per linear foot (plf) using the worst-case tributary width. If the 22-foot slab spans entirely to this wall on one side:

2. Calculate Wall Compressive Capacity

Using the standard American Concrete Institute (ACI 318) Design Equations, we evaluate the design strength (

) of a 7-inch wall under standard 3,000 psi concrete. The equation accounts for both the thickness (t = 7 in) and the height stiffness reductions (h = 96 in):

Because the design capacity (67,885 plf) is vastly higher than the maximum potential roof load (8,800 plf), structural crushing or buckling is not a risk.

3. Handle Post-Tensioned Stress & Lateral Loads

Because the wall is above ground, it will not experience soil pressure. However, two critical engineering factors must be managed:

• PT Slab Shortening: As the tendons in the roof slab are stressed, the slab will physically contract horizontally. Because your wall is rigid and tied to the roof, this contraction creates severe shear and bending stresses at the top joint. Engineers typically use slip joints or flexible dowel configurations to prevent the wall from cracking during post-tensioning.

• Wind Forces: The 7-inch wall will act as a shear wall to resist sideways wind pressures acting against the building.

✅ Structural Verification Summary

The 7-inch concrete wall is structurally adequate. The vertical roof load delivers 8,800 plf to the wall, which is well below the wall's safe design limit of 67,885 plf.

If you would like to review further, let me know:

• Will this wall function as a shear wall to resist lateral wind or seismic loads for the building?

• Do you already have a rebar detailing layout scheduled for the wall-to-slab connection?

That is the correct way to handle a post-tensioned (PT) building envelope. Wrapping the 3" polyiso over the slab edge once the tendons are stressed and anchored provides a continuous, unbroken thermal barrier that completely eliminates thermal bridging.

Because you are attaching cladding over the stressed PT anchor zone, there are a few critical structural and waterproofing steps you must follow during installation.

1. Protect the PT Anchor Pockets

Before applying the polyiso insulation over the slab edge, you must properly seal the PT tendon tails and anchoring pockets:

• Grout the Pockets: Cut the excess tendon tails back inside the anchor pocket. Coat the pocket with a rust-preventative bonding agent, and pack it tightly with a high-strength, non-shrink structural grout.

• Waterproofing Cap: Apply an elastomeric waterproofing membrane over the grouted pockets. This prevents any moisture that gets past your cladding from reaching the steel tendons, which would cause catastrophic corrosion over time.

2. Fastening Cladding Over the PT Zone

Fastening your exterior cladding through 3" of insulation directly into the edge of a post-tensioned slab requires precise execution to avoid damaging the high-tension tendons:

• Map the Tendon Paths: Clearly mark the locations of the PT tendons and anchor heads on the slab edge before covering them with insulation.

• Avoid Mechanical Fasteners in the Anchor Zone: Never drill mechanical concrete anchors or screws directly into or within a few inches of the PT anchor heads. The risk of striking a live tendon is high.

• Alternative Attachment Strategies:

  • Adhesive Backing: Use high-bond, exterior-grade construction adhesives to secure furring strips or rigid insulation to the slab edge in the anchor zones.

  • Structural Framing Bypass: Use vertical furring strips that are anchored into the solid 7" concrete wall below the slab edge and into the structure above the slab edge, letting them safely bridge across the sensitive PT zone without hitting it.

3. Maintain Air and Moisture Continuity

• Tape the Insulation Seams: Use a premium flashing tape to seal the horizontal seam where the wall polyiso meets the roof slab polyiso. This ensures your building's air barrier remains completely airtight.

• Drainage Plane: Ensure your Water-Resistive Barrier (WRB) or taped polyiso layer allows any incidental moisture to drain down the face of the insulation and escape out through weep holes at the bottom of the cladding.

To help you map out the final construction steps, let me know:

• What specific cladding material (e.g., fiber cement siding, metal panels, or stucco/EIFS) are you using?

• Will your cladding require vertical wood furring strips or a engineered metal clip system for installation?