Concrete · Slabs & post-tensioning
Post-tensioning friction & draw-in losses calculator
Force along a parabolic tendon from curvature and wobble friction, and the length affected by anchorage draw-in.
Standard: AS 3600 Cl 3.4.2 (friction) · anchorage draw-in
Inputs in the worked example
The live calculator starts with these values; change any of them and every result, check and diagram updates instantly.
| Input | Value | Unit |
|---|---|---|
| Jacking force | 1500 | kN |
| Tendon length (single end stressed) | 24 | m |
| Drape (sag) | 350 | mm |
| Friction curvature coefficient μ | 0.2 | |
| Wobble βp | 0.016 | rad/m |
| Draw-in | 6 | mm |
| Tendon modulus | 195000 | MPa |
| Tendon area | 1100 | mm² |
Worked example — results
| Result | Value | Unit |
|---|---|---|
| Total angle change 8e/L | 0.117 | rad |
| Force at dead end P0 e^−μ(α + βpL) | 1,357 | kN |
| Friction loss at dead end | 9.53 | % |
| Loss rate per metre | 5.96 | kN/m |
| Draw-in affected length √(Ep Ap δ/p) | 14.70 | m |
| Force at anchorage after draw-in | 1,325 | kN |
- ✓ Friction loss below 20%
- ✓ Draw-in length within tendon
Method, formulas and assumptions
- Total angle change 8e/L = 8*350/1000/24 = 0.11667 rad
- Force at dead end P0 e^−μ(α + βpL) = 1500*EXP(-0.2*(0.11667+0.016*24)) = 1357.1 kN
- Friction loss at dead end = 100*(1-1357.1/1500) = 9.5283 %
- Loss rate per metre = (1500-1357.1)/24 = 5.9552 kN/m
- Draw-in affected length √(Ep Ap δ/p) = SQRT(1.950e+5*1100*6/5.9552)/1000 = 14.701 m
- Force at anchorage after draw-in = 1500-2*5.9552*MIN(14.701,24) = 1324.9 kN
Related concrete calculators
- One-way slab — Bending steel, bar spacing and deemed-to-comply span/depth for a one-way slab.
- Two-way slab — Moments in both directions for a rectangular slab simply supported on four edges.
- Composite slab (steel deck) — Positive moment capacity of a composite slab with the profiled deck acting as reinforcement, vs factored…
- Prestressed beam fibre stresses — Top and bottom fibre stresses at transfer and service for a rectangular section with eccentric prestress.
- Slab punching shear (AS 3600) — Critical shear perimeter at dom/2, concrete capacity fcv, and moment-transfer reduction for an interior…
- Post-tensioned flat slab: load balancing, stresses, punching — Interior panel of a bonded post-tensioned flat plate. The tendon drape balances a chosen share of the…
- Load balancing (parabolic tendon) — Equivalent upward load wp = 8Pe/L², balanced fraction of the permanent load, and average precompression.
For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.