Concrete · Slabs & post-tensioning
Load balancing (parabolic tendon) calculator
Equivalent upward load wp = 8Pe/L², balanced fraction of the permanent load, and average precompression.
Standard: Prestressed concrete load balancing (Lin)
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 |
|---|---|---|
| Effective prestress force | 1200 | kN |
| Drape | 300 | mm |
| Span | 12 | m |
| Permanent load to balance | 25 | kN/m |
| Section area | 400000 | mm² |
Worked example — results
| Result | Value | Unit |
|---|---|---|
| wp = 8 P e/L² | 20.00 | kN/m |
| Balanced fraction | 80.00 | % |
| Average precompression P/A | 3.00 | MPa |
- ✓ Balance 60–100% of permanent load
- ✓ Precompression 1.0–3.5 MPa (typical slabs/beams)
Method, formulas and assumptions
- wp = 8 P e/L² = 8*1200*300/1000/12^2 = 20 kN/m
- Balanced fraction = 100*20/25 = 80 %
- Average precompression P/A = 1200*1000/4.000e+5 = 3 MPa
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…
- Post-tensioning friction & draw-in losses — Force along a parabolic tendon from curvature and wobble friction, and the length affected by anchorage…
For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.