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How to use the post-tensioned slab calculator

One strip of a PT flat slab: what each input means, how to read the losses and the balanced load, and why the jacking check reads 100 %.

All guides Inputs Result Calc sheet Walk-through Mistakes

1. What the calculator does

The post-tensioned slab strip calculator designs one strip of a post-tensioned flat slab between column lines. From the strands it works out the jacking force and the losses, the precompression and the load the tendons balance, then checks the service stresses, the ultimate moments, deflection, the bonded top bars at the columns, punching at the columns and the floor’s natural frequency.

The code routes are SANS 10100-1 + ACI 318-19, Eurocode 2 (EN 1992-1-1) + ACI 318-19 and SANS 51992-1-1 (SA Eurocode, 2023 draft annex) + ACI 318-19. The badge says which: SANS 10100-1 section 5 for prestressed concrete, with ACI 318-19 rules for two-way prestressed slabs where the SANS text has none. The design result names the route per check (for example “punching links (SANS route)”).

PT slab strip calculator header with the PASS badge — The header: PASS 100 %, governed by the jacking force ≤ 0.75 f_pu.

2. The inputs, group by group

Input	What it means	Typical value
Spans	Column centres for up to three spans (0 = no third span).	8000, 8000 mm
Slab	Depth h and the panel width l₂ the strip carries.	250 mm, 8000 mm
Tendons	Strand size, strands per metre and whether stressed from one end or both.	12.9 mm, 4 per metre, one end
Loads	Superimposed dead and imposed load, characteristic.	1.5 and 2.5 kN/m²
Concrete	f_cu and the strength at transfer f_ci; the stress class (no tension, or limited tension).	35 / 25 MPa, class 2
More inputs	The P/A from the tendons in the other direction, humidity, partitions, fire rating and a second direction.	as needed

PT slab inputs: spans, slab, tendons, loads and concrete — The inputs. The panel width is the width of slab the strip carries, usually the column spacing across.

3. How to read the result

The sketch shows the strip on its columns with the load, the hogging and sagging moments and the column reactions.

PT slab strip sketch with the moments and the column reactions — The slab strip: two 8 m spans, M_hog 75.0 and M_sag 52.9 kNm/m, reactions 376 / 779 / 376 kN.

Read the design result from the top: the force per strand at jacking, after transfer and after all losses (P₀ / P_i / P_e), the losses and the precompression P/A, then the balanced load as a share of the self-weight. Balancing about the self-weight is a common target. Then the ULS moments against the resistance, the deflection, the bonded top bars over the columns and the punching links.

Design result card for the PT slab strip — Design result: 139.5 / 127.4 / 115.6 kN per strand, 17.1 % losses, P/A 1.85 MPa, balanced load 6.50 kN/m² (108 % of self-weight), 2.4 mm deflection, 14 Y12 top bars at the columns.

The badge names the governing check. Here it is the jacking force at 100 %: the strands are stressed to the jacking limit (0.75 f_pu) on purpose, so this check sits at 100 % in most designs. Read the next one down in the checks once signed in. Anything the engine could not run shows as NOT CHECKED.

Design checks summary for the PT slab strip — Design checks: SANS 10100-1 · ACI 318-19 · 24 pass.

4. Calc sheet and schedule

The result (pass / fail and utilisation) stays free for everyone. Every check with its clause, the calc sheet and the bending schedule are open to signed-in users. Sign in free: 3 designs a month (one calculator, open for 24 hours). Unlimited with a calculator pack.

The schedule card reads: “Sign in (free) to get the bending schedule and the tendon profile: 3 designs a month”, with CSV on a paid plan. The full detail drawings are part of the paid plan.

The bending schedule and tendon profile card.

5. A 2-minute walk-through

Open the calculator: two 8 m spans, 250 mm slab, four 12.9 mm strands per metre, stressed from one end.
Read the precompression (1.85 MPa) and the balanced load (108 % of the self-weight).
Drop to three strands per metre and watch the balanced load and the service stresses.
Stress from both ends and see the losses fall.
Set the P/A in the other direction under More inputs, as the two-way checks need it.

6. Common mistakes

Reading the jacking check as a problem. 100 % there is normal.
No P/A in the other direction. Two-way checks such as punching use the precompression both ways.
Panel width as the strip width. It is the width of slab between the column lines across.
Forgetting transfer. f_ci is the strength on the day the strands are stressed, lower than f_cu.

8. Learn more

Try it nowThe calculator opens with the walk-through inputs above. The result is free; sign in for the checks, calc sheet and schedule.
Open the PT slab calculator

A guide to using the calculator, not a design. Check every value against the current code and your own project, and have the design checked and signed by a registered professional engineer.

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