Post-tensioned slab strip
A post-tensioned one-way slab, or one strip of a two-way slab, by load balancing on continuous spans with overhangs, to ACI 318-19: uplift, hyperstatic moments, service stresses, transfer, strength with fps, minimum steel, shear and deflection. F is entered: you supply the losses, or the tendon supplier does.
Inputs of the slab #
- Slab: one-way, or a strip of a two-way slab. For two-way: Class U at 6 √f'c, 125 psi of precompression, and the bars of 8.6.2.3. The strip's width and its share of the panel come from your equivalent frame: this type does not distribute moment across the panel.
- Spans and tendon: up to 8. e at each support above mid-depth, e at midspan below it. A tendon is a parabola in each span. Reverse curves over supports are not modeled: the small downward kick is left out.
- Overhangs.
- Design section at the supports: the support centerline, prefilled, or the face of support with its width. ACI 318-19 7.4.2.1 permits the face for a slab built integrally with its supports, and does not require it, so the centerline is the safe side. The sheet and the report say which is in use. The example below labels its moments as at the face, yet they agree with centerline values; until that is resolved the lower number is not the default.
- Loads: own weight on by default, superimposed dead, live. Live load runs on all spans, alternate spans and adjacent spans.
- Tendons: unbonded or bonded, strand area, spacing, fse after losses (175 ksi is common for unbonded strand), fpu, γp. f'ci, force at transfer: your estimate of what the losses take.
- Bars: top and bottom per foot at one depth d.
- Deflection limit: span over the value you enter.
Auto-design varies the thickness and strand spacing. Tendon positions are kept.
Checks performed #
| Check | Basis |
|---|---|
| wp = 8 F a / L² | Load balancing |
| M2 = Mpt − F e, linear between supports | 5.3.11 |
| Service tension and class | Table 24.5.2.1: U, T, C one-way; U two-way |
| Service compression | 24.5.4 at 0.60 f'c, all load |
| Transfer | 24.5.3: 0.60 f'ci; 3 √f'ci as an advisory |
| Precompression | 8.6.2.1 two-way; a practice guide (advisory) one-way |
| φMn | 22.2, 20.3.2.3 or 20.3.2.4.1; tendons and bars as one couple |
| Mu | 1.2D + 1.6L or 1.4D, pattern, plus 1.0 M2 |
| Minimum bars | 7.6.2.3 unbonded one-way; 1.2 Mcr bonded (7.6.2.1); 8.6.2.3 two-way |
| One-way shear | 2 λ √f'c b d, simple and on the safe side |
| Deflection | Unbalanced load, uncracked, immediate |
Limitations #
- Losses are not calculated. Enter fse. No friction, seating or long-term loss analysis.
- Class T and C slabs are flagged but their crack control (24.3) and cracked deflection are not checked.
- Not designed: anchorage zones (25.9), tendon spacing, bar lengths, punching (use Punching shear), long-term deflection.
- Uniform loads only, on prismatic spans. No point loads, no drop panels. One-way slabs per strip.
- 0.45 f'c under sustained load is not separated out: the check uses all load at 0.60 f'c.
Verification examples #
Verification status: 23 values compared in 2 examples (2 published, 0 hand calculations from the cited equations): 20 within tolerance, 3 recorded deviations. Thin: only 2 published examples. Code basis: partial. Only some provisions were read in the target edition; the rest rests on older editions, examples or summaries.
| Example | Source | Values |
|---|---|---|
| Four 20 ft spans, 6 in. slab | ACI SP-17(14), One-way Slab Example 3, pp. 65 to 73 | Aps, F, F / A, wp; M2; fps, a, φMn; Mu with M2; Vc |
To ACI 318-14, with moments from a program and rounded by the source, compared at 5 %. The text says 0.25 in. at the exterior support but uses a drape of 3.25 in., which needs 0.5 in.: run at 0.5 in. It runs as stated, with 12 in. supports and moments at the faces (the option, not the default), and the negative moments at the second and middle supports are then about 15 % below the printed values, a documented deviation on the less conservative side: the printed negative moments agree with this type at the centerline, though the source says at the face. Only this one example was run. SP-17 Two-way Example 3 and the 318-19 examples of ACI MNL-17(21) have not been: the two-way route is hand checked only. PTI Technical Notes in the library need OCR.