Draped post-tensioning tendons impose a calculable upward equivalent load on a slab, and the basic load-balancing equation w = 8·P·d / L² remains the working design tool engineers apply to two-way flat plates in 2026 [S3][S6].
For a 26-kip effective prestress per tendon on 5 ft centers, with a parabolic drape of 5 in over a 22 ft clear span, the upward balanced load is 8 × 26 × 5 / (22 × 22) ≈ 2.15 kip/ft across the 5 ft strip, which works out to about 0.43 kip/ft² (≈43 psf) of upward distributed load; this is the figure designers subtract from dead load before sizing for service stresses [S6].
Three PT design methods, one balanced-load equation
Load balancing is one of three structurally accepted approaches to post-tensioned member design, alongside the rigorous method and the straight method; load balancing is the most common for two-way building slabs because it reduces the problem to familiar flexure analysis of a net (unbalanced) load case [S2].
The method was popularized by T. Y. Lin in the early 1960s and translates a parabolic tendon profile into a uniform uplift per unit length; in a two-way slab, the orthogonal draped tendons add their contributions, and the design only has to handle whatever gravity load exceeds the combined upward force [S2][S3]. A separate, more granular reality check sits alongside: hyperstatic (secondary) moments from indeterminate action must still be computed and added to the service load combination U = 1.00 DL + 1.00 PT + 1.00 LL, otherwise the design is not code-compliant even when the equivalent load looks right [S2]. For two-way systems, the balanced load per unit area is w_bal = w_px + w_py, with each direction's contribution following w_p = 8·P·d / L² where P is the prestress force per unit width [S3].
Drape, cover, and the numerical inputs that drive the answer
Drape is the vertical offset between the tendon high point and low point inside the slab; in a typical first-principles example the simply supported beam case uses e1 = 0 at the supports and e2 = 550 mm at midspan, giving a 550 mm drape that the engineer then converts into a uniform equivalent uplift [S1].
Bottom cover drives the available drape in a real slab: post-tensioned slab design guidance calls for 3/4 in (≈19 mm) minimum bottom cover on restrained interior spans and 1-1/2 in (≈38 mm) on unrestrained exterior spans, while the top of the tendon typically runs about 3/4 in below the top mat at midspan to maintain fire-rating cover [S5]. Standard mono-strand unbonded tendons for this work are 1/2 in diameter, 270 ksi, area 0.153 in², anchored at each end by a ductile iron plate roughly 2-1/4 in × 5-1/4 in that transfers 33 kip per tendon into the concrete [S5]. The ACI 318-05 Equivalent Frame Method remains the manual analysis tool of choice for these two-way flat plates, with ACI 318 Section 13.7 governing the equivalent-frame procedure (Section 13.7.7.4-5 excluded) [S5].
Layout choice: banded, distributed, and the 2023 dual-banded option
Traditional U.S. practice since the 1950s has tendons "banded" together in one direction and "uniformly distributed" in the orthogonal direction, with 60–75% of the prestress placed in the column strip and the balance in the middle strip, and average effective precompression P/A held between 200 and 250 psi (1379–1724 kN/m²) [S4].
PTI Technical Note 22, issued in October 2023, formally recognizes a "dual-banded" layout in which both directions are banded at the column strip with no uniformly distributed tendons; the note frames this as permissible under 2021 IBC Section 104.11 or ACI 318-19 Section 1.10, since ACI 318-19 does not explicitly address the dual-banded case [S4]. The earlier distributed-banded approach dates to the 1968 Watergate Apartments project in Washington, DC, and was the first known U.S. flat plate to use the layout; the University of Texas at Austin ran the first multi-panel tests on it in the early 1970s, with Slab I (one-third scale, 70/30 column-to-middle strip distribution) and Slab II as the reference specimens [S4].
Load-balancing selection: who should use it, who should not
Load balancing is the right tool for routine two-way slab buildings where the engineer can compute hyperstatic moments and the design's main drivers are deflection, crack control, and P/A service stress; it is also widely used for simple determinate beams where drape, span, and prestress force are easy to enumerate [S1][S2].
For large or irregular projects where economy matters and cracking or post-cracking response needs to be tracked closely, the rigorous method is preferred even though it costs more computational effort; engineers who only occasionally design post-tensioning tend to hand the work to specialists rather than run load balancing themselves [S2]. The straight method is the fallback when a designer only needs to verify a single member inside a non-PT project; it does not exploit load balancing, but uses the same flexure and stress checks the engineer already knows [S2]. A practical sizing rule: place all the balance in the short-span direction (w_bal = w_py) to minimize prestressing steel, but distribute the prestress proportional to load distribution once you start checking unbalanced service cases, otherwise the slab will crack in the long-span direction [S3].
Common service checks and failure-mode pitfalls
Service-level checks compare the residual unbalanced load (DL + LL minus the tendon equivalent uplift) to allowable stresses; at the ultimate limit state, the tendons go back in to resist demand forces, and the design cannot be based on the missing-tendon model from load balancing [S2].
Anchor-zone detailing is one of the most common service-life failure sources in unbonded mono-strand slabs: a single 1/2 in 270-ksi tendon drops 33 kip into a 2-1/4 in × 5-1/4 in plate, so consolidation, location accuracy, and supplementary anchorage-zone reinforcement are not optional [S5]. Imperfect load balancing is the rule rather than the exception in real structures, because superimposed dead load and live load are rarely perfectly uniform; any unbalance still has to satisfy the service stress envelope, and visible cracking in the field is the typical symptom when it does not [S3]. Engineers should not assume that load balancing covers hyperstatic moments automatically: the U = 1.00 DL + 1.00 PT + 1.00 LL service combination is necessary but not sufficient, and ACI 318 prescribes the secondary-moment computation that must be added before the member is code-compliant [S2].
What to track next: codes, tendon cover, and alternative cabling
Track the ACI 318-19 dual-banded provisions as they move through code cycles; PTI Technical Note 22 (October 2023) is the current PTI reference and any update to the underlying ACI sections would supersede its framing [S4].
For procurement or detailing questions, compare the standard 1/2 in 270-ksi mono-strand layout against larger 0.6 in strand retrofits when slab thickness or column spacing is being optimized; the same Equivalent Frame Method under ACI 318 Section 13.7 governs both, but anchor-zone reinforcement scales with the per-tendon force, which is roughly 33 kip for 1/2 in at 270 ksi and 48 kip for 0.6 in at the same grade [S5]. On the equipment side, drum-and-spool stressing jacks and the corresponding load cell calibrations used to verify jack force during stressing are part of the same quality chain that load-balancing calculations depend on, and any new spec should reference the actual jack certificate rather than the nominal strand force. The load cell module used in the calibration bench and the electronic load on the QC rack both need the same traceability chain that the post-tensioning design relies on for its 33-kip anchor transfer figure.
Related analysis: Scaffold Tower Castor Load Ratings and Total-Lock Brake Selection: 2026 Spec Map.