Design code guide
ACI 318 Chapter 23 — the strut-and-tie method
Chapter 23 is where ACI 318 puts the strut-and-tie method: the design route for the parts of a member where plane sections do not remain plane — deep beams, corbels, pile caps, pier caps, frame corners, anchorage zones. Once you have chosen it, the whole design reduces to three strength checks (strut, tie, nodal zone) plus anchorage.
This page maps the clauses onto the checks a designer actually performs, and states the factor values as they are applied in AStrutTie.
What each clause settles
| Clause | What it settles | Where you meet it |
|---|---|---|
| §23.2 | General requirements — when the method applies, geometry of the truss | Choosing and drawing the model |
| §23.3 | Design strength — φFn ≥ Fu for every strut, tie and nodal zone | Every check in the report |
| §23.4 | Strut strength — effective compressive strength fce, tables for βs and βc | Strut width check |
| §23.5–23.6 | Strut reinforcement — crack-control steel that lets a bottle strut keep the higher βs | Rebar tab, βs selection |
| §23.7 | Tie strength — steel area, placement of the reinforcement about the tie axis | Required area As |
| §23.8 | Tie anchorage — the bar must develop its force within or beyond the nodal zone | Anchorage length check (with Ch. 25) |
| §23.9 | Nodal zone strength — effective strength and the βn table | Nodal-face bearing check |
Strut strength (§23.4)
A strut is checked as an effective compressive strength acting over the strut's width. Written the way it is applied, the required width follows directly from the member force:
| Strut condition | βs |
|---|---|
| Prismatic — uniform field, boundary strut | 1.00 |
| Bottle-shaped, with crack-control reinforcement | 0.75 |
| Bottle-shaped, without crack-control reinforcement | 0.40 |
| Strut in a tension member or tension flange | 0.40 |
- βc = confinement factor, √(A₂/A₁) capped at 2.0 (Table 23.4.3(b)). With no bearing spread it is 1.00 — that is the default.
- The bottle strut is the one that punishes you: without the crack-control steel across it, βs drops from 0.75 to 0.40 and the required width nearly doubles.
- For seismic load combinations AStrutTie applies the additional 0.8 factor on βs and βn (ACI route only).
Tie strength and anchorage (§23.7, §23.8)
A tie is sized so the reinforcement reaches its design yield strength at the ultimate load. The concrete's tensile strength is ignored.
- Bars are placed symmetrically about the line of action of the tie force and run the full length of the tie.
- Anchorage (§23.8.2) is the crux: the bar must develop its full force by the point where the centroid of the tie leaves the extended nodal zone — checked against the development length of Chapter 25 (§25.4).
- Hooks, headed bars and mechanical anchorages are the usual answers when a straight bar will not develop in the space available.
Nodal zones (§23.9)
The nodal zone is the volume of concrete that turns the forces around. Its usable strength drops as more ties frame into it, because the bars must be anchored through the zone and bond cracking softens the concrete.
| Nodal zone | Framing members | βn |
|---|---|---|
| CCC | Compression only | 1.00 |
| CCT | One tie | 0.80 |
| CTT | Two or more ties | 0.60 |
- Each face of the node is checked separately — AStrutTie reports the governing face rather than one global pass/fail.
- Bearing plate size and the extended nodal zone change the available area, so geometry is as much of a lever here as the factor is.
318-19, 318-25 — what changed
AStrutTie runs the strut-and-tie checks to ACI 318-25 §23. The strength values are unchanged from 318-19 — φ = 0.75, fce = 0.85 βs βc f'c, and the same βs and βn tables (verified against both texts, 2026-07-08). The 318-25 revisions to the method are in detailing rather than in the effective strengths, so a model checked under one gives the same strut, tie and nodal results under the other.
Development length (Chapter 25) is a separate matter and was revised — anchorage results can differ between editions.
What AStrutTie checks for you
Every one of these appears in the calculation report with the clause reference and the factor values that were used — nothing is applied silently.
- Strut width against fce = 0.85 βs βc f'c, with the βs condition stated per strut
- Required tie steel As = Fu / (φ fy), and the provided arrangement
- Bearing on every nodal face, with the governing face reported
- Anchorage length at the nodes (§23.8.2 with Chapter 25)
- Shear reinforcement contribution for the crack-control condition on bottle struts
- The seismic 0.8 factor on βs and βn when a seismic combination governs
See it on a real design
Our worked examples take a pier coping, a pile cap and an abutment footing from problem statement to code check, with every number shown.
This page summarises how the code is applied in AStrutTie; it is not a substitute for the code itself. Clause numbers refer to ACI 318-25 §23 (values identical to 318-19). Always design to the edition adopted by your jurisdiction.