Design code guide
AASHTO LRFD §5.8.2 — the strut-and-tie method
For bridge substructure D-regions — pier caps, footings, pile caps, anchorage zones — AASHTO LRFD puts the strut-and-tie method in §5.8.2. The 8th edition (2017) replaced the older §5.6.3 treatment outright, and the change is not cosmetic: strut strength is now written with a concrete efficiency factor ν that depends on concrete strength and on whether crack-control reinforcement is present.
AStrutTie implements the 8th-edition route. Here is what that means in practice.
Limiting compressive stress (§5.8.2.5)
The usable concrete stress at a node face or in a strut is written as the concrete strength times two factors — a confinement modifier and an efficiency factor:
- ν is the concrete efficiency factor. It is not one number — it depends on the face being checked and on the reinforcement.
- Without crack-control reinforcement meeting §5.8.2.6, ν = 0.45 on every face. That is the single biggest penalty in the method.
The efficiency factor ν (Table 5.8.2.5.3a-1)
The strut-to-node interface expression is the one that surprises people: it falls as concrete strength rises. At about 4 ksi (28 MPa) it is already at the 0.65 ceiling; by roughly 8 ksi (55 MPa) it has dropped to the 0.45 floor. Higher-strength concrete does not buy proportionally more strut capacity here.
| Face being checked | Node type | ν |
|---|---|---|
| Bearing face · back face | CCC | 0.85 |
| Bearing face · back face | CCT | 0.70 |
| Bearing face · back face | CTT | same expression as the strut-to-node interface |
| Strut-to-node interface | all node types | ν = 0.85 − f'c/20 (ksi), bounded to 0.45 ≤ ν ≤ 0.65 |
| Any face | crack-control reinforcement not provided | 0.45 |
Resistance factors (§5.5.4.2)
| Action | φ |
|---|---|
| Compression in strut-and-tie models | 0.70 |
| Tension in strut-and-tie models — reinforced concrete | 0.90 |
- The split is wider than ACI's uniform 0.75: compression is treated more conservatively, tension less so.
- Consequence in practice — moving the same model from ACI to AASHTO usually makes struts and nodes govern more often, and ties less often.
Crack-control reinforcement (§5.8.2.6)
The reinforcement requirement is not a detailing afterthought — it is the gate to the higher efficiency factors. Provide it and you use the table; omit it and every face falls to ν = 0.45, which is close to halving the strength of a strut-to-node interface that would otherwise sit at 0.65.
- AStrutTie carries the crack-control condition per member, with a project-level default, so a partially reinforced region is not silently given the better factor.
- When any strut framing into a node lacks the reinforcement, the node is treated conservatively.
7th edition vs 8th edition
The 7th edition's §5.6.3 used a different formulation for strut strength. It was superseded, not amended — an old spreadsheet built on it will not reproduce 8th-edition results, and the difference is not a small factor. AStrutTie implements the 8th edition (2017) §5.8.2 route only, and the calculation report states the clause it used.
What AStrutTie checks for you
Each check names its clause and prints the factors it used — including which ν branch applied and why.
- Strut and node face stress against fcu = m ν f'c, with the ν branch stated per face
- The strut-to-node interface expression evaluated at your concrete strength, with the 0.45 / 0.65 bounds shown
- Crack-control reinforcement condition per member, and the conservative fallback when it is absent
- Required tie steel with φ = 0.90 and strut/node checks with φ = 0.70
- Anchorage of ties at the nodes
Bridge D-regions, worked through
Pier copings, pile caps and abutment footings — the same members AASHTO §5.8.2 is written for — designed step by step.
This page summarises how the specification is applied in AStrutTie; it is not a substitute for the specification. Clause numbers refer to AASHTO LRFD Bridge Design Specifications, 8th edition (2017).