Seismic detailing
A special moment frame is not designed by stress — it is detailed so that it can deform without losing strength. The provisions are geometric and proportional: a minimum beam width, a maximum reinforcement ratio, a confinement spacing tied to the bar diameter, a strong-column/weak-beam ratio and a joint shear limit. A frame can pass every stress calculation and still fail them.
| Check | Value | Requirement |
|---|---|---|
| Beam width | 400 mm | ≥ 0.3h = 180 mm and ≥ 250 mm |
| Proportions | Pass | §18.6.2.1 |
| Reinforcement ratio | 0.0083 | ≤ 0.025 |
| Steel ratio | Pass | §18.6.2.1 |
| Hoop spacing | 135 mm | min(d/4, 6db, 150) |
| Strong-column ratio | 1.60 | ≥ 1.2 |
| SCWB | Pass | §18.7.3.2 |
| Joint coefficient γ | 15 | 3 confined faces |
| Joint capacity | 1.676e+4 kN | 0.85 γ √f’c Aj |
| Joint demand | 900 kN | 5% utilised |
| Joint shear | Pass | §18.8.4.1 |
All four detailing checks pass. The hoop spacing over the hinge is 135 mm, set by d/4, and the strong-column ratio is 1.60.
bw ≥ 0.3h and ≥ 250 mm · §18.6.2.1.
As/(bw h) ≤ 0.025 · §18.6.2.1.
s ≤ min(d/4, 6db, 150) · §18.6.4.4.
ΣMnc ≥ 1.2 ΣMnb · §18.7.3.2.
A clear path from inputs to results.
A special moment frame is not designed by stress — it is detailed so that it can deform without losing strength. The provisions are geometric and proportional, and a frame can pass every stress calculation and still fail them.
How to use
- Enter the beam dimensions and reinforcement.
- Set the concrete strength.
- Add the column and beam moment capacities at the joint.
- Read the strong-column ratio and the joint shear utilisation.
The formula
Reference: ACI 318-19 Chapter 18: §18.6.2, §18.6.4, §18.7.3 and §18.8.4.
Worked example
A 400 × 600 mm beam at a three-face joint
- 400 mm wide, 600 mm deep, d = 540, 25 mm bars.
- 2,000 mm² of steel, 30 MPa concrete.
- ΣMnc 800 kN·m, ΣMnb 500 kN·m, 900 kN joint shear.
- 1
The proportions pass: 400 ≥ 0.3 × 600 and ≥ 250 mm.
- 2
The reinforcement ratio is 2,000/(400 × 600) = 0.0083, inside the 0.025 limit.
- 3
The hoop spacing is min(135, 150, 150) = 135 mm, set by d/4.
- 4
The strong-column ratio is 800/500 = 1.6, above 1.2.
- 5
The joint capacity with γ = 15 is 16,760 kN against 900 kN of demand.
All four checks pass · 135 mm hoops · 5% joint utilisation.
Interpreting the result
The strong-column/weak-beam ratio is the most important of the four. If it fails, the frame forms a storey mechanism instead of beam hinges, and a storey mechanism is a collapse rather than a repair.
Assumptions & limitations
- A special moment frame in a high seismic category.
- A single joint, with the moment capacities given.
- No slab contribution to the beam capacity.
- No axial load effect on the joint capacity.
Frequently asked questions
Why 1.2 on the strong-column ratio?
Because the beam is expected to overstrength by about 20% through strain hardening, so the column has to be stronger than the nominal beam capacity to stay elastic.
Why is the hoop spacing so tight?
Because the plastic hinge needs confinement to keep the core concrete from spalling, and the spacing is what provides it. min(d/4, 6db, 150 mm) is the rule.
What about a beam-column joint with no beam on one side?
That is an exterior joint, confined on fewer faces, so γ is lower and the joint capacity falls — often the governing check.
What is γ?
The joint shear coefficient, which depends on how many faces of the joint are confined by beams: 20 for four, 15 for three, 12 for two.
Does an ordinary frame need this?
No. Ordinary and intermediate moment frames have much less onerous detailing, and are permitted in lower seismic categories.
Methodology last reviewed 2026-10-22. Results are informational and not financial or engineering advice — see our disclaimer.