Free Steel Brace Frame Calculator — CBF/SCBF Design
Design concentrically braced frames (CBF) and special concentrically braced frames (SCBF) for seismic and wind loading. The calculator checks brace section compactness, slenderness limits, expected strength, gusset plate stability, and connection overstrength per AISC 341-22, AS 4100 Section 8, EN 1993-1-1, and CSA S16 Section 27.
Typical configurations: X-bracing, chevron (inverted V), single-diagonal, and two-story X-bracing. Brace sections are typically HSS, WT, double-angle, or wide-flange shapes.
How to Use
- Select brace type (SCBF, OCBF, or wind-only frame).
- Enter brace geometry: length, angle, section properties.
- Define seismic parameters: R factor, overstrength factor Omega_o.
- Calculate required brace strength and expected brace strength.
- Check gusset plate: Whitmore section, block shear, yield line.
- Verify connection overstrength meets AISC 341 requirements.
Design Code Requirements
| Check | AISC 341-22 | AS 4100 | EN 1993-1-1 | CSA S16 |
|---|---|---|---|---|
| Brace slenderness (kl/r) | F2.5a (≤200 SCBF) | Cl 8.4 | Cl 6.3.1 | Cl 27.2 |
| Brace compactness | D1.1 (SCBF Seismic) | Cl 8.3 | Table 5.2 | Cl 27.3 |
| Expected brace strength | A3.2 (RyFyAg) | Cl 8.5.2 | Cl 6.2 | Cl 27.5 |
| Connection overstrength | F2.6 (1.1RyFy*Ag) | Cl 8.7.3 | Cl 6.2 | Cl 27.6 |
| Gusset plate stability | F2.5b (CJP at corner) | Cl 8.7.5 | Cl 6.2 | Cl 27.7 |
Brace Slenderness Limits (AISC 341-22)
| Frame Type | Max kl/r | Compactness | Expected Strength |
|---|---|---|---|
| SCBF (Seismic) | 200 (200 preferred ≤120) | Highly ductile | RyFyAg |
| OCBF (Moderate) | 200 | Moderately ductile | RyFyAg |
| CBF (Wind-only) | 300 | N/A | Fy*Ag |
Frequently Asked Questions
What is the difference between SCBF, OCBF, and ordinary braced frames? SCBF (Special Concentrically Braced Frame) has the strictest ductility requirements per AISC 341 — braces must be highly ductile sections with maximum slenderness kl/r = 200 and width-to-thickness ratios meeting highly ductile limits. OCBF (Ordinary CBF) permits moderately ductile sections with slightly relaxed limits. Wind-only braced frames have no seismic ductility requirements.
What is the Whitmore section for gusset plates? The Whitmore effective width is the width of the gusset plate that is effective in resisting the brace force, determined by projecting 30-degree lines from each side of the connection length. The Whitmore section is used to check gusset plate yielding and buckling. For standard gusset plates, the Whitmore width typically exceeds the actual plate width, so gross section yield governs.
What is expected brace strength (RyFyAg) and why is it used? Expected brace strength accounts for the fact that actual yield strength exceeds specified minimum yield. Ry is the ratio of expected yield to specified minimum yield (typically 1.1-1.3 for common steels). The connection must be designed for the expected strength (or 1.1RyFy*Ag for SCBF connections per AISC 341 Section F2.6) to ensure the brace yields before the connection fails.
Is this brace frame design calculator free? Yes, completely free with unlimited calculations.
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Disclaimer (educational use only)
This page is provided for general technical information and educational use only. It does not constitute professional engineering advice. All structural designs must be verified by a licensed Professional Engineer (PE) or Structural Engineer (SE). The site operator disclaims liability for any loss or damage arising from the use of this page.