Steel Braced Frame Design — SCBF, OCBF & Brace Connection Detailing

Concentrically braced frames resist lateral forces through axial tension and compression in diagonal brace members. They provide high stiffness with relatively light members compared to moment frames, making them the most common lateral system in low- to mid-rise steel buildings. AISC 341-22 defines three categories: Special Concentrically Braced Frames (SCBF), Ordinary Concentrically Braced Frames (OCBF), and non-seismic braced frames designed only to AISC 360.

Brace configuration comparison

Braced frames are categorized by how diagonal members are arranged within a bay. The choice of configuration affects lateral stiffness, architectural impact, ductility, and construction cost. The table below summarizes the six most common brace configurations.

Selecting a brace configuration

The choice depends on four competing demands:

1. Structural performance — X-bracing and eccentric bracing offer the highest ductility for seismic applications. Single diagonals are acceptable for wind-governed designs.
2. Architectural compatibility — Chevron and V-bracing allow openings that X-bracing blocks. Single diagonals offer the most flexibility.
3. Construction cost — X-bracing is simplest to fabricate. Eccentrically braced frames are the most expensive due to link detailing requirements.
4. Code restrictions — K-bracing is prohibited in SCBF. Some configurations have additional requirements in high-seismic design categories.

Brace member selection

Brace members are primarily axial members, so their design is governed by compressive buckling capacity. AISC 341-22 Section F2.5a limits the slenderness of SCBF braces:

KL/r ≤ 200    (SCBF, AISC 341-22 Section F2.5a)
KL/r ≤ 200    (OCBF, AISC 341-22 Section F1.5a)

For SCBF, braces must also be compact or nearly so: width-to-thickness ratios must satisfy the highly ductile limits of AISC 341 Table D1.1. Round HSS braces require D/t ≤ 0.053 E/Fy = 30.7 for A500 Gr C (Fy = 50 ksi). Square HSS require b/t ≤ 0.64 sqrt(E/Fy) = 15.4.

Common brace sections: Round HSS (most popular for moderate loads), Square HSS, Wide-flange shapes (for heavy braces), and double angles (for light frames).

Brace design per AISC 360

Tension-only bracing (AISC Chapter D)

In tension-only bracing design, the compression brace is assumed to have buckled and contributes zero resistance. The entire lateral force is resisted by the tension diagonal alone. This simplifies design significantly but results in larger brace forces and higher drifts.

Per AISC 360-22 Chapter D, the tensile capacity is:

phi_t × Pn = phi_t × Fy × Ag     (yielding limit state, phi_t = 0.90)
phi_t × Pn = phi_t × Fu × Ae     (rupture limit state, phi_t = 0.75)

Where Ag is the gross area and Ae is the effective net area at the connection. Tension-only design is most common for wind bracing and non-seismic applications. For seismic design, AISC 341 requires both braces to be designed for compression.

Slenderness limit for tension members: KL/r ≤ 300 is the AISC 360 Table D1 recommended maximum. There is no mandatory limit for tension members, but exceeding 300 increases the risk of sag, vibration, and flutter under wind loads.

Compression bracing (AISC Chapter E3)

For compression braces designed per AISC 360-22 Chapter E, the nominal compressive strength is:

Pn = Fcr × Ag

Where Fcr is determined based on the column slenderness ratio:

• Inelastic buckling (KL/r ≤ 4.71 × sqrt(E/Fy)):

Fcr = 0.658^(Fy/Fe) × Fy

• Elastic buckling (KL/r > 4.71 × sqrt(E/Fy)):

Fcr = 0.877 × Fe

Where the elastic critical stress is:

Fe = pi² × E / (KL/r)²

The resistance factor for compression is phi_c = 0.90.

Effective length factors for brace configurations

The effective length factor K depends on the brace configuration and end conditions:

For X-bracing with out-of-plane restraint at the crossing point (which is typical when the two braces are welded or bolted together at the intersection), the effective length for out-of-plane buckling may be taken as half the full diagonal length, significantly increasing capacity.

Slenderness limits summary

Brace connection design overview

Gusset plate design principles

Gusset plates transfer brace forces to beams and columns through a combination of welds, bolts, and plate bending. The design of gusset plates for braced frames follows these principles:

1. Force path — The brace axial force must be transferred from the brace member, through the gusset plate, into the beam and column flanges or webs via a clear load path.
2. Plate thickness — The gusset plate must resist the applied forces without buckling. For compression braces, the gusset is checked as an unsupported compression element. A common rule of thumb is that gusset thickness should be at least equal to the brace wall thickness.
3. Edge distances and bolt spacing — Minimum edge distances per AISC 360 Table J3.4 and J3.3 must be maintained. For SCBF, bolt holes should be drilled (not punched) to avoid stress concentrations that could initiate fracture.
4. 2t clearance zone — For SCBF gusset plates, a clearance zone equal to twice the gusset plate thickness (2t) must be provided at the end of the brace. This allows the gusset to flex and accommodate brace buckling without tearing.

Whitmore section

The Whitmore section (first described by R. E. Whitmore in 1952) defines the effective width of a gusset plate at the end of a connection. It is used to check for gusset plate yielding and block shear failure.

The Whitmore width (Lw) is constructed by drawing 30-degree lines from the outermost bolts in the first and last rows of the connection. The effective width is measured perpendicular to the brace axis at the last row of bolts:

Lw = bolt spacing + 2 × (row spacing × tan 30°)

The Whitmore section is checked for:

• Tension yielding: phi × Fy × Lw × t ≥ Pu
• Compression buckling: Treat the Whitmore section as an unsupported column with an effective length based on the gusset geometry

Uniform Force Method (UFM)

The Uniform Force Method, formalized in AISC Steel Construction Manual Part 13, is the standard approach for analyzing the forces in brace-to-gusset connections. It distributes the brace force to the beam and column connections based on the geometry of the gusset plate.

Key features of the UFM:

• The brace force is resolved into horizontal and vertical components at the beam-column intersection point (the "work point").
• The distribution of forces between the beam connection and column connection depends on the eccentricities (e.g for the gusset plate relative to the beam and column.
• When e_b + e_c = e_gusset, the distribution is balanced and there is no moment on the gusset — this is the ideal configuration.
• The method eliminates the need to assume arbitrary force distributions and produces consistent, code-compliant results.

The UFM is applicable to all brace configurations and is the basis for the design examples in the AISC Steel Construction Manual.

Chevron (inverted-V) brace configuration

When two braces meet at the midpoint of a beam (chevron configuration), the beam must be designed for the unbalanced vertical force that occurs after one brace buckles. Per AISC 341-22 Section F2.3, the beam at the brace intersection point must resist:

• Downward force from the tension brace: Ry × Fy × Ag × sin(theta)
• Upward force from the buckled compression brace: 0.3 × Fcre × Ag × sin(theta) (post-buckling residual)

The net unbalanced force is large and typically requires a heavy beam at the brace intersection level.

Capacity design for connections

SCBF connections must be designed for the expected brace capacity, not the applied load. Per AISC 341-22 Section F2.6c, the required connection strength in tension is:

Pu,connection = Ry × Fy × Ag    (expected yield strength of brace)

For A500 Gr C HSS, Ry = 1.4 (AISC 341 Table A3.2). For the HSS 8.625x0.500: Pu,connection = 1.4 × 46 × 12.76 = 822 kips. The gusset plate, welds, and bolts must all resist this force.

In compression, the connection must resist the lesser of Ry × Fy × Ag and 1.14 × Fcre × Ag (expected post-buckling compression capacity).

Multi-code comparison

AISC 341-22 (USA): Three categories — SCBF (R = 6, Omega_0 = 2), OCBF (R = 3.25, Omega_0 = 2), non-seismic. KL/r ≤ 200 for both. SCBF requires highly ductile members and capacity-designed connections.

AS 4100-2020 / NZS 3404 (Australia/NZ): Concentrically braced frames are classified by the structural ductility factor mu. Category 1 (limited ductility, mu = 2) is roughly equivalent to OCBF. Category 3 (mu = 4) requires detailing comparable to SCBF. Brace slenderness limits follow AS 4100 Section 6, and phi = 0.90 for compression. Key differences from AISC:

• AS 4100 uses a modified slenderness ratio lambda_n rather than KL/r directly
• The compression capacity formula uses a member capacity reduction factor alpha_b (0.5 to 1.0) depending on section type
• Connection design follows AS 4100 Section 9 with capacity factor phi = 0.80 for bolts and 0.80 for welds
• NZS 3404 adds additional seismic provisions for capacity design of connections, similar in principle to AISC 341

EN 1993-1-1 / EN 1998-1 (Eurocode): Concentrically braced frames are designed to ductility class DCM or DCH. DCH requires q = 4 (behavior factor) and brace slenderness lambda_bar ≤ 2.0. EN 1998 Section 6.7 requires that braces in X-configuration satisfy a balanced strength condition: the difference in tension and compression capacities between stories must not exceed 25%. Connection overstrength factor is gamma_ov = 1.25. Key differences from AISC:

• EN 1993 uses imperfection factors (buckling curves a through d) instead of a single formula
• The Eurocode approach separates member checks (EN 1993) from seismic detailing (EN 1998)
• Capacity design uses the overstrength factor gamma_ov applied to the brace yield capacity
• Beam design for chevron configurations follows EN 1998-1 Section 6.7.4 with specific rules for the unbalanced force

CSA S16-19 (Canada): Moderately ductile concentrically braced frames (Type MD, Rd = 3.0) and limited ductility (Type LD, Rd = 2.0). CSA S16 Clause 27.5 limits KL/r ≤ 200 and requires capacity design of connections for Cu = Ag × Ry × Fy. Key differences:

• CSA S16 uses Rd and Ro factors (ductility-related and overstrength-related force modification factors) instead of R and Omega_0
• The effective length factor for X-bracing may be reduced per CSA S16 Commentary
• Connection design follows CSA S16 Clause 13 with specific seismic requirements in Clause 27

Worked example — designing an X-brace for a 20-ft bay

Given:

• Bay width = 20 ft, story height = 14 ft
• Diagonal brace length L = sqrt(20² + 14²) = sqrt(400 + 196) = sqrt(596) = 24.4 ft = 293 in
• Factored lateral shear per brace V_u = 85 kips (from seismic load combination)
• Brace angle theta = arctan(14/20) = 35.0 degrees
• Factored brace axial force P_u = V_u / cos(theta) = 85 / cos(35.0) = 85 / 0.819 = 103.8 kips
• Material: A500 Gr C Round HSS, Fy = 46 ksi, Fu = 62 ksi
• System: Non-seismic braced frame (AISC 360 only, no seismic provisions)

Step 1 — Determine design approach: Since this is a non-seismic frame, we use tension-only design. The compression brace is assumed buckled, and the tension brace resists the full lateral force.

Step 2 — Target minimum radius of gyration: With K = 1.0 for pin-pin connection and target KL/r ≤ 200:

r_min = KL / 200 = 293 / 200 = 1.47 in.

Step 3 — Try HSS 6.000x0.280: A = 4.80 in², r = 2.02 in, D/t = 21.4.

• KL/r = 293 / 2.02 = 145 < 200 OK

Step 4 — Check tension capacity per AISC Chapter D:

• Yielding: phi × Pn = 0.90 × 46 × 4.80 = 198.7 kips > 103.8 kips OK
• Rupture (assume Ae = 0.85 × Ag for end connection): phi × Pn = 0.75 × 62 × (0.85 × 4.80) = 0.75 × 62 × 4.08 = 189.7 kips > 103.8 kips OK
• Governing capacity = 189.7 kips > 103.8 kips OK

Step 5 — Check compression capacity (for completeness, even in tension-only design):

• Fe = pi² × 29000 / 145² = 13.6 ksi
• 4.71 × sqrt(29000/46) = 118.3. Since 145 > 118.3, elastic buckling governs:
• Fcr = 0.877 × 13.6 = 11.9 ksi
• phi × Pn = 0.90 × 11.9 × 4.80 = 51.4 kips

Compression capacity is 51.4 kips. If full participation of both braces is desired, the HSS 6.000x0.280 is insufficient for the 103.8 kip compression demand and a larger section would be needed.

Step 6 — Connection design force (tension-only, non-seismic):

Since this is a non-seismic frame, capacity design is not required. The connection is designed for the factored load:

Pu,connection = 103.8 kips

For a welded gusset plate connection: Required gusset plate area = 103.8 / (0.90 × 36) = 3.20 in² (using A36 plate). With gusset width of 6 in at the Whitmore section: t_min = 3.20 / 6 = 0.53 in. Use 5/8 in plate.

Result: Use HSS 6.000x0.280 with 5/8 in A36 gusset plate for the X-brace.

Common mistakes engineers make

1. Ignoring the 2t linear clearance at gusset fold lines. SCBF gusset plates must accommodate brace buckling by providing a 2t clearance zone (where t = gusset thickness) at the end of the brace. Omitting this clearance zone causes the gusset to tear or the brace to fracture at the connection during buckling.

2. Using bearing-type bolts in brace connections. AISC 341-22 Section F2.6c(3) requires slip-critical bolts in brace connections for SCBF. Bearing-type connections can slip under seismic loading, causing sudden stiffness changes and unpredictable frame behavior.

3. Failing to check the beam at chevron brace intersections. The beam at the chevron intersection must resist the unbalanced vertical force after one brace buckles. Many designers size the beam only for gravity loads and miss this critical seismic demand.

4. Exceeding D/t limits for HSS braces. Round HSS braces with D/t above the highly ductile limit will fracture at the midpoint during cyclic buckling before developing full ductility. This is the single most common SCBF brace failure in testing.

5. Assuming both braces in X-bracing share load equally. In reality, the compression brace buckles at a fraction of its nominal capacity and sheds load to the tension brace. Design should account for this behavior, especially in seismic loading where repeated cycling further degrades compression capacity.

6. Neglecting out-of-plane buckling of gusset plates. Gusset plates that are too thin or have excessive unsupported length can buckle out of plane, causing sudden connection failure. The Whitmore section must be checked for compression buckling with an appropriate effective length.

7. Using K-bracing in seismic design categories D through F. K-bracing is explicitly prohibited in SCBF per AISC 341-22 F2.4a because the buckling of a compression brace creates an unbalanced shear in the column. This has caused collapses in past earthquakes.

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FAQ

What is the difference between SCBF and OCBF?

SCBF (Special Concentrically Braced Frame) and OCBF (Ordinary Concentrically Braced Frame) are both concentrically braced frame types defined in AISC 341-22. SCBF has much more stringent detailing requirements — including highly ductile member limits (AISC 341 Table D1.1), capacity-designed connections, and slip-critical bolting — but receives a higher response modification coefficient (R = 6 vs. R = 3.25 for OCBF). This means SCBF can be designed for lower seismic forces but requires more careful fabrication. OCBF has fewer detailing requirements and is appropriate for lower seismic design categories or where the additional ductility is not needed.

When should I use tension-only vs. tension-compression bracing?

Tension-only bracing is appropriate for wind-governed designs and non-seismic applications where the compression brace buckling does not compromise the structural system. It simplifies design because only the tension diagonal needs to be sized for the full lateral force. Tension-compression bracing (where both braces are designed for compression) is required for seismic applications per AISC 341, because both braces must participate in energy dissipation during cyclic loading. Using tension-only design in seismic regions results in excessive drift and poor hysteretic performance.

What is the Whitmore section and when is it used?

The Whitmore section is an effective width used to check gusset plate stresses at the end of a bolted or welded connection. It is constructed by drawing 30-degree lines from the outermost fasteners and measuring the intercepted width. The Whitmore section is used to check for plate yielding (in tension) and plate buckling (in compression) at the last row of fasteners. It was first described by R. E. Whitmore in 1952 and is now standard practice per the AISC Steel Construction Manual.

Why is K-bracing prohibited in seismic design?

K-bracing is prohibited in SCBF because when a compression brace buckles, the unbalanced force is applied directly at the midpoint of the column rather than at a beam-column joint. This creates a large shear force and bending moment in the column at a location where there is no beam to resist it, potentially causing column failure and progressive collapse. This failure mechanism was observed in several buildings during past earthquakes and led to the explicit prohibition in AISC 341.

How do I determine the effective length of an X-brace?

For X-bracing, the effective length depends on whether out-of-plane restraint is provided at the intersection point. If the two braces are connected at the crossing point and the connection provides adequate out-of-plane restraint, the effective length for out-of-plane buckling may be taken as half the full diagonal length. If no out-of-plane restraint is provided (braces simply cross without connection), the full diagonal length should be used. For in-plane buckling, the effective length is typically taken as the full diagonal length with K = 1.0, unless a more detailed analysis justifies a reduction.

What phi factors apply to brace design per AISC 360?

AISC 360-22 specifies the following resistance factors for brace design: phi_t = 0.90 for tensile yielding, phi_t = 0.75 for tensile rupture, phi_c = 0.90 for compression, phi_v = 0.75 for bolt shear, and phi_w = 0.75 for weld shear. For AISC 341 seismic design, connection elements are designed for the expected brace strength (Ry × Fy × Ag) using these same phi factors.

Related references

• How to Verify Calculations
• Seismic Design Basics
• Gusset Plate Design
• Compact Section Limits
• Brace Connection Design
• Framing Systems
• Bolted Connection Design
• Welded Connection Design
• Column Buckling
• Steel Member Design

Disclaimer

This page is for educational and reference use only. It does not constitute professional engineering advice. All design values must be verified against the applicable standard (AISC 360, AISC 341, AS 4100, EN 1993, or CSA S16) and project specification before use. Structural design should be performed by a licensed professional engineer with knowledge of the specific project conditions and local building code requirements. The site operator disclaims liability for any loss, injury, or damage arising from the use of this information.

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Design guides

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• Steel Connection Calculator Guide
• Weld Design Checklist
• EN 1993-1-8 Bolted Connection Worked Example

Reference pages

• Shear Tab