UK Braced Frame Design — Concentric Braced Frames per EN 1993-1-1 + UK NA

Design of concentric braced frames (CBF) for UK steel buildings per EN 1993-1-1 with UK National Annex. Covers brace slenderness limits, capacity design of connections and beams, brace section selection (CHS, UC, and angles), and a worked example for a UK multi-storey braced bay in S355 steel.

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Concentric Braced Frame Design

Concentric braced frames (CBF) provide lateral stability through diagonal bracing members that intersect the beam-column joint centreline. They are the most common lateral load-resisting system for UK steel buildings up to 15 storeys.

Brace Slenderness Limits

EN 1993-1-1 Clause 6.3.1 and the UK NA impose limits on brace slenderness to ensure ductile behaviour:

Capacity Design Principle

The capacity design approach ensures that:

1. Brace yields first — designed for the design seismic/maximum wind load
2. Connections are overstrength — designed for 1.1 × γov × Npl,Rd of the brace (where γov = overstrength factor = 1.25)
3. Beams and columns — designed for the maximum forces from the yielding brace

Brace Section Selection

Worked Example — UK Braced Frame

Frame details:

• 5-storey braced bay, 6.0 m wide × 4.0 m storey height
• Wind load at each floor level: 30 kN (total 150 kN at base)
• Brace arrangement: Single diagonal (X-bracing in each bay)
• Brace length: √(6.0² + 4.0²) = 7.21 m

Step 1 — Brace Design Force

Base shear per brace: VEd,total = 150 kN

Brace force per X-brace: NEd = 150 / (2 × cosθ) where θ = atan(4.0/6.0) = 33.7°

NEd = 150 / (2 × 0.832) = 90 kN per brace in tension (the other brace is in compression, but X-bracing allows tension-only design for wind reversal)

Step 2 — Brace Section Selection

Try CHS 88.9×5, S355:

• A = 13.2 cm² = 1,320 mm²
• i = 2.99 cm = 29.9 mm
• Npl,Rd = 1,320 × 355/1.0 = 469 kN

Step 3 — Brace Slenderness Check

For the compression brace (wind from left to right):

Lcr = 7,210 mm (full length, pinned at both ends)

λ̄ = (7,210 / 29.9) / 76.4 = 241.1 / 76.4 = 3.16

λ̄ = 3.16 > 2.0 → Exceeds the slenderness limit!

The CHS 88.9×5 is too slender. Try CHS 193.7×8:

• A = 46.5 cm², i = 6.62 cm = 66.2 mm
• λ̄ = (7,210 / 66.2) / 76.4 = 108.9 / 76.4 = 1.43
• λ̄ = 1.43 ≤ 2.0 — OK (satisfies DCM slenderness limit)

Npl,Rd = 4,650 × 355/1.0 = 1,651 kN

Buckling resistance: χ for curve a, λ̄ = 1.43: χ ≈ 0.45 (from χ-λ̄ tables) Nb,Rd = 0.45 × 1,651 = 743 kN — OK for compression brace (unlikely to reach this in tension-only X-bracing design).

Step 4 — Connection Design (Capacity Design)

Connection design force = 1.1 × γov × Npl,Rd = 1.1 × 1.25 × 1,651 = 2,270 kN

Connection bolts: 6 × M24 Class 8.8 = 6 × 203.3 = 1,220 kN (in tension) — inadequate. Use 8 × M24: 1,626 kN — still below 2,270 kN. Consider CHS 193.7×10 or alternative.

This shows that capacity-designed connections for heavy braces become very large.

Step 5 — Beam and Column Design

Beam at braced bay: designed for the brace vertical component at ULS: V_Ed,beam = NEd × sinθ = 90 × sin(33.7°) = 90 × 0.555 = 50 kN per brace This is typically within the beam capacity for a 533UB section.

Design Resources

• UK Steel Grades Reference — EN 10025-2 grade selection for UK projects
• UK Steel Mechanical Properties — fy, fu, and elongation tables
• UK Universal Beam and Column Sizes — UB/UC section dimensions and properties
• UK Bolt Capacity Tables — Class 8.8 and 10.9 bolt resistance
• UK Beam Design Guide — EN 1993-1-1 flexure, shear, and LTB
• UK Connection Design Guide — EN 1993-1-8 bolted and welded joints
• All UK Steel Design References — complete library

Frequently Asked Questions

What brace slenderness limits does the UK NA impose?

The UK NA to BS EN 1993-1-1 limits brace slenderness to λ̄ ≤ 2.0 for compression members in braced frames. This corresponds to Lcr/i ≤ 153 for S355 (λ1 = 76.4), or Lcr/i ≤ 174 for S275 (λ1 = 86.8). The purpose is to limit brace slenderness so that the brace can develop a minimum compression resistance and provide adequate frame stiffness. For tension-only bracing, no slenderness limit applies.

What is the capacity design approach for brace connections?

Capacity design requires that the brace connection be stronger than the brace itself to ensure ductile yielding of the brace before connection failure. The connection design force is 1.1 × γov × Npl,Rd, where γov = 1.25 (material overstrength factor per UK NA). The 1.1 factor accounts for strain hardening. For a CHS brace with Npl,Rd = 1,651 kN, the connection must resist 2,270 kN — typically requiring either a very large bolted connection or a full-strength welded connection.

What is the most common UK brace section for CBF?

CHS (circular hollow section) hot-finished to BS EN 10210-1 in S355J2H is the most common brace section for UK concentric braced frames. CHS provides the best buckling resistance (curve a, α = 0.21) and equal strength about all axes. UC sections are used for heavy braces where CHS connection detailing becomes complex. Back-to-back angles are used for light bracing in low-rise frames.

How are beams and columns designed in a braced bay?

Beams and columns in a braced bay must be designed for the maximum forces from the bracing system at ULS. The brace axial force introduces both horizontal and vertical components into the frame. The beam at the braced bay must resist the vertical component (NEd × sinθ), and the column must resist the accumulated brace forces from all storeys above. In UK practice, the beams in braced bays are typically designed as identical to non-braced bay beams (same section), with a capacity check confirming adequacy. Columns at braced bays are typically heavier than non-braced bay columns to resist the accumulated lateral loads.

Related Pages

• EN 1993 Steel Design Overview
• European Steel Properties
• EN 1993 Beam Design Guide
• EN 1993 Column Buckling
• EN 1990 Load Combinations
• UK Steel Chemical Composition
• UK Steel Charpy Values

Educational reference only. All design values are per BS EN 1993-1-1:2005 + UK National Annex and BS EN 10025-2:2019. Verify all values against the current editions of the standards and the applicable National Annex for your project jurisdiction. Designs must be independently verified by a Chartered Structural Engineer registered with the Institution of Structural Engineers (IStructE) or the Institution of Civil Engineers (ICE). Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent professional verification.