Braced Frame Configurations

K-bracing is not permitted in seismic design (EN 1998-1 Cl. 6.7.1) because brace buckling induces large moments at column mid-height, risking column failure.

Behaviour Factors q for CBF (EN 1998-1 Table 6.2)

For DCH, braces must have bar_lambda >= 1.3 to ensure ductile behaviour (yield in tension before buckling).

Capacity Design (EN 1998-1 Cl. 6.7.4)

Connections and adjacent members must resist the overstrength brace force:

N_ov,Rd = 1.1 x gamma_ov x N_pl,Rd,brace

Where gamma_ov = 1.25 (material overstrength), 1.1 = strain-hardening factor.

Worked Example — 3-Storey CBF with HEA 200 Braces

3 storeys at 4.0 m, 3 bays at 6.0 m. X-bracing both directions. S355, HEA 200. DCM, q = 4.0.

Overstrength connection design: N_ov,Rd = 1.1 x 1.25 x 2128 = 2926 kN

Drift failure means increased brace size or additional braced bay required.

Brace Section Selection Guide

Design Applications

Common Design Scenarios

This reference covers structural design scenarios commonly encountered in structural steel design practice:

• Strength verification: Check member or connection capacity against factored loads per the applicable design code
• Serviceability checks: Verify deflections, vibrations, and other serviceability criteria
• Code compliance: Ensure design meets all provisions of the governing standard
• Connection detailing: Verify weld sizes, bolt quantities, and edge distances

Related Design Considerations

• System behavior: consider the interaction between members and connections
• Load paths: verify that forces can be transferred through the structure to the foundations
• Constructability: check that the design can be fabricated and erected practically
• Cost optimisation: evaluate alternative sections or connection types for economy

Worked Example

Problem: Verify a typical steel member for the following conditions:

Typical span: 6.0 m | Load: service loads per applicable code | Section: common section in this category

Design Check:

1. Determine governing load combination (ULS or SLS per EN 1990)
2. Calculate maximum internal forces (moment, shear, axial)
3. Compute nominal capacity per code provisions
4. Apply resistance/safety factors
5. Verify interaction if combined forces exist

Result: Use the results from the Steel Calculator tool to verify design adequacy.

Design Applications

Common Design Scenarios

This reference covers structural design scenarios commonly encountered in structural steel design practice:

• Strength verification: Check member or connection capacity against factored loads per the applicable design code
• Serviceability checks: Verify deflections, vibrations, and other serviceability criteria
• Code compliance: Ensure design meets all provisions of the governing standard
• Connection detailing: Verify weld sizes, bolt quantities, and edge distances

Related Design Considerations

• System behavior: consider the interaction between members and connections
• Load paths: verify that forces can be transferred through the structure to the foundations
• Constructability: check that the design can be fabricated and erected practically
• Cost optimisation: evaluate alternative sections or connection types for economy

Worked Example

Problem: Verify a typical steel member for the following conditions:

Typical span: 6.0 m | Load: service loads per applicable code | Section: common section in this category

Design Check:

1. Determine governing load combination (ULS or SLS per EN 1990)
2. Calculate maximum internal forces (moment, shear, axial)
3. Compute nominal capacity per code provisions
4. Apply resistance/safety factors
5. Verify interaction if combined forces exist

Result: Use the results from the Steel Calculator tool to verify design adequacy.

Frequently Asked Questions

What European Standard governs structural steel design?

EN 1993 (Eurocode 3: Design of Steel Structures) is the primary standard for structural steel design in Europe. EN 1993-1-1 covers general rules for buildings, EN 1993-1-8 addresses connection design, and EN 1993-1-2 covers fire design. The standard uses limit state design with partial safety factors (γM). National Annexes adapt parameters to each member state. Companion standards include EN 10025 for hot-rolled products, EN 1090 for execution, and EN 1994 for composite design.

What are the common steel grades used in European construction?

The most common steel grades for European construction are S235, S275, S355, S420, and S460 per EN 10025-2. S355 (minimum yield 355 MPa for t ≤ 16 mm) is the most widely used for structural applications. S275 is used for secondary members. S420 and S460 are quenched and tempered high-strength steels for weight-critical applications. Weathering steel (S355J2W) and fine-grain structural steels (EN 10025-3 and -4) are also available.

How does EN 1993 compare to other international steel design codes?

EN 1993, AISC 360 (US), AS 4100 (Australia), and CSA S16 (Canada) all use limit states design principles but differ in key details. EN 1993 uses partial safety factors (γM0 = 1.00, γM1 = 1.00, γM2 = 1.25) rather than resistance factors (φ). Buckling curves in EN 1993 follow the European Column Curve system (a0 to d) with 5 distinct curves, compared to AISC's single curve. EN 1993-1-8 has comprehensive connection design provisions including the component method for moment connections.

Frequently Asked Questions

Key differences between DCM and DCH for CBF?

DCM: braces Class 1-3, bar_lambda <= 2.0, pinned beam-to-column joints acceptable. DCH: braces must be Class 1, bar_lambda 1.3-2.0, beam-to-column joints must be rigid or specifically detailed. Both use q = 4.0.

Why is K-bracing prohibited in seismic design?

K-bracing (per EN 1998-1 Cl. 6.7.1) induces unbalanced horizontal force at column mid-height when one brace buckles, causing large bending moments and potential column plastic hinging. Only X, V, and inverted-V bracing permitted.

Related Pages

• Moment Frame Design
• Framing Systems
• Seismic Design
• Brace Connection
• All European References

Educational reference only. CBF design per EN 1993-1-1:2005 and EN 1998-1:2004. Verify National Annex. Results are PRELIMINARY - NOT FOR CONSTRUCTION without independent verification.

Design Resources

Calculator tools

• Bolted Connection Calculator
• Weld Capacity Calculator
• End Plate Moment Connection Calculator
• Fin Plate Shear Connection Calculator
• Gusset Plate Calculator

Design guides

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

Reference pages

• EU Shear Tab
• UK Moment Connection