European Steel Framing — EN 1993 Structural Systems

Steel framing systems provide the primary lateral and gravity load-resisting structure for European steel buildings. EN 1993-1-1:2005 Sections 5-6 covers design requirements for various system types including braced frames, moment-resisting frames, and portal frames. Global analysis methods include first-order elastic, second-order elastic, and plastic hinge analysis depending on frame type and slenderness.

Frame selection depends on building height, seismicity, wind loads, and architectural requirements. Each system has specific ductility, stiffness, and detailing requirements per EN 1993-1-1 and the relevant National Annex.

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Code Reference: EN 1993-1-1:2005 Sections 5-6

EN 1993-1-1 Sections 5 (Structural Analysis) and 6 (Ultimate Limit States) provide the framework for all steel structural systems. EN 1993-1-1 Clause 5.2 defines first-order and second-order analysis requirements. EN 1993-1-1 Clause 5.3 covers global and member imperfections.

Steel Framing Systems Overview

In European practice, the braced frame is the most common system for low-to-mid-rise steel buildings (up to 10 storeys). Portal frames dominate single-storey industrial buildings (warehouses, factories, hangars). Moment-resisting frames are used where architectural freedom from bracing is required. Concrete-core braced systems are the dominant solution for high-rise steel buildings in the UK (e.g., the City of London).

Global Imperfections (EN 1993-1-1 Clause 5.3)

EN 1993-1-1 requires consideration of both global (frame) and local (member) imperfections:

Initial sway imperfection:

[ \phi = \phi_0 \times \alpha_h \times \alpha_m ]

For a 5-storey building (h = 17.5 m) with 4 braced bays (m = 4): (\alpha_h = 2/\sqrt{17.5} = 0.478), but minimum is 2/3, so (\alpha_h = 0.667) (\alpha_m = \sqrt{0.5(1 + 1/4)} = \sqrt{0.625} = 0.791) (\phi = (1/200) \times 0.667 \times 0.791 = 1/379)

The initial sway imperfection (\phi) determines the equivalent horizontal forces that must be applied at each floor level for the structural analysis.

Second-Order Effects (EN 1993-1-1 Clause 5.2)

EN 1993-1-1 Clause 5.2.1 requires second-order effects to be considered when the frame's sensitivity to sway is significant. The sensitivity parameter (\alpha_{cr}) is:

[ \alpha*{cr} = \frac{F*{cr}}{V*{Ed}} = \frac{H*{Ed}}{V*{Ed}} \times \frac{h}{\delta*{H,Ed}} ]

Where (F*{cr}) is the elastic critical buckling load, (V*{Ed}) is the vertical load, (H*{Ed}) is the horizontal reaction, (h) is the storey height, and (\delta*{H,Ed}) is the first-order sway displacement.

For braced frames, the horizontal bracing system is designed for (\alpha*{cr} \geq 10) in most cases. For unbraced moment frames, (\alpha*{cr}) is typically 4-8, requiring second-order analysis.

Frame Classification — Braced vs Unbraced

EN 1993-1-1 Clause 5.2.1(4) classifies frames based on their sway sensitivity:

For sway frames, the effective buckling length of columns increases significantly: a column in a sway frame has (L*{cr} = 2.0 \times L) for pinned bases and (L*{cr} = 1.0-1.5 \times L) for fixed bases, depending on the beam-to-column stiffness ratio. Non-sway (braced) frames use (L_{cr} = 0.7-1.0 \times L).

Worked Example — Bracing System Design

Problem: Design a concentric X-bracing system for a 6-storey steel frame. Storey height 3.5 m, bay width 6.0 m. Wind load per storey: wk = 30 kN. Steel grade S355. Building is non-sway per EN 1993-1-1.

Step 1 — Wind load combination (EN 1990): Ultimate horizontal force per storey: (F*{Ed,storey} = 1.5 \times 30 = 45) kN Total base shear: (V*{Ed} = 6 \times 45 = 270) kN

Step 2 — Brace force (X-bracing, tension-only): Brace length: (Lb = \sqrt{3.5^2 + 6.0^2} = 6.95) m Angle to horizontal: (\theta = \arctan(3.5/6.0) = 30.3^\circ) Force per brace (2 braces per bay, assume tension-only design): (N{Ed,b} = 270 / (2 \times \cos 30.3^\circ) = 270 / (2 \times 0.864) = 156) kN

Step 3 — Section selection: Try CHS 114.3 (\times) 5.0 in S355: A = 1,720 mm², i = 38.7 mm Lcr = 6,950 mm (tension: no buckling, but slenderness check for handling) (\lambda1 = 93.9 \times \epsilon = 93.9 \times 0.814 = 76.4) (\bar{\lambda} = (6,950 / 38.7) / 76.4 = 179.6 / 76.4 = 2.35) For tension: (N{t,Rd} = A \times fy / \gamma{M0} = 1,720 \times 355 / 1.0 = 611) kN (> 156) kN — OK.

Step 4 — Deflection check (serviceability): Wind load per storey at SLS: wk = 30 kN Approximate apex deflection from brace elongation: (\delta_h = \delta_b / \cos \theta) Brace axial elongation: (\delta_b = (30 \times 10^3 \times 6,950) / (1,720 \times 210,000) = 0.58) mm per brace Storey drift: (\delta_h = 0.58 / 0.864 = 0.67) mm Total building drift (6 storeys): ~4.0 mm Limit H/500 = 21,000/500 = 42 mm — OK.

Design Resources

• EN 1993 Steel Grades
• European Steel Properties
• EN 1993 Bolt Capacity
• IPE/HEA/HEB Beam Sizes
• EN 1993 Column Buckling
• All European References

Frequently Asked Questions

What framing systems are recognized by EN 1993? EN 1993-1-1 covers braced frames (with concentric or eccentric bracing), unbraced moment-resisting frames, portal frames, and framed tube systems. The Eurocode provides guidance for both elastic and plastic global analysis across all system types. For seismic design, EN 1998-1 supplements these with ductility classification (DCL, DCM, DCH) and specific q-factors for each system type: moment-resisting frames (q = 4-6.5), concentrically braced frames (q = 2.5-4.8), and eccentrically braced frames (q = 5-6.5).

How does EN 1993 handle frame imperfections? EN 1993-1-1 Clause 5.3 requires consideration of both global (frame sway) and local (member bow) imperfections. The global initial sway imperfection (\phi = \phi_0 \times \alpha_h \times \alpha_m) with basic value (\phi_0 = 1/200). Member bow imperfections depend on the buckling curve and are typically L/200 to L/300 for hot-rolled sections. Imperfections are introduced in the analysis model as equivalent geometric imperfections (not as loads). For second-order analysis, the imperfections must be applied in the most unfavourable direction and combined with the load effects.

What is the difference between a sway frame and a non-sway frame per EN 1993-1-1? A non-sway (braced) frame has (\alpha*{cr} \geq 10), meaning second-order PA effects are less than 10% of first-order effects. A sway (unbraced) frame has (\alpha*{cr} < 10) and requires explicit second-order analysis. The classification dramatically affects column design: non-sway columns use effective lengths Lcr (\leq 1.0 \times L) (typically 0.7-1.0L), while sway columns use Lcr up to 2.0-3.0 (\times) L depending on end fixity. The (\alpha*{cr}) parameter is calculated as (\alpha*{cr} = H*{Ed} \times h / (V*{Ed} \times \delta_{H,Ed})) using the first-order sway displacement under horizontal loads.

When should plastic global analysis be used instead of elastic analysis? Plastic global analysis per EN 1993-1-1 Clause 5.4 is appropriate for braced frames and portal frames where ductility demands are predictable. It requires Class 1 sections in all plastic hinge locations and adequate rotation capacity. Elastic global analysis is used for moment frames in moderate-to-high seismicity regions, frames with significant second-order effects, and frames where section classification is Class 3 or 4. Plastic analysis is most efficient for portal frame design (the dominant European application), where it typically reduces the rafter and column sizes by 15-25% compared with elastic design.

What are the drift limits for steel frames in European practice? EN 1993-1-1 recommends inter-storey drift h/300 and total drift H/500 under serviceability wind loads. For seismic drift, EN 1998-1 Clause 4.4.3.2 limits inter-storey drift: 1.0% of storey height for buildings with brittle non-structural elements, 1.5% for buildings with ductile non-structural elements. For wind-excited tall buildings (H > 50 m), occupant comfort criteria based on peak acceleration (typically 0.05-0.10 m/s² for 5-year return wind) may govern the required frame stiffness. The UK NA specifies drift limits based on cladding type: h/300 for flexible cladding, h/500 for brittle cladding.

Reference only. Verify all values against the current edition of EN 1993-1-1:2005 Sections 5-6 and the applicable National Annex. This information does not constitute professional engineering advice.