Types of Torsion

Torsional Section Properties

Closed Sections (CHS, RHS, SHS)

For closed hollow sections, St. Venant torsion dominates and warping effects are negligible.

Open Sections (I-sections)

For I-sections, both St. Venant and warping torsion contribute.

Torsional Properties — Standard Sections

CHS Sections

I-Sections

Combined Bending and Torsion (Clause 6.2.7)

For sections subject to combined bending and torsion:

(M_Ed / M_c,Rd)^2 + (B_Ed / B_Rd)^2 + (T_Ed / T_Rd)^2 <= 1.0

Where:

• M_Ed = design bending moment
• M_c,Rd = design bending resistance
• B_Ed = design bimoment (warping)
• B_Rd = warping resistance = f_y x W_w
• T_Ed = design torsional moment
• T_Rd = design torsional resistance

Worked Example — Eccentrically Loaded Cantilever Beam

Cantilever beam, 3.0 m span. HEA 200, S355 steel. Point load at tip: 20 kN, 150 mm eccentricity. T_Ed = 20 x 0.15 = 3.0 kNm.

St. Venant contribution: T_T,Ed = 3.0 x (24.8 / (24.8 + 47100)) = 0.0016 kNm (negligible) Warping contribution: T_W,Ed = 3.0 - 0.0016 = 2.998 kNm (dominant)

Bimoment at support: B_Ed = 2.998 x 3.0 / 2 = 4.50 kNm2

Combined check: (60/207)^2 + 0 + (3.0/3.5)^2 = 0.084 + 0 + 0.73 = 0.81 < 1.0 OK

The torsional component dominates the interaction despite being only 3.0 kNm.

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

What is the difference between St. Venant torsion and warping torsion?

St. Venant (uniform) torsion occurs when warping is unrestrained, with resistance through shear stress circulation (governed by J). Closed sections (CHS, RHS) have high J and resist torsion efficiently. Warping torsion occurs when warping is restrained, developing normal stresses (governed by I_w). Open sections (I-beams) predominantly resist torsion through warping action.

When is torsional design required per EN 1993-1-1?

Torsional design per Clause 6.2.7 is required for: edge beams supporting cantilever slabs, eccentrically loaded beams, crane runway girders with lateral loads, curved beams, and spandrel beams. For typical simply supported I-beams with concentric loading, torsional effects are small and may be neglected.

Related Pages

• Beam Design
• HSS Section Properties
• Column Design
• All European References

Educational reference only. Torsional design per EN 1993-1-1:2005 Clause 6.2.7. Verify combined interaction with applicable National Annex. Results are PRELIMINARY - NOT FOR CONSTRUCTION without independent verification.

Design Resources

Calculator tools

• Torsion Analysis Calculator

Reference pages

• EU Torsion