Torsion Checks in Steel Beam Design

Introduction

When designing steel beams subject to torsion in MasterSeries, users may encounter messages such as:

“Torsion deflection not checked”

This typically occurs when the torsional loading or restraint conditions fall outside the scope of the underlying SCI design guidance, rather than due to a limitation in the software itself.

This note explains when torsion checks are carried out, why they may be omitted, and how to address this in your model.

See Also

• 📄 Beams in torsion
• 📄 Defining Torsional Loads

1. Scope of Torsion Design (SCI Guidance)

The torsion design implemented in MasterSeries is based on SCI design guidance, which applies only under specific conditions.

Torsion checks are carried out when:

• (a) Constant torsion exists along the member, or

• (b) The torsion diagram sums to zero (self-equilibrating torsion)

If neither condition is satisfied (e.g. cantilevers with applied torsion):

❗ Torsion design will not be performed

This is an inherent limitation of the design methodology, not the software. In such cases, further assessment may require:

• Alternative design methods

• Reference to SCI or specialist guidance

Analysis vs. Design: How Torsional Deflections Are Calculated

MasterSeries uses two distinct calculation levels to evaluate section rotation under torsional loading:

• In MasterFrame (1D Global Analysis Stage):

  • MasterFrame calculates nodal rotational deflections (θx​ or θy​ in degrees/radians) using pure St. Venant torsion: 
    θ=T⋅L​ / G⋅J
    where T is the applied torque, L is member length, G is shear modulus, and J is the St. Venant torsional constant.
  • For standard straight 1D line elements, MasterFrame does not consider warping stiffness during the initial linear elastic frame analysis. As a result, raw analysis outputs can show larger rotational displacements for open sections.

• In MasterKey Steel Design (Member Design Stage):

  • When you transfer the member into MasterKey Steel Design, the module evaluates section warping stiffness and non-linear cross-sectional warping stresses in accordance with SCI P385 / SCI P057.
  • Because warping resistance inhibits section twisting, the torsional rotations calculated in the Steel Design module are generally smaller and more accurate than the raw St. Venant rotations reported in MasterFrame tabular analysis outputs.

The 2-Degree Twist Limit (Theta Limit) & SCI Scope Requirements

• The 2° Theta Limit: In the steel design brief (Axial + Moment brief), the software includes a default Theta Limit of 2.0∘. This limit originates from long-standing SCI guidance (SCI P385 / P057) to prevent excessive twisting that could cause serviceability issues or dislodge supported masonry cladding.
• 

• "Torsion Deflection Not Checked" / Scope Limitations:

  • SCI P385 limits code-mandated design formulas strictly to two primary torsional moment diagram shapes:

    1. Uniform (constant) torsion along the unbraced length.
    2. Torsion diagrams where the net area under the diagram sums to zero.
  • If intermediate supports, asymmetrical framing, or point torques produce a non-uniform torsion diagram that does not sum to zero, MasterSeries outputs a note stating that torsion design/deflection cannot be checked because it falls outside the scope of SCI published guidance.

Applying Torsional Loads (Torq ecc) & Shear Centre Rules

To induce torsion in MasterFrame line elements:

1. Open Loads > Member Loading.
2. Insert a Torq ecc. command into the load list prior to the applied loads.

3. Shear Centre Alignment (S):

   • Specify offsets in the local major (ey​) or minor (ex​) axes in meters.
   • Critical Rule: Eccentricity is measured strictly from the Shear Centre (S) of the section, not the geometric centroid (C). For asymmetrical open profiles (such as PFC channels or L-angles), the shear centre lies outside the material profile.
4. All loads listed below the Torq ecc. entry inherit the specified offset and generate a torsional moment (T=F×e).

Causes of Excessive Torsional Deflection & Practical Solutions

When a beam fails the torsional deflection check (exceeding 2∘) or reports instability:

1. Open Section Profile (UB / UC / PFC / Angle): Open sections have very low St. Venant torsional constants (J) and deform severely under modest eccentric loads.

   • Solution: Replace the open section with a closed hollow profile (SHS, RHS, or CHS), which possesses exponentially higher torsional rigidity.

2. Missing Rotational End Restraints (θx):

   • Applying torsional loads to a beam with pinned supports that lack torsional rotation restraints causes global matrix instability.
   • Solution: In Nodal Static Supports, ensure rotational restraint (θx​ / Moment X) is enabled at support nodes to anchor the torsional reaction.

3. Lack of Secondary Tying / Cross-Framing:

   • In multi-bay structures, secondary beams or purlins framing into a primary beam provide torsional restraint. If secondary beams are released or missing, the primary beam twists freely.
   • Solution: Verify member end releases on connecting cross-beams or introduce horizontal ties.

4. Ignoring Unintended / Minor Torsion:

   • If torsional forces are incidental (e.g. minor secondary framing interaction) and do not threaten primary stability, check "Ignore Torsion" in the Axial + Moment design brief pane.

End Warping Restraint Settings

In MasterKey Steel Design, the torsion check option provides two end warping conditions:

• "Torsion with Ends Free to Warp" (Standard Recommended Setting): Assumes standard bolted endplate or cleat connections where flanges can warp freely. This increases calculated torsional rotation and represents realistic steelwork fabrication.
• "Torsion with No End Warping": Assumes full physical restraint against flange warping (e.g. beam cast directly into heavy solid concrete). This significantly reduces calculated rotation but is rarely achievable in standard steel connections.

Special Case: Curved Members on Plan

For members that are curved on plan (such as curved balcony edge beams):

• Curved line members experience strong composite bending-torsion interaction.
• MasterSeries provides a dedicated analysis setting under Analysis > Global Analysis Options > Include Warping Stiffness in Curved Members. Ticking this option incorporates non-linear warping matrix terms into the MasterFrame analysis solver, accurately evaluating torsional deflections for 3D curved geometry.