Steel Design FAQs

In MasterSeries technical support, queries regarding the MasterKey Steel Design module (integrated with MasterFrame and MasterPort) represent a large volume of helpdesk cases. Most inquiries center around effective lengths and restraints, nominal column moments, local bow imperfections, torsion design, and secondary member modeling.

Here is a technical overview of the most common FAQs for the steel design module, detailing the software mechanics, practical solutions, and key ticket references.

1. Effective Lengths, Lateral Restraints & The "Full Rest" Pitfall

• User Query: "How do I set up LTB vs. axial buckling effective lengths, and why shouldn't I use 'Full Rest' under wind uplift?"

• MasterSeries Mechanics:

  • Axial Buckling (Lx​,Ly​/Lz​): Global compressive buckling lengths are defined on the Axial + Moment Design Data tab. In Eurocode terminology, Ly​ denotes major axis buckling and Lz​ denotes minor axis buckling. Entering 0.00m defaults to the physical member length.
  • Lateral-Torsional Buckling (LTB): Intermediate minor-axis restraints are set on the Lateral Restraints tab using portion lengths (distances between restraints, e.g., 1.8m purlin spacing).
  • The "Full Rest" Dropdown: Selecting "Full Rest" in the Axial + Moment tab assumes continuous restraint and suppresses all LTB checks globally across the entire member.

2. Nominal Column Moments (Mnom​) & Beam Eccentricities

• User Query: "Why aren't nominal column moments appearing in my column design output?"

• MasterSeries Mechanics:

  • In simple construction (SCI P358 / P365 under Eurocode 3), horizontal beam shear reactions (VEd​) are assumed to act at a nominal lever arm 100 mm off the face of the column flange or web.
  • Nominal moments (Mnom​=VEd​×0.100m) are calculated post-analysis during the steel design phase and do not alter 1D frame analysis equilibrium.

• Why Mnom Fails to Generate (The #1 Support Cause):

  • If a member end release (pin) is applied to the top node of the column itself rather than to the incoming horizontal beams, the software assumes the column top is a pinned cap plate and suppresses Mnom​.

• Practical Solution:

  1. Remove member end releases from the column head and assign end releases to the ends of the incoming horizontal beams.
  2. To adjust offsets beyond 100 mm (e.g. for welded toe-plates), configure custom offsets under Design > Beam Eccentricities for Nominal Column Moments.

3. Local Bow Imperfections (NEd​>0.25Ncr​)

• User Query: "What does the warning 'NEd > 0.25 Ncr, required local bow load type NOT applied' mean and how do I resolve it?"

• MasterSeries Mechanics:

  • Under Eurocode 3 (BS EN 1993-1-1 Cl. 5.3.2 / Method M4), when applied axial compression exceeds 25% of the elastic critical buckling load (NEd​>0.25Ncr​), in-member geometric non-linearity (small P−δ) must be accounted for explicitly via initial member curvature (e0​) combined with a 2nd-order (P−Δ) analysis.

• Practical Solution & Prerequisites:

  1. Assign Member Attribute: Go to Properties > Member Attributes > More Attributes in MasterFrame and assign the Local-Bow attribute. Enter 0 for major axis (y) or 1 for minor axis (z).
  2. Enable P−Δ Analysis: Ensure 2nd-order P−Δ analysis is active for the governing ULS load case.
  3. Analytical Subdivision: The physical member must be subdivided into at least 3 internal nodes (4 analytical parts) using Modify Geometry > Modal Auto-Connect so flexural bowing is captured.
  4. Alternative: You can tick "Ignore Local Bow Check" inside the steel design brief to rely on standard code empirical buckling curves (χ reduction factors).
  5. Note: Merging multi-storey columns into a single "Supermember" artificially inflates slenderness and triggers false local bow warnings; split columns storey-by-storey at floor levels.

4. Torsion Design & Deflection Limits (θ Limit / 2∘ Rule)

• User Query: "Why does my beam fail the torsion check with a 'Theta Limit > 2°' warning, or state 'Torsion deflection not checked'?"

• MasterSeries Mechanics:

  • MasterFrame calculates 1D nodal rotation (θ) using St. Venant torsion (TL/GJ), whereas MasterKey Steel Design evaluates non-linear cross-sectional warping stiffness per SCI P385 / SCI P057.
  • The default Theta Limit of 2.0∘ originates from SCI guidance to prevent serviceability or cladding damage.
  • "Torsion Deflection Not Checked": SCI P385 limits code-mandated design formulas strictly to uniform torsion or torsion diagrams where the net area under the diagram sums to zero. Non-conforming torsion diagrams output a note stating that torsion falls outside the published SCI scope.

• Practical Solutions:

  • For open sections (UB/PFC) failing in twist under eccentric loads, switch to a closed hollow profile (SHS/RHS/CHS), which possesses exponentially higher torsional rigidity.
  • Ensure rotational support restraints (θx​) are active in Nodal Static Supports to anchor torsional reactions if applicable.
  • When applying eccentric member loads, insert a Torq ecc. command in Loads > Member Loading (measured from the section Shear Centre (S), not the geometric centroid).

5. Purlins and Side Rails

• User Query: "How do purlins and side rails function in MasterSeries, and can I design proprietary cold-formed purlins?"

• MasterSeries Mechanics:

  • Purlins and side rails are not active 1D structural line elements in the MasterFrame stiffness matrix. Their primary role is to define discrete lateral and torsional restraint points on primary rafters and columns.

• Limitations & Recommendations:

  • MasterSeries does NOT design proprietary thin-gauge cold-formed Z/C purlins (EN 1993-1-3); purlins must be sized using manufacturer load tables.
  • To prevent artificial out-of-plane rafter torsion in 3D frame models, model lightweight 1D Tension Ties along key purlin lines across the roof to transfer horizontal sway forces to vertical bracing bays.

6. Merged Supermembers vs. Cranked / Faceted Beams

• User Query: "Why does merging members into a Supermember cause massive buckling failures, and how do I handle cranked rafters?"

• MasterSeries Mechanics:

  • Merged Supermembers: Merging continuous beam spans or multi-storey column lifts into a physical Supermember causes MasterSeries to default all buckling length checks (Lx​,Ly​,LLTB​) to the total combined length of the merged assembly. You must manually define portion lengths or override Lx​/Ly.
  • Cranked / Faceted Beams: Non-collinear cranked beams (such as portal rafters or stair stringers) cannot be merged into a Supermember. MasterSeries evaluates each straight segment independently. To design for global buckling across the full cranked span, manually override Lx​ and Ly​ to the total overall span length and adjust the LTB multiplier (e.g. 2.0⋅L) relative to the segment length.