Effective Length Settings in Steel Design

When using the Steel Design modules in MasterSeries (e.g. MasterPort, MasterFrame Steel Design), several key inputs control how member buckling and stability are assessed. Correct use of these inputs ensures that the design checks reflect your engineering intent.

In MasterSeries (MasterKey Steel Design), correctly configuring effective lengths and lateral/torsional restraints is essential for accurate flexural and axial buckling checks.

The software separates axial compression buckling parameters from Lateral-Torsional Buckling (LTB) restraint settings. Below is a detailed technical guide on how effective length settings work in MasterSeries, their location, mechanics, and key support recommendations.

The Axial + Moment Design Data Tab

This primary tab controls global effective lengths for axial compressive buckling (Nb,Rd) and overall bending/LTB effective length factors (Mb,Rd).

Major & Minor Axis Buckling Lengths (Lx,Ly/Lz):
- British Standards (BS 5950): Ly designates the major axis buckling length, and Lz designates the minor axis buckling length.
- Eurocode 3 (BS EN 1993-1-1): In Eurocode terminology, Ly represents the major axis and Lz represents the minor axis. (Note: In earlier versions, the interface retained Lx and Ly notation across both codes for consistency, but recent updates present standard code terminology).
Numeric Overrides & Factors (Kx,Ky):
- Default (0.00m / 1.0L): Setting lengths to 0.00m or multipliers to 1.0 defaults the buckling length to the physical analytical member length L.
- Custom Length Overrides: You can type an explicit numeric length in metres (e.g., entering 6.1 m for a member segment) or adjust effective length factors (Kx,Ky). This is particularly useful when an analytical member segment is shorter or longer than its unbraced buckling length.
"Full Rest" (Fully Restrained):
- Setting the LTB effective length dropdown to "Full Rest" assumes continuous lateral restraint along the compression flange (e.g. composite deck or concrete floor slab) and suppresses LTB checks in the output.

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Caution on Wind Uplift / Load Reversal

Do not select "Full Rest" if the beam or rafter experiences wind uplift that puts the bottom flange into compression. "Full Rest" suppresses LTB checks globally across all load cases. Instead, leave LTB set to 1.0L and specify top-flange purlin restraints on the Lateral Restraints tab.

The Lateral Restraints (Minor Axis) Tab

This tab defines discrete intermediate restraint positions along the member to break down the minor-axis buckling and LTB unbraced lengths into smaller portion lengths.

Portion Lengths vs. Absolute Coordinates:
- Inputs are entered as portion lengths (the distances between restraints, such as purlin or side-rail spacings), not absolute coordinates from the start node.
- For example, entering 1.8m into Equal Spacing instructs MasterSeries to divide the member into 1.8 m segments between lateral restraints.
Auto-Restrain at Connected Members:
- Ticking "Auto Restrain at Connected Members" automatically detects nodes where secondary framing members intersect the minor axis and sets the portion lengths accordingly.
Restraint Side Selection (Top, Bottom, Both Flanges):
- In MasterSeries 2024+, you can explicitly set the Restraint Side to Top Flange, Bottom Flange, or Both Flanges.
- Specifying Top Flange allows the software to treat the top flange as restrained under gravity loads while automatically evaluating unrestrained bottom-flange LTB under uplift load cases.

Supermembers (Merged Members) vs. Segmented Members

Merged Supermembers:
- Merging continuous beam spans or multi-storey column lifts into a single physical Supermember causes MasterSeries to default all buckling checks (Lx,Ly,LLTB) to the total combined length of the merged assembly.
- Action Required: You must manually enter portion lengths on the Lateral Restraints tab or override Lx/Ly in the Axial + Moment tab; otherwise, the software evaluates the entire combined height/length as an unrestrained member.
Cranked / Faceted Members:
- Non-collinear cranked beams (such as stair stringers or portal rafters) 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 set the LTB effective length factor (e.g., 2.0L or 3.0L) relative to the individual segment length.

Torsional Restraints & Unrestrained Flanges (Annex BB / Appendix G)

Unrestrained Compression Flanges: Near portal frame haunches, apexes, or cantilevers, gravity hogging or wind uplift puts the bottom flange into compression where top purlins offer no direct restraint.
Torsional Stays:
- MasterSeries evaluates unrestrained compression flange buckling in accordance with Annex BB (Eurocode 3) / Appendix G (BS 5950).
- To resolve bottom-flange buckling failures, introduce Torsional Stays (knee braces connecting the bottom flange to a top purlin) on the Torsional Restraints tab. Torsional stays must coincide with a defined top purlin position.

Torsional Buckling (NcrT,NcrTF) & KT Factor

For axial compressive torsional (NcrT) and torsional-flexural (NcrTF) buckling checks on columns or struts, MasterSeries historically defaults to evaluating the full unrestrained column height.
If intermediate floor framing or side-rail stays provide torsional restraint, you can override the torsional effective length factor KT in the Axial + Moment tab to reflect the reduced distance between torsional restraints.