Purlins and Side Rails in Steel Design

Purlins and Side Rails are only considered in so far that they only designed to control the lateral torsional buckling (LTB) design checks for your structural members.

You can not directly define the physical properties or section types of purlins or rails themselves, but rather, it allows you to define the restraint conditions that these secondary members (like purlins or side rails) would provide to primary beams or rafters (read more below).

Therefore they are not technically modelled within your 3d frame and therefore can not be exported.

The 📄 Lateral Torsional Buckling (LTB) Inputs are a crucial component within the MasterKey: Steel Design module, specifically designed to control the lateral torsional buckling (LTB) design checks for your structural members. It does not directly define the physical properties or section types of purlins or rails themselves, but rather, it allows you to define the restraint conditions that these secondary members (like purlins or side rails) would provide to primary beams or rafters.

Analysis vs. Design: Purlins Are Non-Structural / Graphical Only

• Wireframe Analysis Model: MasterFrame and MasterPort analyse 1D wireframe elements along their mathematical centroids/shear centers. Purlins and side rails are not modeled as active 1D structural line elements in the frame stiffness matrix, nor do they appear as physical 3D objects connected to beam flanges.
• Design Restraint Function: The primary role of purlins and cladding rails in MasterSeries is to define discrete lateral and torsional restraint points that control the Lateral-Torsional Buckling (LTB) and minor-axis flexural/axial buckling lengths of primary rafters and columns.

Configuring Restraints in MasterKey Steel Design

You can define these at design time in the 📄 Lateral Torsional Buckling (LTB) Inputs

To apply purlin or side rail restraints to a rafter or column:

1. Open the member's design brief in MasterKey Steel Design (under the Axial with Moment brief).
2. Open the Lateral Restraints tab.

3. Input Spacings / Portions:

   • Enter an Equal Spacing value (e.g. 1.8m for roof purlins or 1.4m for side rails).
   • Alternatively, specify custom Variable Portion Lengths across up to 3 distinct zones to account for local spacing changes near eaves or ridge zones.

4. Specify Restraint Side:

   • Set Restraint Side to Top Flange, Bottom Flange, or Both Flanges.

5. Auto-Restrain Feature:

   • Ticking "Auto Restrain at Connected Members" automatically detects nodes where secondary horizontal frame members intersect the minor axis and populates the portion lengths accordingly.

Torsional Stays & Load Reversals (Annex BB / Appendix G)

• Top vs. Bottom Flange Compression: Standard gravity loads put the top flange of a portal rafter into compression, where top-flange purlins provide effective lateral restraint. However, near haunch eaves, apex zones, or under wind uplift, the bottom flange goes into compression.

• Torsional Stays:

  • To stabilize an unrestrained compression flange, you must define Torsional Stays (knee braces connecting the bottom flange back to a top purlin).
  • Open the Torsional Restraints tab (or Annex BB / Appendix G stability check). Torsional stays must coincide with a top purlin position.
  • Click the Stay@ button on the graphical member diagram to toggle stays on or off at purlin nodes.

Column Restraint via Perimeter Concrete/Masonry Walls

• "Start At" Parameter: If a steel column is cast inside a partial-height concrete wall or masonry parapet (e.g. a 6.0m wall on an 11.0m column), you do not need to model the wall as an FE shell.
• Setup: In the column's Lateral Restraints tab, enter 6.0 in the Start at X - End 1 (m) field. MasterSeries suppresses buckling checks below 6.0m and evaluates buckling strictly over the remaining unrestrained height above the wall.

See Also: