Are Purlins Considered in the Frame Analysis?

While MasterPort uses the location of purlins/rails for checking the main frame members, MasterSeries modules currently do not carry out the design or analysis checks for the purlins or side rails themselves as independent secondary beams carrying load (though their type and size can be specified for drafting/graphical purposes).

In the context of the overall structural analysis of the portal frame, their primary influence on the main frame analysis is through defining restraint positions for design checks.

Purlins and cladding rails can be added graphically to produce a visual 3D representation of the structure. In sophisticated models, especially 3D structures, longitudinal transfer members can be included in the analysis to simulate the diaphragm action of the roof cladding and purlins/cladding rails, allowing bracing systems at either end of the structure to share wind loads. Read more here: 📄 Modelling the Real World Effects of Roof Cladding on a Portal Frame

Purlins and Frame Analysis

The most significant role of purlins and cladding rails within the MasterSeries workflow is defining the points of lateral and torsional restraint necessary for the verification of the primary members (columns and rafters) against out-of-plane buckling.

• Restraint Spacing: The spacing of purlins and rails dictates the restraint spacings used in the steel design module. These locations are critical for calculating the buckling length of the main frame members, particularly for lateral torsional buckling checks.
• Automatic Restraint Optimisation: MasterPort modules provide tools to automatically set and optimise the location of purlins/cladding rails, which in turn define the restraint positions and add torsional stability stays where required (e.g., using Annex BB, previously Appendix G, checks in the Eurocode). For example, initial defaults for roof purlins are often 1.8 m centres and cladding rails are 1.4 m centres.