What is the "Prevent out of plane instability" factor for and when would I use it?
Not version-specific
Tekla Structural Designer
Environment
Not environment-specific
Answer
Briefly, the setting is for use when buckling is causing solution problems in 2nd Order Analysis. For more about 2nd Order Analysis see the related article What are the "2nd Order" Analysis options on the Analyze Ribbon in TSD?
An example of when such buckling problems might occur is in members subject to axial loads which are relatively slender about the minor axis - such as sloping rafters - where secondary members (such as purlins or rails) which would reduce the buckling length are not included in a model. For the full context of when you would usually use this setting, see the Related Article I have analysis Warnings and/ or Errors about buckling. Why is this happening and what do I do about it?A remedy that can be used in this case (for Steel, Cold Rolled/Formed, Timber and General material members) is to activate the Prevent out of plane instability factor. This is listed in the member properties and is activated by a simple check-box as shown below.
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When activated, the minor axis inertia of the member is multiplied by the factor, thus increasing its minor axis (theoretical elastic) buckling capacity. This only occurs in the 2nd Order analysis solver model. The default value for the factor is 20. This is just a sensible initial value, not a 'magic' number or one that will work in all circumstances. This behaviour also applies to the pre-size sections assigned during Steel Auto-design using 2nd order analysis. Hence the setting can be used to assist when buckling is occuring in the 2nd Order analysis that is part of Steel Auto-design. The increased minor axis inertia is NOT used in design checks.
Since the setting adds ‘artificial’ stiffness to the model, the engineer should take care to ensure that it does not inappropriately increase the lateral stiffness of the structure as a whole leading to inaccurate results.