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The following settings pertain to the beam stability factor CL from NDS 3.3.3.
Unsupported length Lu ends at points of zero moment
Since NDS C3.3.3.3 refers to the compression edge of the member when determining whether it is laterally supported, many designers measure the unsupported length Lu on a multi-span beam only over those distances that the member is in compression, that is, treating points of zero moment as if they were lateral support points.
Other analysts question this approach; for example the CSA Handbook of Steel Construction does not allow it based on work done by Schmitke and Kennedy, 1985. The Canadian Wood Council’s Introduction to Wood Design for the 2011 edition, example 7.5, does not include the points of zero moment, however a note at the end of this example says that as an alternative approach, they may be used.
This setting allows you to specify which of these approaches to use to analyse the beam, by including points of zero moment as if they were lateral support points, or not including them.
Built-up member width b for slenderness ratio
Since research has shown that nailed and bolted beams have at most 30% composite action effect in terms of resisting torsional buckling, the program allows you to choose whether to use the full member width or the single ply width as b in the expression for the slenderness ratio RB in NDS 3.3.3.6.. The slenderness ratio is used to compute the lateral stability factor. It is recommended to use single ply width unless adhesives are used to laminate the members together.
Note that this setting applies only to lateral stability; design for weak axis bending does not incorporate composite action and treats each ply separately when calculating the section modulus S.