Unsupported Length lu

The unsupported length l_u from O86 7.5.6.4.1 determined for each span as the smaller of the following distances:

• between the two points of bearing bracketing the span that have been designated as having lateral support at the point of bearing. For a simple span beam, these are just the two supports at each and of the beam.
• the user-input lateral support spacing
• if you have chosen to allow this in the Beam Stability group of the Design Settings, the distance between points of zero moment within a span (counterflexure) or between a laterally supported point of bearing and a point of zero moment.

Points of zero moment

Since O86 7.5.6.4 refers to the compression edge of the member, many designers measure L_u 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, 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, zero moment may be used.

Multi-span members:

For multi-span systems, if the maximum moment occurs within a span, then L_u is taken for the span in which the maximum moment occurs. If the maximum moment occurs at a support, the largest l_u from either side of the support is taken.