Instability in Frame Analysis

How different node types, bracings and bracings out-of-plane is affecting the instability calculations in Frame Analysis

Important! A node defined using the tool Member will automatically be braced with regards to flexural buckling out-of-plane. If no bracing with regards to flexural buckling is requested, Unsupported joint can be used. The node will in that case have enough stiffness to be sufficient supported to be assumed as pinned with regards to lateral torsional buckling. 

In the case above both the buckling length with regards to flexural buckling out-of-plane and lateral torsional buckling length is 2m.

In this case the buckling length with regards to flexural buckling is 6m, while buckling length for lateral torsional buckling is assumed to be 2m.

Continuous bracing

In the window above continuous support on element 1 and 3.

Single bracing

In the window above bracings with spacing 1m at the top level of element 2 is given. 

To show lateral supports graphically the option shown below needs to be activated.


This will grant the following feedback:

When continuous supports has been applied in both edges, no flexural or lateral torsional buckling out-of-plane can occur. When one of the edges has been continuously supported no flexural buckling out-of-plane can occur, but since the free edge can buckle, lateral torsional buckling may happen.

When single lateral supports has been defined, flexural buckling out-of-plane and lateral torsional buckling between the supports occur. For pure flexural buckling out-of-plane, the buckling length will be calculated with regards to any supports along the length. In case of torsional-flexural buckling, the buckling length will be calculated for each edge and the maximum value will be used. For lateral torsional buckling the capacity will be calculated for edges that are compressed in the current load combination only.

Point bracings along a edge in tension is not part of the capacity with regards to lateral torsional buckling.

For an element with the moment distribution and point bracings as above there will be no regards taken to the rightmost support, since it is at an edge in tension.

The element will therefor be partitioned in three parts with lengths 2m, 2m and 4m.

Buckling stiffeners

In the window for Buckling stiffeners it is possible to assign possible buckling stiffeners in the web of the element. If buckling stiffeners are applied it is possible to raise the capacity of web buckling for cross sections with slender webs. Buckling stiffeners can only be applied for open cross sections.



According to the picture above two buckling stiffeners is given for element 2. 

With the option Details all given data for the design will be shown graphically like for member 2 below.



Several of the capacities that are shown in the window are calculated with some kind of instability in mind. More information how the instability are reducing the capacity can be shown in Capacity.

From the window above it is possible to read that with regards to lateral torsional buckling there are supports/bracings at 1m and at 2m from the element starting node. Flexural buckling out-of-plane are calculated. Flexural buckling out-of-plane is calculated with a buckling length of 1m with regards to mentioned supports. Torsional buckling is calculated for the whole length of 4m since atleast one flange is unsupported. Shear buckling of the web is calculated with regards to the stiffeners at 1.5 and 3.0m along the element.



Joakim is the author of this solution article.

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