Determining the buckling length is straightforward in simple cases, but it becomes much more complicated when working with frames. Most software leaves it up to the user to define these lengths correctly, and this is often where mistakes are made.
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Buckling Length from Rules of Thumb
There is an old rule of thumb widely used by many engineers. It suggests that the buckling length can be taken as the distance from the support to the top of the building. This method is quick and easy to apply, which is why it’s still commonly used today.
This method is not very accurate nor conservative.
Buckling Length from Eigenvalue Analysis
A more accurate way to determine the buckling length is through an eigenvalue analysis. The critical load from the analysis can then be converted into an equivalent critical length using the Euler equation.
This approach is much better, but the downside is that it is dependent on the applied load.
The lengths from the eigenvalue analysis are very different from those given by the rule of thumb, which does not seem conservative.
Imperfections
Buckling can also be handled by applying imperfections and performing second-order analysis. This results in increased internal forces.
The verification can then be carried out using cross-section checks without applying reduction factors.
PolyFrame
PolyFrame automatically applies an imperfection based on the first eigenmode and performs a second-order analysis, thereby handling stability checks automatically.
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