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I study seismic performance of steel braced frames. To do so, I model a 2D frame but in order to capture the compression out-of-plane flexural buckling of the braces, I perform a full 3D analysis. Lateral support at work points were added to account for global stability. In order to trigger the out-plane buckling of the braces, I need to set a Straightness out-of-plane imperfection about 4 mm at the middle of the brace. Are there any possibility to add this feature to the one element?

If you want to assign central imperfection, you need a central node. It means your braces should be discretized. But in this case you cannot use truss elements, otherwise your model will be singular. On the contrary you should model them as HongedBeams

Is this the case?