Have a look at the layer Intersection Priority number (in the layer settings) of the layer that the beams are in.
Is it set to 0 (zero)?
If so the beams will not trim.
It could be that you are using a different layer combination in plan and 3D - the IP number for each layer is saved with the layer combination.
Barry.
One of the forum moderators.
Versions 6.5 to 27
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