We located the following response to your question. See below:

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Reply From: Daniel Membry, Autodesk Discussion Forums
Date: Jan/03/06 - 23:42 (CST) NEW!
Reply
Re: Trimming Structual Steel Shapes in MDT6
If your using the power parts - steel shapes, then suppress the fillets (round corners - inside and out) chamfer the edge you want to mitre (45 deg for a 90 deg corner) then unsuppress the fillets.
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