RhinoBIM

RhinoBIM is a Ning community focusing on extending Rhino with BIM features: http://www.rhinobim.com/

Currently, they have a structural-oriented extension available as "Work-In-Progress" for registered members of the community.

RhinoBIM is a suite of a number of planned plug-ins to enhance Rhino 5.0 for the Architectural, Engineering and Construction Industries. Developed by Virtual Build Technologies LLC, the first module now available is called RhinoBIM Structure, for 3D structure modeling, Clash & Clear analysis and structural analysis.
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The first tool of the RhinoBIM suite of products has been released as a Work-In-progress (WIP). You must be signed up as a member of RhinoBIM in order to download the WIP and give it a test drive.