March 25, 2015
How are Unbraced Lengths coordinated between RISAFloor and RISA-3D?
In a RISAFloor model the columns and beams can be assigned unbraced lengths, or the program may calculate them automatically. This unbraced length information can then be used in RISA-3D. A great example is the frame below:
The beams along Grid Lines 1 and 2 have fixed ends. Therefore their bottom flanges are in compression, so they need to be braced. The 12” deep gravity beams framing into them should provide sufficient bottom flange restraint for the lateral beams, so the Segment command was manually entered for the lateral beams:
The program now considers the gravity beams (spaced at 7.5 feet o.c.) to brace the lateral beams:
When the model is transferred into RISA-3D, the gravity beams are no longer present though. Not even a node exists at their end locations:
How can we ensure that the RISA-3D model knows about the gravity beams providing bracing? In the RISA-3D Members spreadsheet the unbraced lengths all report “Floor”. This means that RISA-3D is using the unbraced length information from RISAFloor:
When solving in RISA-3D the unbraced lengths from RISAFloor are then used for those members: