Ram SS Large Nodal Displacement

Question

I have a 2 story building where the lower portion of the floor plan must cantilever out due to existing conditions. I modeled the cantilever using a combination of gravity extensions and stubs as shown in the floor plans. When I run RAM SS and go through the Frame module I am getting large nodal displacements on the order of between 500 and 2000 inches in the gravity load cases. The building behaves normally under wind and seismic so I must be missing something with the cantilever. Should I fix the base of these columns?

Seth Guthrie · Answer

It looks like you might have a pin where you intended a fixed connection. Double check the beam fixity. While stub cantilevers default to being fixed at the backspan it's possible for this to be changed or reassigned as a pin leading to such problems. If the braces are tension only, then also see: https://communities.bentley.com/products/ram-staad/w/structural_analysis_and_design__wiki/3058/ram-frame---tension-only-faq

Seth Guthrie · Answer

Not usually. In Ram Frame the gravity beams are not actually part of the finite element model. It's only the reactions of those beams applied as point loads to the lateral members that we use. A gravity stub cantilever then apply\ies a shear reaction and a moment at the support. 
 There is a way to set a gravity beam fixity in Ram Concrete, and there is a way to do this through Ram DataAccess, but neither would apply in Ram Frame analysis. 
 See also: https://communities.bentley.com/products/ram-staad/w/structural_analysis_and_design__wiki/6513/ram-ss---analysis-types-tn

Seth Guthrie · Answer

Use View - Foundation Nodes to confirm support points at the base of the frame, especially if the bottom of the column is moving a lot. If there is none, check the Modeler - Datacheck. That might happen if the frame sets upon gravity framing for example. 
 If it's the top of the column that's moving a lot then check if it's tied to the rest of the model with a diaphragm using View - Disconnected nodes. If it's not connected, then it's free to swing or pivot like a gate. Remember, the gravity members are not in the finite element model so if you are relying on gravity beams to hold things together, those beams need to be lateral too.

Seth Guthrie · Answer

A Pseudo-flexible diaphragm has no stiffness. A 2-D frame connected to a pf diaphragm and pined at the base can easily fall over, or swing out in this case. As noted on the wiki, https://communities.bentley.com/products/ram-staad/w/structural_analysis_and_design__wiki/3173/ram-frame---pseudo-flexible-diaphragms-faq, adding some small rotational fixity at the base is one good solution.