Issue: moment being transferred at pinned connection in model

[Anonymous]

I am currently trying to model a four span continuous beam that frames between two walls across three columns. I have place pinned-pinned releases at each column and the beam. There is a slab above that is modeled as a rigid diaphragm. RLINKS are enabled. 

When viewing the moment diagram for the beam, there are "jumps" at the column locations that indicate that some moment is being transferred to the pinned beam to column connection. This is not what is expected to happen. I have attached a screenshot of the moment diagram and my project file. 

Also, Type 1 instabilities occur at most nodes in RX, RY, RZ. I'm very new to Robot, so I'm not exactly sure what is causing these.

Could somebody please help me troubleshoot this problem?

Any help is greatly appreciated. Thank you!

[Anonymous]

I should also mention that the rigid-diaphragm slab has full stiffening because when it was partially stiffened in the XY the calculation would freeze.

[EduardoMeruvia · Accepted solution]

Hi devan.m,

Just had a look at your model and my comments would be as follows;

- The instability warnings you are getting is due to the pinned-pinned releases you have on your columns ALL which have also a PINNED support. A pin support on a node with a pinned release is a mechanism, and thus you structure would be unstable. 

- To fix the instability warning change the pinned-pinned columns releases to a pinned-FIXED with the fixed end at the pinned support node. This will elimate your instability warnings in your model. 

- Regarding the moment distribution on your 4 span continuous beam, I think the moment shown in the diagram is correct and the reason that you get a small difference at the columns interface is due to the different spans length this beam is spanning to i.e. each span attracts different magnitudes of load which creates different moments. Also note in figure below how the difference in moments is proportional to the difference in spans. You could do a hand calculation to verify this. 

I've attached your model with fixed resales on the link below for your reference. 

Ed

[Anonymous]

Ed,

Thank you so much for your very informative and helpful reply! Everything makes a lot more sense now. I really appreciate your help.

Devan