I'm modeling a performing arts center with BRBFs for the lateral system. When running the model in RAM Frame I've been getting errors about the model being unstable causing it not to run. I followed the steps listed in this link, http://communities.bentley.com/products/structural/structural_analysis___design/w/structural_analysis_and_design__wiki/ram-frame-troubleshooting-tn, and can reach step 5 but not go any further.
We've modeled a very similar performing arts center before using regular braced frames and that model had no issues running, so I'm not sure if the issue is the BRBFs not providing enough stability or I'm missing something glaringly obvious in the settings.
Thanks
There is an option in RAM Frame to exclude the BRBF's when analyzing the gravity load cases (Criteria - General). If the column bases and beam ends are pinned and there are no shear walls, this will result in no lateral stiffness and result in an instability when attempting to analyze those load cases. A similar problem occurs for tension-only members, which are also excluded when analyzing gravity load cases.
The frequently asked question, "Why am I getting large deflections under gravity loads?" on the web page below discusses this and some suggestions for avoiding the instability:
http://communities.bentley.com/Products/Structural/Structural_Analysis___Design/w/Structural_Analysis_and_Design__Wiki/ramss-tension-only-faq
Please post back if this does not apply to your model or if you follow the suggestions and continue to see the error.
I tried fixing all column bases to see if that would help and the same errors keep occuring. The most common error I've been getting is the one in the pictures attached, which is for a node in a frame column at a small partial level where nothing is framing into it.
communities.bentley.com/.../Instability-Error.png
communities.bentley.com/.../Instability-Error-2.png
The model runs for all loads cases if P-Delta is turned off. However, I see some large deflections associated with local effects of the semirigid diaphragm. See below.
Here are my recommendations:
1. Change the diaphragms on the Tech Mezz Level to Flexible/None diaphragms and set the Exposure in RAM Frame - Loads - Exposure to "None". These diaphragms are small partial diaphragms connected to only a few frame columns and not appear to be subject to wind loading. The mass of these diaphragms has already been combined to a diaphragm on an upper level.
2. In RAM Frame - Criteria - Diaphragm, check the box for including out-of-plane stiffness of semirigid diaphragms with one-way decks. The semi rigid diaphragms have a low flexural stiffness due to a small effective thickness and a relatively low Elastic Modulus. The effect of including the out-of-plane stiffness should be small. However, since the top of the columns are fixed, you may see a small moment transfer between the diaphragms and columns. If this is a concern, you can release moment at the top of the columns that are connected to a semirigid diaphragm.
3. Change the wind and seismic load cases to use specified frequencies and periods instead of calculated values. The P-Delta associated instability is closely tied to the Eigen Analysis used to determine these values. The Eigen Analysis is not completed if the frequencies and periods in each direction are specified. You can either use specified values from an analysis run without P-Delta or use a frequency of 1 Hz for wind load (rigid structure equations used) and T= Ta for seismic load.
4. Change the solver type in RAM Frame - Criteria - Analysis to the In-Core Direct Solver. The direct solver is more rigorous than the sparse solver and the more rigorous solution method will help bypass the error.
Answer Verified By: Josh Randall
I have a similar problem with my model. I tried these recommendations and am still receiving an "Unstable Structure Detected" in RAM Frame. I uploaded my file.
Karl - I've run into a similar issue. After turning off P-delta, It looks like the solution is simply related to meshing because the Dead Load displacement shows huge deflections in an area that has gravity columns. In the gravity columns module, they are being design for gravity loads, so I'm confused how to get this to run properly in Ram Frame. Do you have any thoughts?
I uploaded the file as "RAM Building 5-30-23"
Edit: I changed all the elements supporting the discontinuous roof frame to lateral instead of gravity and the model runs with P-delta on but only if using In-Core direct solver, which takes several minutes for a relatively small building. I feel like something else must be the issue.