Hi,
I am modeling a three-story steel Buckling Restrained Brace Frame structure in RAM Structural System. I have added spread and combined footing at the foundation level which is the base level. The second floor level has composite steel framing system with concrete over metal deck. The middle level is a partial roof system with both composite and non-composite framing system. The top level is non-composite framing system with metal deck. The lateral system of the building has buckling restrained brace frame system.
After modeling the structure, I did the DataCheck and no errors were found. Gravity beam and column design also works perfectly. I have created both ELF load cases and Response Spectrum Analysis load cases for dynamic analysis. The model shows no errors while running the load cases without P-Delta effects. All the dynamic analysis completes without any error with P-Delta option on for dead loads.
I am getting an 'Unstable Structure' error when running the model for Live Load case with P-Delta option on. I have all the diaphragms defined as semi-rigid diaphragms. Some of the areas in the model has live loads upto 125psf. The model shows no errors if I use a live load upto 50psf.
As noted in some of the other discussions in the community, I tried several model checks including the following.
1. Eigen analysis - tried all different options (eigen vectors, ritz vectors).
2. Tried all the different solver types
3. Check different diaphragm types'
4. Tried parametric checks on geometric tolerances, convergence margins etc.
5. Check for any disconnected nodes through model data output - I have chevron braces in few locations and saw the beam nodes where the braces connected are shown as disconnected. I have redrawn the members and getting the same error message.
Could you please provide me insights on any additional ways to check from where the instability is occurring ?
The following is the error message I received. I checked around the locations as noted in the error message, but didn't find disconnected members or illegal releases or boundary conditions.
Thanks for your help in advance,.
Jeena
