Use the Report - Analysis Log to get further details. Warnings are listed at the beginning of the report under "Summary of Numerical Behavior of Model". The most common illustrated below:
1. Local, instabilities. The instability wiki discusses some common causes for these.
Typically they occur where all the frame members at a joint are pinned leaving the node free to rotate.
Starting with version 15.02 when using a semi-rigid diaphragm with the Criteria - Diaphragm settings such that out-of-plane stiffness is ignored, analysis log instabilities are common, for most all of the diaphragm nodes. The program used to apply a small out-of-plane stiffness factor to the nodes of all semi rigid diaphragms to avoid instability in earlier versions. With version 15,02 that factor was reduced further to solve another issue, but the side effect is more instability messages. For flat diaphragms, it's safe to ignore these messages since no out of plane forces are being applied to he diaphragm anyway. For very thin semirigid diaphragms with negligible out-of-stiffness to begin with, using the option to consider out-of-plane stiffness is also an option to eliminate the warnings.
2. Loads on walls partially ignored. This happens when an opening in a wall crosses a story line when there are floor decks/loads or line loads modeled on the floor that is met or crossed. The plan view does not show these openings so the elevation view should be used to review these conditions. Note, it’s only the loads from one-way decks, lines loads and wall self-weight affected by the warning, loads from meshed 2-way decks are still considered. Wall self-weight is affected because wall weight is calculated for the wall segment and applied as a line load at the top of the wall.
3. Rigid end zone warnings. These tend to happen when the length of the Rigid End Zone based on intersecting beam and column dimensions is greater than the center to center length of the member. In those situations, the original center to center length is used in the stiffness matrix for the affected members.
The next section, "Summary of Equilibrium Check of Model" includes the total reactions for each loading. The program includes the nodal reactions of all columns and walls even if they terminate higher than the base, so the sum of the reactions can be checked against the total applied loads to confirm equilibrium, Small deviations are expected when performing P-Delta analysis.
Note, the summation of moments is now reported about that global model origin so that overturning equilibrium can also be evaluated. In older versions (< v14) the reported moments were a simple summation of the column base moment reactions. Refer to the Modeling wiki for more on story height and how it affects the elevation of rigid diaphragm lateral loads
Structural Product TechNotes And FAQs
RAM Instability In Finite Element Analysis