The problem is as described in the subject. As per the following post, if the structure is in equilibrium instability error should not arise.https://communities.bentley.com/products/ram-staad/w/structural_analysis_and_design__wiki/8063/lifting-analysis
I have attached the model. Can you please advice on the cause of instability.
Staad Version: 21.00.02.43FundamentalLiftingTest.std
The structure is unstable as the assembly is hanging from a single support that is pinned. The pinned support at node 1000 is a hinge with no ability to resist any rotation about any of the axes. So if you apply any lateral load along global X or Z at the nodes like 117,131 or 151, the structure will not be able to resist that load and the load will simply get lost. If you try adding a unit lateral load at node 117 for example, you will see how the solution would result in abnormal displacements due to this unstable arrangement. You may be able to get an analysis for just vertical loads but the software will always generate zero stiffness/instability warnings and try to add weak springs internally to stabilize the solution.
There is no lateral load acting on the structure. It is pure self-weight and as per the article in the link I have mentioned, it quotes the following:
If one wishes to avoid using stabilizing rotational springs, the structure has to be analyzed in the equilibrium position.
For this, the eccentricity needs to be determined (the horizontal distance of CG from the X and Z coordinates of the point of suspension). The center of gravity of loading can be found in the output file with the help of the PRINT CG command .
From this data, Θ be computed . Next, rotate the structure by that angle “Θ” , so that the load is aligned with a line passing through the support coordinate. Thus, you are now analyzing the arrangement at the equilibrium position.
The structure is in the equilibrium position as mentioned in the article. So I do not understand the reason for the instability unless the article is inaccurate.