I am trying to model what would appear to be a simple steel lifting frame with 1 bracing member. I have so far created a model but I am getting quite a few warnings when attempting to analyse about zero stiffness in members etc. (I think this is due to using tension only members for the cables). I have attached the Staad file and the CAD drawing of what I am trying to model. All I am trying to do is model a frame with all pinned connections held in place by tension cables to determine all forces and moments acting on the structure to make sure that it matches my hand calculations. The lifting frame is meant to hold up a force of 240kN from the crane hook (the node at the top) or a force of 60kN at each of the corner frame nodes. I applied 60kN to the corner nodes and just put a pinned support at the crane hook. I would love to be able to get this model working.
Any assistance would be greatly appreciated. Steel Lifting Frame_edited.std
For lifting analysis, you should use Non-linear cable analysis. Also you have used ground supports for frame which I removed as while lifting the only support should be the crane hook point. I have used the member truss command for the angle sections to simulate their pinned end connections. Please find attached the modified model. You can now match your hand calculations with considering the self weight of the structure.
MadhanSteel Lifting Frame_edited_r.std
Answer Verified By: Ricardo Graham-Ward
Thank you for the help so far Madhan. The file is running without warnings/errors now but I do have a few questions. When I run the analysis in Staad, it is showing that there is almost no axial force acting in the bracing member. This must be incorrect as my hand calculations has shown it to be in tension with a considerable axial force (unless my hand calculations are incorrect?). Please assist with this issue.
When you lift this platform with the four slings attached to the corners, all the members in the platform will be subjected to compression. I don't expect any members in tension except the lifting slings. Here is the axial force diagram which is as expected.
The following changes were made to the file
*1 3 12
was changed to
*5 FIXED BUT FX FZ MX MY MZ
5 FIXED BUT MX MY MZ
*PERFORM CABLE ANALYSIS BASIC SAGMINIMUM 0.7 STABILITY 5 145
PERFORM CABLE ANALYSIS BASIC STABILITY 6 145
The cable analysis runs fine with the basic analysis engine and results match pretty nicely with those coming from advanced cable analysis. If you are required to carry out nonlinear cable analysis frequently, I would suggest that you get the license for the advanced analysis engine. It has a far superior and advanced cable analysis algorithm and does not require tweaking the stiffness parameters.
A modified file is attached for your reference.