I am preparing a lift analysis using cables and 2 spreader beams. I have tried multiple support types and tension members (for the cables) which resulted in unstable conditions. My latest attempt is using "Fixed but" supports and cables with a non linear cable analysis. The structure appears to be stable and the results look like I would expect. In the output, I notice the program is noting zero stiffness (directions 4,5,6) at the supports (crane hook) and ignoring the the moments. I also have an instability at one of the spreader beam attachment points and STAAD added a weak spring. In my model, the lifting points are geometrically centered, but NOT aligned with the center of gravity. I'm assuming the rigging contractor will adjust his lift points according to the COG. STAAD file is attached.
- Since this is my 1st time using this analysis type am I doing it right?
- Is the added spring on the spreader beam connection due to unbalanced loading (not aligned with COG)?
- When the program says "weak spring added" - how strong/weak is it?
5542 - ChemPro GTE Lift 4.std
The added spring stiffness in the unstable direction is just sufficient to append division by zero in calculations and run the analysis. Best is to check the displacements/rotations - if they are in reasonable limits. Please see the weak spring warning explained by Sye in this thread:
In most cases it is enough to run a linear elastic analysis for lifting scenario. You can add PRINT CG command to get the coordinates of center of gravity printed to Output file. In your model there is a few inches shift in distance between lifting points and CG. If there will be people holding ropes on site - this may be considered by adding supports like: FIXED BUT FY MX MY MZ - check the support force values to adjust the number of such rope holding points to correspond man power.