Rigid members – upper limits for stiffness
Category: Structural dynamics
Problem
How to quantify “too large” a variation in stiffness?
According to Bladed’s “Project Info Options” manual :
"Bladed modal analysis cannot cope with a support structure which contains too large a variation in element stiffness. It is not recommended that users model rigid members by entering a very large stiffness value. It is better to make some of the stiffer elements rigid.”
Solution
The condition is that the matrix “reciprocal condition number” must be > 1e-15.
This basically means that the maximum allowable ratio of the smallest and largest eigenvalue of the stiffness matrix is 1e15.
If Bladed doesn’t fail then the matrix should be well enough conditioned to give accurate results.
Keywords Offshore, rigid members, stiffness, modal analysis, support structure