Reply To: Springs in modal analysis.

[peteroznewman]

Hi Javat,

Animate the first and second modes. What is the principal direction of movement?  That is the direction where there is zero stiffness.

To eliminate all rigid body motion, you need a minimum of stiffness in 6 directions, and that is only sufficient if the directions are not redundant.

For example, if you have a frame that is attached to ground at 3 points, and ground is the X-Y plane with Z vertical, the first point can have stiffness in 3 directions (X, Y, Z).  The second point must be separated from the first point along the X axis and needs stiffness in the Y and Z directions.  The third point must be separated from the first point along the Y axis and needs stiffness in the Z direction. This is the minimum to avoid a zero frequency mode.

You can have springs with stiffness in 3 directions on 3 or 4 attachment points, that is fine and may increase the frequencies of some modes.

COMBIN14 is a 1D spring element, so to get stiffness in 3 directions, you need 3 springs. KEYOP(1) = 1 is for nonlinear damping. To get no rigid body modes, you would need 6 springs oriented in the correct directions as described above. If you wanted stiffness in 3 directions at 4 attachement points, you would need 12 COMBIN14 springs.

Use one COMBIN250 at each attachment point which is a spring element that can have stiffness in 6 DOF (X, Y, Z, Rx, Ry, Rz).