Seismosoft | Earthquake Engineering Solutions

During dynamic nonlinear analysis I included additional Rayleigh damping and after certain number of steps (0,8s) analysis terminates with fractures only in columns. When I exclude additional damping, analysis terminates without fractures or yielding in any member of the frame. Is there another way of overcoming this problem (it is obviously an error, because load (acceleration) is too low to cause fracture in columns)

Have you defined correctly the damping parameters, i.e. are the period values that you input the periods of the 1st and 2nd eigenmode in the direction of the earthquake?

Seismosoft Support

Yes, I did. I defined also damping ratio - 5% for both modes.
Sandra M.

Can you please send us the model at support@seismosoft.com, so that we can have a look? This behaviour is rather strange.
Thank you.
Seismosoft Support

Hi Sandra,

The fracture was caused by unbalanced damping forces in the Y direction, when you deactivate the masses in that direction (in the Gravity and Mass page of the settings). See the deformed shape of your structure to understand.
If you support the out-of-plane directions (add restraints: y+rx+rz to all the upper nodes) you will get the behaviour you want.

SeismoSoft Support

As I said in the e-mail, when I define 'Global Mass Directions' as 'X,Z,RY' (frame is in XZ plane) analysis terminates after few steps. But, when I include all (X,Y,Z,RX,RY,RZ) mass directions, analysis terminates without any problem. That is strange, but results are OK.
Anyway, thank you for suggestion, I will see what happens when I define X,Z,RY and with restrained upper nodes.

Sandra

Hi Sandra,
Just a clarification because in our previous message it was a bit vague: with 'upper nodes' we meant all the nodes that are not already supported at the base of the building, i.e. restrain evey out-of-plane deformation.

Regards,
SeismoSoft Support