Earthquake loads @ FE codes and software: How are they applied?

[Jonathan29]

Hi to everyone,

I am struggling to understand the actual way the earthquake loads are applied to a FE model when either a commercial structural analysis software (like SAP2000, Abaqus or Ansys) or a FE code, like OpenSees, is used.

Especially, I am wondering if the earthquake loads (being in the form of strong ground motion accelerations) are applied to the base nodes (supports) of a structural model or they are applied in each one of the nodal masses that a model includes.

I would be grateful if you could help me to clarify this issue.

Best,

Jonathan

 

[BUGGAR]

What do the software instructions say?

 

[Jonathan29]

Hi Buggar.

I did not see your reply earlier because I had a problem with my account.

For example, SAP2000 documentation mentions that:

"Acceleration loads are used to simulate the ground motion of a time-history acceleration record. CSI software assumes complete fixity for all supports, then automatically computes acceleration loads at each node and structural object."

and

"CSI software automatically generates and envelopes a set of acceleration loads for translation along, and rotation about, each global axis. These acceleration loads are applied at each node and object to simulate ground motion during time-history and translation-only response-spectrum analyses.
Acceleration loads may be manually assigned to specific nodes and objects, then applied through a load case. Acceleration loads in the program are included as a percentage of gravity, g. Therefore, to manually assign acceleration loads to selected members, first divide the acceleration by g. Then enter the percentage in the Scale Factor box in the Loads Applied area of the Load Case Data form."

So, based on the above, is it reasonable to conclude that, at least, in SAP2000, the seismic acceleration loads are applied at each one of the nodal masses of the entire structure and not only to the base node (where the supports are defined)? So, do they keep the base fixed and vibrate all the masses above? I would assume the opposite, but probably it is the same. Do you agree?

Thanks again for the interest

Best

Jonathan

 

[Retrograde]

There are applied as forces at the nodal masses. The forces are taken as the mass matrix multiplied by the earthquake acceleration vector.