I am currently running a spectral analysis of a model of a steel frame structure using Robot. The structure is attached to a supporting structure.
I have supports representing where the structure is connected to its supporting structure.
I have input the (envelope of the) secondary response spectra for the supporting structure at the location of the supports.
Then I take the output displacements from specified nodes. I have had a look in the Robot help, but I can't find whether this output displacement is the relative or absolute (i.e displacement of the node relative to the supports + the input displacement) displacement.
Please can you let me know which one is output.
Thank you for your help
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Thank you for your reply Artur,
Since posting this I had a think about how I could work this out myself.
The displacements of the nodes at the supports (fixed supports) are always 0. Even for the spectral analyses.
Therefore I concluded that the displacement given are relative to the input motion not absolute displacements.
Please let me know if I'm mistaken.
Thank you for your help.
If by relative you mean with the respect to the ground or the points where the smaller structure is 'attached' to the 'larger' one then you are right. The nodal displacements given by Robot are with respect to the supports.