Hello All,
I’m performing dynamic analysis on a piping system that initiates at top of building A (anchored to the floor), continues on building B (there is one branch off the main header that’s anchored to the top floor at bldg. B) and terminates underground (Modeled as an anchor support with some stiffness).
I have got all the modified response spectrum and movements for both buildings as well as the ground shock and displacement. Per the structural engineer, top floors at both buildings can experience 10” lateral movement in all directions ( ± X & Z). Ground movement would not be more than 1”.
This case is eventually similar to the case that was briefly discussed in one of the past webinars by Mr. David Diehl.
What is puzzling me is that when I run the analysis and check the displacements, I don’t see either ± signs on the displacements or a large differential displacement (20”) since I was expecting that building A moves to the right while building B moves to the left. Is this because of an errors in my inputs like I didn’t specified the SAM displacements with signs? Or maybe the structures periods/ stifnesses are close and they somewhat move together?
I really appreciate your inputs/advises.
Thanks,

Yes, support motion can be a major player in systems with more than one shock component. You have your deflections entered and I assume your supports that you are moving are included in your node numbers defined for these shocks.
Check your Control Parameters. Two of these are related to this support movement. See below.
Response spectrum analysis has no sign. Modal results are combined absolutely or SRSS. Inherent to this method of analysis, we know the maximum response (provided in the response spectrum) expected for each mode of vibration be we have no timing information to relate one modal max to another. The response spectrum method envelopes all expected modal maxima and you will see no signs.
(Each set of support motion is evaluated independently of any other set of support motion and then all are combined with the modal results by the method identified in the Control Parameters.)