For our dynamic analysis (earthquake, spectrum), we applied an enveloped spectra. Later, we decided to include seismic anchor motion by applying a displacement to one anchor node (7630). All other node groups were set to zero displacement.

We compared results between:
a. Enveloped spectra applied to all joints (no seismic anchor motion - anchor movement column left blank)
b. Enveloped spectra + SAM = 8mm at one anchor node
c. Enveloped spectra + SAM = 0.001mm at one anchor node

On a particular portion of the pipeline, the pipe maximum displacement is:
a. 7mm
b. 23mm
c. 19mm

We are curious why displacement of c. is closer to b. than a., even with SAM of only 0.001mm? A little surprised that the ISM method yields different results than the USM method in this case.

Thanks in advance for any insight.