Seismic anchor motion question

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.

If you have defined start/stop nodes, then you're actually running ISM, not envelope spectra. If you run ISM and DO NOT define anchor movements then the software will estimate this value and use it.

If omitted, the program estimates this value as the displacement derived from the response spectrum entry corresponding to the lowest frequency. Leaving this value blank does not mean the value is zero.