Can a modal dynamics step solve for acceleration response below the lowest mode?

[AdamB85]

I'm performing a modal dynamics analysis in Abaqus with the requisite frequency step beforehand. Both steps are set to 1-4000Hz. The first mode isn't found until ~90 Hz and when I plot either total or spatial acceleration from the modal dynamics step for a node anywhere on the geometry, I'm only given output from 90-4000 Hz. I need that low frequency response output though. Is this something obvious I am overlooking?

 

[FEA way]

I think that you mean steady-state dynamics since (transient) modal dynamics analysis is in time domain.

In SSD step settings, you can control the number of points at which the results are given. From the documentation:

If INTERVAL=EIGENFREQUENCY, this is the number of points at which results should be given, including the end points, from the lower limit of the frequency range to the first eigenfrequency in the range; in each interval from eigenfrequency to eigenfrequency; and from the highest eigenfrequency in the range to the high limit of the frequency range.

 

[AdamB85]

I was actually mistaken, this is not for a modal dynamics step but rather a random response. So unfortunately I don't see the interval option in the step settings.

I checked the documentation for random response and it does appear that it uses the extracted natural frequencies to compute the dynamics outputs - is there any way around this so I could compute the response for that lower frequency range?

 

[FEA way]

There's no INTERVAL parameter for random response but the number of points is defined in the same way:

Number of points between eigenfrequencies at which the response should be calculated, including the end points, from the lower limit of the frequency range to the first eigenfrequency in the range

You should also check the output frequency settings.