Modes Beyond End of Spectrum
Folks,
I'm running ANTYPE,MODAL followed by ANTYPE,SPECTR. The Help Documentation states that I should extract modes in the upstream modal solution, that are 50% higher than the spectrum data. For example, if my spectrum ends at 100Hz, I should extract modes up to 150Hz. The documentation states that the spectrum solver uses the last data point of the spectrum, for modes beyond the spectrum. So I think that means a mode at 140Hz would use the spectrum acceleration at 100Hz. That seems like it would alter the results though. Can someone explain the rationale for the solver's approach to use the last spectrum data point for modes beyond the spectrum? Is it conservative?
Matt Ridzon, PE, MSME
Sr. Engineering Analyst
Email matt@prime-engineer.commailto:matt@prime-engineer.com
Mail 266 Main St, Burlington, VT 05401
Web www.prime-engineer.comhttp://www.prime-engineer.com/
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Matt,
It is. On the far right-hand of the spectrum, the acceleration is assumed to flatten out to a fixed value. Your curves may refer to something called ZPA or zero period acceleration, which is a rigid body acceleration felt by all super-high frequency modes above your cutoff.
What's not conservative, though, is that your model probably didn't account for all modal mass at your cutoff frequency. You need to add that back in and multiply the "missing mass" by the ZPA as an extra mode to be combined with all your other modes. I'm short on time so I forget what commands/flags one needs to throw to have ANSYS automatically account for the missing mass, but if ANSYS isn't doing it automatically for you via some clever flags, you need to ensure you deal with it yourself.
My seismic experience comes from the nuclear world. Not sure if that's what you're doing today or not, but either way, you should google "Standard Review Plan 3.7.2" and "Regulatory Guide 1.92" and you will get some background on these questions and related questions you've been asking lately.
Regards,
David J. Gross, P.E., ASME Fellow | Dominion Engineering, Inc.
Director, Federal Services
12100 Sunrise Valley Drive, Suite 220 | Reston, VA 20191
office 703.657.7300 | desk 703.657.7311 | mobile 301.580.3066
dgross@domeng.com | domeng.com
-----Original Message-----
From: Matthew Ridzon, PE via Xansys xansys-temp@list.xansys.org
Sent: Monday, March 16, 2026 4:30 PM
To: XANSYS Mailing List Home xansys-temp@list.xansys.org
Cc: Matthew Ridzon, PE Matt@prime-engineer.com
Subject: [Xansys] Modes Beyond End of Spectrum
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Folks,
I'm running ANTYPE,MODAL followed by ANTYPE,SPECTR. The Help Documentation states that I should extract modes in the upstream modal solution, that are 50% higher than the spectrum data. For example, if my spectrum ends at 100Hz, I should extract modes up to 150Hz. The documentation states that the spectrum solver uses the last data point of the spectrum, for modes beyond the spectrum. So I think that means a mode at 140Hz would use the spectrum acceleration at 100Hz. That seems like it would alter the results though. Can someone explain the rationale for the solver's approach to use the last spectrum data point for modes beyond the spectrum? Is it conservative?
Matt Ridzon, PE, MSME
Sr. Engineering Analyst
Email matt@prime-engineer.commailto:matt@prime-engineer.com
Mail 266 Main St, Burlington, VT 05401
Web www.prime-engineer.comhttp://www.prime-engineer.com/
[A blue hexagon with white letters Description automatically generated] PRIME ENGINEERING LLC
This message (including any attachments) may contain confidential, proprietary, privileged and/or private information. The information is intended to be for the use of the individual or entity designated above. If you are not the intended recipient of this message, please notify the sender immediately, and delete the message and any attachments. Any disclosure, reproduction, distribution or other use of this message or any attachments by an individual or entity other than the intended recipient is prohibited.
In linear frequency domain analyses, the response at a given input frequency (say 100 Hz) is the summation of the response at all modes. A mode at 90 Hz will have some contribution to the response at 100 Hz. So will a 110 Hz mode. In practice, for lightly damped structures, the amplitude of response of a mode usually drops off 'fairly' rapidly away from the frequency of that mode. Some industries use 2x the max input frequency ... it seems ANSYS is suggesting 1.5x as a start. The frequency at which you truncate also depends on the output of interest ... for example I've found that displacements are more forgiving (tend to need fewer modes) than bending loads.
I too don't recall the missing mass command in APDL, but look for a term like residual mass or residual vectors.
Regards,
Alan
Alan Baker
Associate Chief for APU Dynamics
Engine Systems & Component Analysis
Honeywell | Aerospace
alan.baker2@honeywell.com
-----Original Message-----
From: David Gross via Xansys xansys-temp@list.xansys.org
Sent: Monday, March 16, 2026 3:53 PM
To: XANSYS Mailing List Home xansys-temp@list.xansys.org
Cc: Matthew Ridzon, PE Matt@prime-engineer.com; David Gross dgross@domeng.com
Subject: [External] [Xansys] Re: Modes Beyond End of Spectrum
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Matt,
It is. On the far right-hand of the spectrum, the acceleration is assumed to flatten out to a fixed value. Your curves may refer to something called ZPA or zero period acceleration, which is a rigid body acceleration felt by all super-high frequency modes above your cutoff.
What's not conservative, though, is that your model probably didn't account for all modal mass at your cutoff frequency. You need to add that back in and multiply the "missing mass" by the ZPA as an extra mode to be combined with all your other modes. I'm short on time so I forget what commands/flags one needs to throw to have ANSYS automatically account for the missing mass, but if ANSYS isn't doing it automatically for you via some clever flags, you need to ensure you deal with it yourself.
My seismic experience comes from the nuclear world. Not sure if that's what you're doing today or not, but either way, you should google "Standard Review Plan 3.7.2" and "Regulatory Guide 1.92" and you will get some background on these questions and related questions you've been asking lately.
Regards,
David J. Gross, P.E., ASME Fellow | Dominion Engineering, Inc.
Director, Federal Services
12100 Sunrise Valley Drive, Suite 220 | Reston, VA 20191 office 703.657.7300 | desk 703.657.7311 | mobile 301.580.3066 dgross@domeng.com | domeng.com
-----Original Message-----
From: Matthew Ridzon, PE via Xansys xansys-temp@list.xansys.org
Sent: Monday, March 16, 2026 4:30 PM
To: XANSYS Mailing List Home xansys-temp@list.xansys.org
Cc: Matthew Ridzon, PE Matt@prime-engineer.com
Subject: [Xansys] Modes Beyond End of Spectrum
*** WARNING: This email originated from outside of the organization. Exercise caution when viewing attachments, clicking links, or responding to requests. ***
Folks,
I'm running ANTYPE,MODAL followed by ANTYPE,SPECTR. The Help Documentation states that I should extract modes in the upstream modal solution, that are 50% higher than the spectrum data. For example, if my spectrum ends at 100Hz, I should extract modes up to 150Hz. The documentation states that the spectrum solver uses the last data point of the spectrum, for modes beyond the spectrum. So I think that means a mode at 140Hz would use the spectrum acceleration at 100Hz. That seems like it would alter the results though. Can someone explain the rationale for the solver's approach to use the last spectrum data point for modes beyond the spectrum? Is it conservative?
Matt Ridzon, PE, MSME
Sr. Engineering Analyst
Email matt@prime-engineer.commailto:matt@prime-engineer.com
Mail 266 Main St, Burlington, VT 05401
PRIME ENGINEERING LLC
This message (including any attachments) may contain confidential, proprietary, privileged and/or private information. The information is intended to be for the use of the individual or entity designated above. If you are not the intended recipient of this message, please notify the sender immediately, and delete the message and any attachments. Any disclosure, reproduction, distribution or other use of this message or any attachments by an individual or entity other than the intended recipient is prohibited.
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Hello Mathew, David already mentioned the "missing mass". It works only without superelements. Have a look at "rigresp". My freq/excitation table looks like this at the end:
freq,70 $sv,0.02,1.4
rigresp,on,lindley,1.4
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Folks,
I'm running ANTYPE,MODAL followed by ANTYPE,SPECTR. The Help Documentation states that I should extract modes in the upstream modal solution, that are 50% higher than the spectrum data. For example, if my spectrum ends at 100Hz, I should extract modes up to 150Hz. The documentation states that the spectrum solver uses the last data point of the spectrum, for modes beyond the spectrum. So I think that means a mode at 140Hz would use the spectrum acceleration at 100Hz. That seems like it would alter the results though. Can someone explain the rationale for the solver's approach to use the last spectrum data point for modes beyond the spectrum? Is it conservative?
Matt Ridzon, PE, MSME
Sr. Engineering Analyst
Email matt@prime-engineer.commailto:matt@prime-engineer.com
Mail 266 Main St, Burlington, VT 05401
Web https://urldefense.com/v3/http://www.prime-engineer.com;!!AVBudQ!W8a957BgMTOnfYJaEbV6tHwLOuL_L_GcD1AHc0I9O6j304UnZkNh9_Dzp9p0OLTs9Scrz5MgMc2DcE_5Pp8cfapytjOZgY1A$ <https://urldefense.com/v3/http://www.prime-engineer.com/;!!AVBudQ!W8a957BgMTOnfYJaEbV6tHwLOuL_L_GcD1AHc0I9O6j304UnZkNh9_Dzp9p0OLTs9Scrz5MgMc2DcE_5Pp8cfapytq5jGnhx$ >
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Thank you folks! This makes sense. It seems like the choice to go 1x, 1.5x, 2x, etc. beyond the spectrum is somewhat arbitrary. But it seems rational to go some distance beyond the spectrum and then activate the missing mass to capture the rest.
MMASS is the missing mass command. Workbench Mechanical adds it automatically after the user clicks the button to add it. Yes, this is nuclear seismic, so I have been in the documents that were mentioned. Thank you for the reminder.
-Matt
-----Original Message-----
From: Baker, Alan (E&PS) Alan.Baker2@Honeywell.com
Sent: Monday, March 16, 2026 7:48 PM
To: XANSYS Mailing List Home xansys-temp@list.xansys.org
Cc: Matthew Ridzon, PE Matt@prime-engineer.com; David Gross dgross@domeng.com
Subject: RE: [External] [Xansys] Re: Modes Beyond End of Spectrum
In linear frequency domain analyses, the response at a given input frequency (say 100 Hz) is the summation of the response at all modes. A mode at 90 Hz will have some contribution to the response at 100 Hz. So will a 110 Hz mode. In practice, for lightly damped structures, the amplitude of response of a mode usually drops off 'fairly' rapidly away from the frequency of that mode. Some industries use 2x the max input frequency ... it seems ANSYS is suggesting 1.5x as a start. The frequency at which you truncate also depends on the output of interest ... for example I've found that displacements are more forgiving (tend to need fewer modes) than bending loads.
I too don't recall the missing mass command in APDL, but look for a term like residual mass or residual vectors.
Regards,
Alan
Alan Baker
Associate Chief for APU Dynamics
Engine Systems & Component Analysis
Honeywell | Aerospace
alan.baker2@honeywell.com
-----Original Message-----
From: David Gross via Xansys xansys-temp@list.xansys.org
Sent: Monday, March 16, 2026 3:53 PM
To: XANSYS Mailing List Home xansys-temp@list.xansys.org
Cc: Matthew Ridzon, PE Matt@prime-engineer.com; David Gross dgross@domeng.com
Subject: [External] [Xansys] Re: Modes Beyond End of Spectrum
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Matt,
It is. On the far right-hand of the spectrum, the acceleration is assumed to flatten out to a fixed value. Your curves may refer to something called ZPA or zero period acceleration, which is a rigid body acceleration felt by all super-high frequency modes above your cutoff.
What's not conservative, though, is that your model probably didn't account for all modal mass at your cutoff frequency. You need to add that back in and multiply the "missing mass" by the ZPA as an extra mode to be combined with all your other modes. I'm short on time so I forget what commands/flags one needs to throw to have ANSYS automatically account for the missing mass, but if ANSYS isn't doing it automatically for you via some clever flags, you need to ensure you deal with it yourself.
My seismic experience comes from the nuclear world. Not sure if that's what you're doing today or not, but either way, you should google "Standard Review Plan 3.7.2" and "Regulatory Guide 1.92" and you will get some background on these questions and related questions you've been asking lately.
Regards,
David J. Gross, P.E., ASME Fellow | Dominion Engineering, Inc.
Director, Federal Services
12100 Sunrise Valley Drive, Suite 220 | Reston, VA 20191 office 703.657.7300 | desk 703.657.7311 | mobile 301.580.3066 dgross@domeng.com | domeng.com
-----Original Message-----
From: Matthew Ridzon, PE via Xansys xansys-temp@list.xansys.org
Sent: Monday, March 16, 2026 4:30 PM
To: XANSYS Mailing List Home xansys-temp@list.xansys.org
Cc: Matthew Ridzon, PE Matt@prime-engineer.com
Subject: [Xansys] Modes Beyond End of Spectrum
*** WARNING: This email originated from outside of the organization. Exercise caution when viewing attachments, clicking links, or responding to requests. ***
Folks,
I'm running ANTYPE,MODAL followed by ANTYPE,SPECTR. The Help Documentation states that I should extract modes in the upstream modal solution, that are 50% higher than the spectrum data. For example, if my spectrum ends at 100Hz, I should extract modes up to 150Hz. The documentation states that the spectrum solver uses the last data point of the spectrum, for modes beyond the spectrum. So I think that means a mode at 140Hz would use the spectrum acceleration at 100Hz. That seems like it would alter the results though. Can someone explain the rationale for the solver's approach to use the last spectrum data point for modes beyond the spectrum? Is it conservative?
Matt Ridzon, PE, MSME
Sr. Engineering Analyst
Email matt@prime-engineer.commailto:matt@prime-engineer.com
Mail 266 Main St, Burlington, VT 05401
PRIME ENGINEERING LLC
This message (including any attachments) may contain confidential, proprietary, privileged and/or private information. The information is intended to be for the use of the individual or entity designated above. If you are not the intended recipient of this message, please notify the sender immediately, and delete the message and any attachments. Any disclosure, reproduction, distribution or other use of this message or any attachments by an individual or entity other than the intended recipient is prohibited.
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I put this together in a past life. I think it deals with missing mass. Hope it helps
On Tuesday, March 17, 2026 at 07:27:41 AM CDT, Matthew Ridzon, PE via Xansys xansys-temp@list.xansys.org wrote:
Thank you folks! This makes sense. It seems like the choice to go 1x, 1.5x, 2x, etc. beyond the spectrum is somewhat arbitrary. But it seems rational to go some distance beyond the spectrum and then activate the missing mass to capture the rest.
MMASS is the missing mass command. Workbench Mechanical adds it automatically after the user clicks the button to add it. Yes, this is nuclear seismic, so I have been in the documents that were mentioned. Thank you for the reminder.
-Matt
-----Original Message-----
From: Baker, Alan (E&PS) Alan.Baker2@Honeywell.com
Sent: Monday, March 16, 2026 7:48 PM
To: XANSYS Mailing List Home xansys-temp@list.xansys.org
Cc: Matthew Ridzon, PE Matt@prime-engineer.com; David Gross dgross@domeng.com
Subject: RE: [External] [Xansys] Re: Modes Beyond End of Spectrum
In linear frequency domain analyses, the response at a given input frequency (say 100 Hz) is the summation of the response at all modes. A mode at 90 Hz will have some contribution to the response at 100 Hz. So will a 110 Hz mode. In practice, for lightly damped structures, the amplitude of response of a mode usually drops off 'fairly' rapidly away from the frequency of that mode. Some industries use 2x the max input frequency ... it seems ANSYS is suggesting 1.5x as a start. The frequency at which you truncate also depends on the output of interest ... for example I've found that displacements are more forgiving (tend to need fewer modes) than bending loads.
I too don't recall the missing mass command in APDL, but look for a term like residual mass or residual vectors.
Regards,
Alan
Alan Baker
Associate Chief for APU Dynamics
Engine Systems & Component Analysis
Honeywell | Aerospace
alan.baker2@honeywell.com
-----Original Message-----
From: David Gross via Xansys xansys-temp@list.xansys.org
Sent: Monday, March 16, 2026 3:53 PM
To: XANSYS Mailing List Home xansys-temp@list.xansys.org
Cc: Matthew Ridzon, PE Matt@prime-engineer.com; David Gross dgross@domeng.com
Subject: [External] [Xansys] Re: Modes Beyond End of Spectrum
WARNING: This message has originated from an External Source. This may be a phishing email that can result in unauthorized access to Honeywell systems. Please use proper judgment and caution when opening attachments, clicking links, scanning QR codes, or responding.
Matt,
It is. On the far right-hand of the spectrum, the acceleration is assumed to flatten out to a fixed value. Your curves may refer to something called ZPA or zero period acceleration, which is a rigid body acceleration felt by all super-high frequency modes above your cutoff.
What's not conservative, though, is that your model probably didn't account for all modal mass at your cutoff frequency. You need to add that back in and multiply the "missing mass" by the ZPA as an extra mode to be combined with all your other modes. I'm short on time so I forget what commands/flags one needs to throw to have ANSYS automatically account for the missing mass, but if ANSYS isn't doing it automatically for you via some clever flags, you need to ensure you deal with it yourself.
My seismic experience comes from the nuclear world. Not sure if that's what you're doing today or not, but either way, you should google "Standard Review Plan 3.7.2" and "Regulatory Guide 1.92" and you will get some background on these questions and related questions you've been asking lately.
Regards,
David J. Gross, P.E., ASME Fellow | Dominion Engineering, Inc.
Director, Federal Services
12100 Sunrise Valley Drive, Suite 220 | Reston, VA 20191 office 703.657.7300 | desk 703.657.7311 | mobile 301.580.3066 dgross@domeng.com | domeng.com
-----Original Message-----
From: Matthew Ridzon, PE via Xansys xansys-temp@list.xansys.org
Sent: Monday, March 16, 2026 4:30 PM
To: XANSYS Mailing List Home xansys-temp@list.xansys.org
Cc: Matthew Ridzon, PE Matt@prime-engineer.com
Subject: [Xansys] Modes Beyond End of Spectrum
*** WARNING: This email originated from outside of the organization. Exercise caution when viewing attachments, clicking links, or responding to requests. ***
Folks,
I'm running ANTYPE,MODAL followed by ANTYPE,SPECTR. The Help Documentation states that I should extract modes in the upstream modal solution, that are 50% higher than the spectrum data. For example, if my spectrum ends at 100Hz, I should extract modes up to 150Hz. The documentation states that the spectrum solver uses the last data point of the spectrum, for modes beyond the spectrum. So I think that means a mode at 140Hz would use the spectrum acceleration at 100Hz. That seems like it would alter the results though. Can someone explain the rationale for the solver's approach to use the last spectrum data point for modes beyond the spectrum? Is it conservative?
Matt Ridzon, PE, MSME
Sr. Engineering Analyst
Email matt@prime-engineer.commailto:matt@prime-engineer.com
Mail 266 Main St, Burlington, VT 05401
PRIME ENGINEERING LLC
This message (including any attachments) may contain confidential, proprietary, privileged and/or private information. The information is intended to be for the use of the individual or entity designated above. If you are not the intended recipient of this message, please notify the sender immediately, and delete the message and any attachments. Any disclosure, reproduction, distribution or other use of this message or any attachments by an individual or entity other than the intended recipient is prohibited.
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