Professional and Career Topics

Hello everyone,

I have recently discovered the hazard tool sometimes produces a Multi-Period Design Spectrum (MPDS) for a "bad" site class that has lower spectral accelerations than the MPDS for a "good" site class when using ASCE 7-22. This does not make sense to me. See below for a comparison for a site near Nashville, TN. For example, at a period of 0.2 second, the MPDS for site class E has an acceleration less than that for four "better" site classes.

Can someone explain why this might be? Is it an error, or is it supposed to be this way (I hope not)?

Thank you in advance for your assistance! Best regards,

Douglas O. Bell, PE

The ASCE 7 seismic site class descrbes the overall soil stiffness at the site, with A being most stiff and E being least stiff. Generally, soft sites de-amplify short period vibrations and ampify long-period vibrations. Stiff sites generally amplify short period vibrations and deamplify long period vibrations. 

The response spectrum returned by the Hazard Tool also depends on the sources used in the probabilistic model, and the characteristic motions they can produce, which is why the amplification/de-amplification trend described above is not always true. 

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Daniel Simpson M.ASCE
Principal
Sage Geotechnical
Tumwater WA
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Thank you, Daniel.

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Douglas Bell P.E., M.ASCE
Principal Engineer
TTL, Inc.
Nashville TN
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Original Message:
Sent: 04-13-2026 09:04 AM
From: Daniel Simpson
Subject: Inquiry About MCER Design Spectra for Different Site Classes from ASCE Hazard Tool for ASCE 7-22

The ASCE 7 seismic site class descrbes the overall soil stiffness at the site, with A being most stiff and E being least stiff. Generally, soft sites de-amplify short period vibrations and ampify long-period vibrations. Stiff sites generally amplify short period vibrations and deamplify long period vibrations. 

The response spectrum returned by the Hazard Tool also depends on the sources used in the probabilistic model, and the characteristic motions they can produce, which is why the amplification/de-amplification trend described above is not always true. 

------------------------------
Daniel Simpson M.ASCE
Principal
Sage Geotechnical
Tumwater WA
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