Exception to Site Class F for Liquefiable Soils

Liquefaction During the Christchurch Earthquake [Image Courtesy: Steve Taylor
The wording of Section 20.3.1 of ASCE 7-16 appears to allow the site for some structures not to be classified as Site Class F even if the soil is vulnerable to potential failure or collapse under seismic loading, but that is not the intent of the code. See the response below to a question we recently received.

Q. Soils vulnerable to potential failure or collapse under seismic loading typically must be assigned a site class of F per ASCE 7-16 Section 20.3.1 Item 1. However, if the fundamental period of the structure is equal to or less than 0.5 second, then the site class does not need to be taken as Site Class F as the structure falls under the exception - see highlighted portion of code section below. Would you please confirm that this is correct?

  • 20.3 SITE CLASS DEFINITIONS
    Site class types shall be assigned in accordance with the definitions provided in Table 20.3-1 and this section.
    • 20.3.1 Site Class F. Where any of the following conditions is satisfied, the site shall be classified as Site Class F and a site response analysis in accordance with Section 21.1 shall be performed.
      • 1.    Soils vulnerable to potential failure or collapse under seismic loading, such as liquefiable soils, quick and highly sensitive clays, and collapsible weakly cemented soils.
        • EXCEPTION: For structures that have fundamental periods of vibration equal to or less than 0.5 s, site response analysis is not required to determine spectral accelerations for liquefiable soils. Rather, a site class is permitted to be determined in accordance with Section 20.3 and the corresponding values of Fa and Fv determined from Tables 11.4-1 and 11.4-2.
A. While we see how one can interpret the language of the code in this manner, the intent of the exception in ASCE 7 is NOT to change the site class of the soil. Instead, the intent is only to allow a more straightforward method for determining the site coefficients for short-period structures instead of the more detailed site response analysis usually required for structures on Site Class F soils. And while it is acceptable to determine the site coefficients in a more simplified manner, all other code requirements for a structure on Site Class F soil remain in effect. This is confirmed by reviewing the ASCE 7 commentary on this section and reviewing approved changes to ASCE 7 for the 2022 edition.

Per the ASCE 7-16 Commentary for Section 20.3.1, an exception to performing a site response analysis is allowed for liquifiable soils for short-period structures (< 0.5 seconds). The ASCE 7-16 commentary states “For liquefiable soils in Category 1, an exception to conducting site response analyses was developed… The exception is made for short-period structures …For such structures, it is permissible to determine site coefficients Fa and Fv from Tables 11.4-1 and 11.4-2 assuming that liquefaction does not occur because ground motion data obtained in liquefied soil areas during earthquakes indicate that short-period ground motions are generally reduced in amplitude because of liquefaction.” The commentary goes on to state that “this exception does not affect the requirement in Section 11.8 to assess liquefaction potential as a geologic hazard and develop hazard mitigation measures if required.” In addition, the wording of this section in the upcoming ASCE 7-22 is being modified to clarify this section. See below for the version of the “Exception” that is included in ASCE 7-22 that was recently released for public review. It clearly indicates that the exception only applies to the purpose of determining spectral accelerations.

  • 20.32.1 Site Class F.
    Where any of the following conditions is satisfied, the site shall be classified as Site Class F and a site response analysis, in accordance with Section 21.1, shall be performed.
    • 1.    Soil profile includes Ssoils vulnerable to potential failure or collapse under seismic loading, such as liquefiable soils, quick and highly sensitive clays, and collapsible weakly cemented soils.
      • EXCEPTION: For structures that have fundamental periods of vibration equal to, or less than, 0.5 s, site response analysis is not required to determine spectral accelerations for liquefiable soils. Rather, for the purpose of determining spectral accelerations only, a site class is permitted to be determined in accordance with Section 20.2 3 and the corresponding response spectrum determined from Section 11.4 and the corresponding values of aF and vF determined from Tables 11.4-1 and 11.4-2.
Thus, any site with soils vulnerable to potential failure or collapse under seismic loading must comply with requirements in the IBC and ASCE 7 for Site Class F, no matter what the period of the structure is. The only exception allowed in the code is that a site response analysis is not required for short-period structures. Support for responding to this Q/A was provided by Mr. Chris Kimball of West Coast Code Consultants, Inc. and Mr. Stephen K. Harris of Simpson Gumpertz & Heger.