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Seismic Site Class F due to liquefaction

Seismic Site Class F due to liquefaction

Seismic Site Class F due to liquefaction

(OP)
I'm a geotech in the pacific northwest. We freqently encounter sites that classify as Site Class F due to liquefaction (ASCE 7-16), but for projects with structures Tn<0.5 seconds, we usually use the exception that lets the seismic parameters be determined based on the site calss as if liquefaction were not occuring. And more often than not, we're looking at site class D in absence of liquefation.

Can someone help me understand, what are the structural design implications with this condition (ie technically site class F, but using site class D seismic design parameters), other than different seismic design parameters? (I'll note, becasue the PNW is a high seismic area, we're typically seismic design category D or higher, i believe).

Like, is there some importance factor, or response modification coefficient that changes if you're Site Class F or D, even if the seismic parameters are the same? Does this question even make sense to folks?

Thanks.

RE: Seismic Site Class F due to liquefaction

I'm not in a seismic area, but from class work. Liquifaction occurs when the pore pressure goes to zero (or very small) and the soil has little lateral or vertical restraining value (A real geotechnical guy can jump in to help). I had understood that once this condition was reached there was very little that could be done to correct or alleviate the condition. I suspect that partial liquifaction can occur and some restraint may still exist.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

RE: Seismic Site Class F due to liquefaction

Pls look ASCE 7-16
Commentary C20.3 SITE CLASS DEFINITIONS
C20.3.1 Site Class F. Site Class F conditions are conditions for
which the site coefficients Fa and Fv in Tables 11.4-1 and 11.4-2
may not be applicable for site response analyses required by
Section 11.4.7; they are defined in this section. For three of the
categories of Site Class F soils—Category 1 liquefiable soils,......

Category 1. For liquefiable soils in Category 1, an exception
to conducting site response analyses was developed by Technical
Subcommittee 3, Foundations and Geotechnical Considerations,
of the BSSC Provisions Update Committee and was first published
in the 2000 NEHRP Provisions (FEMA 2001). The
exception is made for short-period structures, defined for purposes
of the exception as having fundamental periods of vibration
equal to or less than 0.5 s. 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, whereas
long-period ground motions may be amplified by liquefaction.
Note, however, 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


Use it up, wear it out;
Make it do, or do without.

NEW ENGLAND MAXIM


RE: Seismic Site Class F due to liquefaction

(OP)
Thank you both for the reply.

I am well aware of the effects of liquefaction and how to design foundations to withstand it, the the effects of liquefaction on strong ground motions. When were are designing projects in the scenario I describe above, we're always saitisfying chapter 11 of ASCE-7 (that is, designing the foundations to withstand the effects of liquefaction).

My question is about the site classification itself, and not the mechanics of liquefaction or how to design a foundation with liquefaction. Does a Site Class F change the approach to structural design of the superstructure? Are certain shear resisting systems not allowed or are certain connections not allowed or required solely due to the seismic site classification?

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