Seismic Site Class F due to liquefaction

[DanielSgeo]

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.

 

[dik]

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

 

[HTURKAK]

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

 

[DanielSgeo]

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?

 

[Struct123ure]

DanielSgeo, your question seems very straight forward to me, so here I go:
Simply put by designing a building in Site Class (SC) D vs F you are significantly reducing the horizontal seismic load that the building is designed for. I think you already knew that but your Sa(0.2, X_D)=0.446 vs Sa(0.2, X_E)=0.482 2%/50 years probability in Niagara Falls; F would be even worse.
The second most important impact are the height restrictions for the buildings seismic force resistant structure. It is based on a combination of I_E*Fa*Sa(0.2). So for instance a conventional steel moment frame has no limit when I_E*Fa*Sa(0.2)<0.35, but when it is more then 0.75 then a restriction of 15m height applies. This would mean you either go to more complex system or change SFRS systems all together.

I have never designed in SC F, but I am sure that there are additional clauses that apply that make the design more difficult both in the building codes and in the material codes (AISC, CSA).

 

[DanielSgeo]

Thank you Struct123ure, the comment on changes to the SFRS is helpful. The only place that the seismic site class comes in there is with the calculation of Fa, right? I_E does not depend on seismic site class or the site coefficeints Fa or Fv?

 

[Struct123ure]

DanielSgeo,
I_E: Importance Factor is not impacted by the SC it is dependent so on the type of building/purpose of building.
Like you said the SC does impact the Fa which you use to modify your Sa(0.2) to get your S(0.2). Sounds more complicated then it is, just take a look at section 4.1.8 Earthquake in OBC(Ontario Building Code) its free online

https://www.buildingcode.online/697.html.

Specifically look at 4.1.8.4. Site Properties you will see the charts to get the F value from peak ground acceleration or velocity.
And regarding liquefaction which this all started with it says that "Site-specific evaluation is required to determine F(T),F(PGA)and F(PGV)for Site Class F." so I would be looking at you to give me those .

OBC has the same Section you do 4.1.8.4.6)"For structures with a fundamental period of vibration equal to or less than 0.5 s that are built on liquefiable soils, Site Class and the corresponding values of Fa and Fv may be determined as described in Tables 4.1.8.4.A., 4.1.8.4.B., and 4.1.8.4.C. by assuming that the soils are not liquefiable." Now I am the one with a question for you why do you upgrade from SC F to SC D? The note above says that the SC can be upgraded out of F but doesn't specify to what SC and if I was a soil engineer I would move it one level better from SC F to SC E. Is there a reason or another clause/reference that tells you to go to SC D because the OBC and/or NBCC does not give anything.

 

[DanielSgeo]

The SC you change to from F if Tn < 0.5 second is just the SC that is calculated in the absence of liquefaction. It can be D or E or F.