How to determine Fa and Fv?

Question

How to determine Fa and Fv?

IRER (Geotechnical)

(OP)

18 Mar 08 00:23

There is a Site Class F site (has liquefaction settlement).
The proposed building meet ASCE 7- 20.3.1 exception. In the exception says "Rather, a site class is permited 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"
What does it means, and in this case how to determine Fa and Fv?
Thank you.

moe333 (Geotechnical) · Answer 1 · 2008-03-18 14:25:16

As stated in ASCE 7- 20.3.1 exception, the Fa and Fv would be based on your site classification assumming there is no liquefaction, probably a D or E site class.

msucog (Civil/Environmental) · Answer 2 · 2008-03-18 18:34:49

i don't have it in front of me but i believe it's saying that if you're in a lower seismic area (less than 0.5g maybe...again, i don't recall exactly), then determine the site class as described in the section based on N values, Vs, etc.

once the site class has been determined, you pull Fa and Fv from the table based on Ss and S1 along with the site class.

muuddfun (Geotechnical) · Answer 3 · 2008-03-18 21:12:14

What region is the site located in? Most of the time the site class F can be determined to be a site class C, D, or E depending on where you are and the accelerations, if the fundemental period of the structure is less that one half second, then the site specific analysis required just sends you back to the tables for site C, D, E etc. Only if it is a longer period than one half do you have to get into the site specific modeling etc. If the accelerations are on the high end a site D is more conservative than an E due to the better response of the stiffer material. So in most likelyhood you can get your site values for Fa and Fv from the code book, or the Earthquake Ground Motion Parameters software from the USGS.

IRER (Geotechnical) · Answer 4 · 2008-03-18 21:42:49

I did the same thing as moe333 says. For this case, if there is no liquefaction problem, the site sould be classfied as "D".
Therefore, I use Site Class D, and get Fa Fv value based on Site Class D.
City Correction Letter says: Per CBC, all site has liquefaction settlement should be classified as "F"
Therefore, I get confused....
or
Should I class the site as "F", and then use Fa Fv value for site class D?
Thank you.

msucog (Civil/Environmental) · Answer 5 · 2008-03-19 19:45:06

i think muuddfun has it right (i think i'm wrong)...fundamental period < 0.5sec. the Fa and Fv are related to the amplification characteristics of the soil so the number used should coordinate with the type soil at the site (i.e. soil classification).

try the usgs link
http://earthquake.usgs.gov/research/hazmaps/design/index.php

also try

FEMA454
http://www.fema.gov/library/viewRecord.do?id=2418

FEMA451B & 451 & NEHRP recommendations
http://www.bssconline.org/

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How to determine Fa and Fv?