Vs30 Map for Determining Site Class

[CaliEng]

Hey All,

In the quest for the determination of a site's Site Class in the lack of a geotechnical report or a geotech report which does not specify testing for site specific shear velocities I have come across this: A map developed by the USGS which uses site slope as a proxy for Vs30. I know, I too was initially skeptical. However, this method is developed by USGS and is used (as far as I can tell) in the background of developing the ShakeMaps for real and earthquake scenarios. The analysis has been calibrated to various site specific measurements with surprising correlation. At the very least it is an interesting approach.

The map is here:

https://usgs.maps.arcgis.com/apps/webappviewer/index.html?id=8ac19bc334f747e486550f32837578e1

The paper is here:

https://pubs.usgs.gov/of/2007/1357/pdf/OF07-1357_508.pdf

I am curious of the community's opinion of using this for determining a site's Site Class. The updatedASCE 7-16 declares that if you are assuming a Site Class D, you have a 20% increase in S[sub]DS[/sub] design values. This clearly makes a huge difference in design, but a site specific geotech analysis may be cost prohibitive or time sensitive for smaller jobs.

See Also:

https://seismicmaps.org/

Toggle between Site Class D and Site Class D -Default to see 20% Increase.
For reference I am in California, many Site Class D locations.

Appreciate your input! Thanks!

 

[MTNClimber]

That map isn't very detailed. I wouldn't trust it to get into a better site class, nor do I think ASCE 7-16 allows it.

 

[r13]

From where ASCE 7 gets their information? Just curios.

I think the USGS site should have an app that provide info for given zip code, or they already have it.

 

[EDub24]

I personally wouldn't use that map. I'm not a geotech but I would imagine there are a lot of variables in determining the soil shear velocity for a specific site. A map like that is too general for this purpose. It might work well when estimating earthquakes etc but it's not specific enough for a small area. It's one thing if a geotech gives me data based off that map since that is his/her area of expertise but it's another thing for me to use that data. When jobs are too small or time sensitive to warrant having a geotech report then you'll just have to suck it and assume a site class D along with the potentially increased Sds.

Also as an FYI it's not a 20% increase of Sds but rather the short-period site coefficient, Fa, used to calculate Sms and therefore Sds. ASCE 7 §11.4.4 says that the Fa can't be taken as less than 1.2 if you assume a site class D. So you would only get an increase in your Sds if your Ss>0.75.