IBC Site Class Determination 2

[JAE]

The new IBC series of codes includes in their seismic sections a parameter for design called the "Site Class" (see IBC 2000 Table 1615.1.1). This involves the structural engineer using a particular classification (Class A through F) based on the soil shear wave velocity, the standard penetration resistance N, and the undrained shear strength. All of these are for the top 100 feet of the soil at teh building site.

My question is this....most geotechnical engineers drill between 20 and 50 feet for their reports. Many of you on this forum are geotechs and I was wondering how each of you deal with determining this for your structural buddies. Do you combine your borings with a general knowledge of the underlying geology of the site? Is there a potential that you could be wrong by doing this?

What I'm after is an understanding of what everyone out there is doing in terms of getting this classification properly set for the building design. It does make a large difference in loads.

 

[eric1037]

vmirat:

I wouldn't take the responsibility of determining the site class if I were you. Put the onus back on the geotech. It's his job. If he won't do it because he doesn't have enough info, then a deep boring or seismic testing will have to be performed.

 

[jdonville]

vmirat,

Sound advice from eric1037. SSC should be part of the geotech's scope. I see it more and more frequently in the RFPs I get.

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

 

[kozera]

I also have a seismic design question.

In considering a pile supported pier, does the top of the soil profile start at the top of the bearing stratum for the piles? or at the top of the bottom ooze through which the piles are driven into? As one can imagine this make a tremendous difference in the site class as defined in Table 1615.1.1 in IBC.
Any help would be appreciated.

 

[TDAA]

I use from the top of the finish grade (typically close enough to the existing) for my calcs. This will take into account the fact that the motion of the overburden soils above the bearing stratum will have an effect on your structure (side of your piers). I will state that for the sites I typically work on there would not normally be a significant difference (only one site class, not from say an E to a B).


I didn't see this thread when it was first out, so would like to comment form a geotech perspective:

I think it is pitiful that so many people have a hard time with this. The code (this section) is not that hard, it is just that many people do not actually read it. It allows for the geotech to use their knowledge of the area and not have drilling to 100 feet. If the soils in the area are known to have issues making them less than a "D", then drill. Other wise it is not worth it. Use the geologic information. Create a spreadsheet to figure the weighted average. Play with it knowing the typical values in your area, and you will see that it gets difficult to apply a higher site class if you have blow counts in the 20's and 30's to any depth, unless you switch to using shear wave velocity.

 

[JAE]

kozera - you might want to start a new thread instead of latching on to this older one - you have a completely different topic and by starting a new thread you'd probably get more direct responses.

 

[cbosy]

JAE,
The geology of the Chicago area is very well mapped and based on the shallow borings and a geologic review, we will recommned a site class but always recommend a deep boring be drilled from a CYA standpoint.

 

[KMR1]

Dear All,

I have a basic question. Hope someone could help.

Within US, let us say for a medium construction project if I want to define a Site Class as D (based on limited drilling data and IBC codes interpretation), how much difference in terms of structural design point of view and costs would it make from any other better classes, say C or B etc (assuming that my classification is more conservative than what the actual site class is!)

Thank you very much!

 

[TDAA]

It can be very substantial, depending on the building, and many other factors. The Site Classification is only a small portion of the equation though.

You should really start a new thread though, to get more direct responces.