Site classification for seismic design in Foundation design. 4

[Chunil]

I want to ask you how to classify site for seismic design ?
For example, if a spread footing is planned to under beneath ground level then site classification should be defined including all ground material from ground level to 30m depth below?
Or just be classified with soil just from the bottom of footing (not from ground level)?

 

[EireChch]

There are many websites, publications, papers etc that clearly spell out how to determine the seismic site class. Methods used and how to do it.

Im sorry to be blunt but.....just google it.

Have a read of a few papers and if you are not sure on something then post.

I think most people are happy to help but at least make the start yourself.

 

[HTURKAK]

This para. copy and paste from 2003 NEHRP Recommended Provisions,


(The parameters used to define the Site Class are based on the upper 30 m (100 ft) of the site profile. Profiles containing distinctly
different soil layers shall be subdivided into those layers designated by a number that ranges from 1 to n at the bottom where
there are a total of n distinct layers in the upper 30 m (100 ft).)

You may look ASCE 7 Chapter 20

or Chapter 3 of the 2003 NEHRP Recommended Provisions at link;

https://www.nehrp.gov/pdf/fema450provisions.pdf

 

[MTNClimber]

I'm not sure where this is clearly documented. I remember researching this when I was a young engineer and didn't find anything. I just did a quick google search and found mixed results.

I don't work in highly seismic active areas and typically frost depth is 5' or less. I normally looked at it from the bottom of footing if the structure doesn't have deep below grade construction. I certainly ignore any loose layers that will be removed during construction (thick topsoil/subsoil deposits). Also keep in mind how site class can change if the planned construction includes deep cuts or fills.

 

[HTURKAK]

You may look You may look ASCE 7 5,10,16 Chapter 20 or free of charge doc. at Chapter 3 of the 2003 NEHRP Recommended Provisions at link;

https://www.nehrp.gov/pdf/fema450provisions.pdf..

Yes... you should normally look at it from the bottom of footing . If the foundation supported on fill, you are expected to use the properties of proposed fill..

 

[MTNClimber]

Can you provide a specific line in either document that states that you should look at it from the bottom of footing? I have read ASCE 7-10 and -16 in the past and don't remember it saying anything that specific. NEHRP seems to just re-iterate ASCE 7.

 

[r13]

I don't think there is specific wording, but structural engineers tend to be more concerned on the ground motion below the footing (virgin soil), rather than the fill above. Also, although the difference is quite small for shallow foundations, which way is more prudent and conservative?

 

[MTNClimber]

How does everyone else look at buildings with 50’ of below grade construction? Are there some engineers here that look at the stratum between 50’-150’?

 

[HTURKAK]

The thread is for 'Site classification for seismic design in Foundation design' and most of the codes classify the site for seismic loadings as per the uppermost 100 ft.. also ASCE 7- 16 section 20 (20.1 SITE CLASSIFICATION The site soil shall be classified in accordance with Table 20.3-1 and Section 20.3 based on the upper 100 ft (30 m) of the site profile....)

If you are looking for settlement analysis, you may need to look more than 50' ..

Ofcourse ,the geotechnical report shall provide also information regarding the general geology of the site to foresee probable risks.

If this reply does not answer to your question, open another thread with your more specific problem.

 

[MTNClimber]

The question in my last post was somewhat rhetorical and aimed at r13 to demonstrate that determining the seismic site class isn’t purely for the soil just under the footing.

Also, I don’t think anyone here really addressed the OP’s question so I’m trying to bait more people to provide their interpretations of ASCE 7 chapters 20 and 21. I provided mine. I think the answer is more complicated than “just read this reference”. I personally think it’s a bit of code interpretation and a bit of engineering judgement too.

Why would I start a new thread when I’m talking about the subject posted by the OP?

Which codes have you determine the site class by looking at the upper 50 ft? Never seen that either.

 

[r13]

...but structural engineers tend to be more concerned on the ground motion below the footing (virgin soil), rather than the fill above. Also, although the difference is quite small for shallow foundations,...

MTNClimber,

As a geotechnical practitioner, you should know the differences/significance of shallow foundation (surface) and basement (deep) foundation, and man made material (fill) and native ground (virgin), in seismic site classification. I wish you to contribute more from your training and believes, rather than rhetorical speaking to attack a person's opposing view.

 

[MTNClimber]

I’m not trying to attack anyone. I’m just trying to have a conversation with other practitioners who have to interpret a seemingly open ended portion ASCE 7. Some here claim it’s well documented, but I beg to differ and have asked someone to show where it is documented. There are some questions posted on this site that should be answered by referring to the code but I don’t think the OP’s question isn’t clearly answered in the code and, in my opinion, deserves a conversation between practicing geotechnical engineers.

If you interpreted my post as an attack, I’m sorry but it wasn’t meant to be.

 

[Chunil]

Thank you very much with your various opinion.
I think this topic should be handled carefully by geotechnical engineer and structual engineer both.
There are more complicated practical problems exists(Pile, Caisson etc.,).
So, this topic needs more clear justification I think.
Thank you guys.
I still wants CLEAREANCE.

 

[r13]

This excerption is copied from a document from IDOT. See P4 of the linked document. Link

Determining Local and Global Site Class Definitions
The geotechnical material within the upper 100 ft. shall be evaluated at each substructure unit
using Methods B and C. As previously mentioned, the summation of the total thickness of all
layers used to compute N , N , or ch su , shall equal 100 ft. The 100 ft. of cumulative layers
should start at the bottom elevation of spread footings, 6 times the pile or shaft diameter/width
below the bottom of abutment or pier footing elevations for pile supported foundations (except
bent type piers), or 6 times the pile or shaft diameter/width below the ground surface elevation
for bent type piers. Soils above these points are not anticipated to have a significant influence
on the dynamic response of the structure.

And, an important note on ASCE7-16 on site coefficients, by S. K. Ghosh, Link

 

[MTNClimber]

Interesting. I haven't worked for IDOT before, nor will I likely ever, but this is the first time I've seen it clearly specified. Good for IDOT! I wonder where the guidance was developed from and why it's not included in ASCE 7?

 

[r13]

Try AASHTO library. Where is BridgeSmith, or bridgebuster!?

 

[MTNClimber]

I believe AASHTO's LRFD Bridge Design Specifications just specify "upper 100 feet of soil profile" with no reference point as to where that starts.

 

[r13]

I don't think you can find the definite word in the codes. Here is a research paper, not directly tied to the subject, but provide the use of shear wave velocity in determining the allowable bear pressure, and how it is measured. I think our geotechnical practitioners should have a glance. Link

 

[epoxybot]

There is this from FEMA 451B - 2007 Link

PDF page 1483

Also consider that a VS30 shear wave velocity test can be performed either in bore holes or with geo-phones at the surface.

A google search for "FEMA 451" yields a host of complimentary FEMA 451 materials.

After reading this bullet point over again, it seems to suggest that Map-based site classifications were developed based on the top 30 meters but as stated do not account for local soil conditions.

Looking further at,

Link

the basis for 30 meters seems to be the availability of truck-mounted boring rigs to which, this is the approximate depth limit of the rig. PDF page 58 - Bucket Auger

 

[r13]

IMO, the average shear wave velocity is a site specific issue, that many factors need to be considered. From structural point of view, the outmost important soil properties to be looked at starts at the soil-foundation interface, similar to bearing capacity and settlement analysis. Let me set an example here, a site with existing datum of +100', and it is determined the upper 5' soil is unfit and needs to be removed, the footing will then be placed on elevation +95. Now, at what elevation the boring can stop that meeting the 100' requirement for Vs[sup]30[/sup] measurement?