IBC Seismic Site Classification 1

[kmmgeotech]

I need HELP. This question keeps coming up and everyone seems to have their own opinion. This is a Building Code issue so I would think there should only be 1 answer.

Do I consider BEDROCK (N=100) in the upper 100 ft of Soil Profile. It greatly impacts the Site Classification. Some peope say YES. The IBC Building Code is VAGUE. It indicates the "N" value to be for cohesionless soils (ie: NOT BEDROCK). On many sites, the Site Classification may be increased from an "E" to "D" if BEDROCK is considered in the equation.

In the New England area, bedrock is commonly encountered within 20-30 ft of grade. Thank You for your response.

 

[jdonville]

kmmgeotech,

Depends on the bedrock. We recently investigated a site where the bedrock was extremely weathered shale. SPT values were sometimes less than 100. We never reached auger refusal even after penetrating over 20 ft into the shale. In this case we were not investigating SSC, but it is instructive nevertheless. If auger refusal is reached, and the rock is coreable with good recovery, then N>=100 is justifiable. Use your judgement.

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

The views or opinions expressed by me are my own and do not necessarily reflect the views or opinions of my employer.

 

[kmmgeotech]

Thanks for your response. Most of the rock in New England is not able to be penetrated with augers (granite, schist, diorire, etc). Possibly a few feet if severely weathered.

To understand your response, I am correct to use "N" = 100 within the upper 100 ft profile if adequate rock is present?

My "hypothetical" site has "N" =10 blows in the upper 60 ft (silty SAND & sandy SILT) then good bedrock thereafter. By including Bedrock in the upper 100 ft, I can increase the Site Class from "E" (N=10) to "D" (N=16). I want to be certain this is CORRECT as the IBC Code makes reference to "N" for cohesionless soils. There is no specific language how to include BEDROCK.

 

[jdonville]

kmmgeotech,

After re-reading my copy of the code, 1615.1.5 reads in part "ds = The total thickness of cohesionless soil layers in the top 100 feet...".

This suggests to me that the Navg value applies to whatever thickness of soil is _above_ the bedrock, and that your "hypothtical" site is SSC of 'E'. 1615.1.5 indicates that SSC of 'A' or 'B' is only applicable if there is less than 10 ft of soil below the foundation bearing surface.

My advice for saying N>=100 in my previous reply may not be correct.

Hope this is helpful.

Jeff


Jeffrey T. Donville, PE
TTL Associates, Inc.

http://www.ttlassoc.com

The views or opinions expressed by me are my own and do not necessarily reflect the views or opinions of my employer.

 

[kmmgeotech]

This is why the IBC Code is very vague and confusing. Everybody seems to have a different interpretation.

In my opinion, a Code should be very CLEAR & SPECIFIC. Thank You for your input but I guess I still do not have a clear interpretation.

 

[damguy]

You may want to read Part 3, Section 3.5 of the NEHRP 2003:

http://www.bssconline.org/NEHRP2003/provisions/P3E.pdf

and Part 3, Section 3.5 of the Commentary:

http://www.bssconline.org/NEHRP2003/comments/C3.pdf

 

[kmmgeotech]

Thank You very much Damguy. Based on the Commentary I was correct in my sitution (ie: accounting for rock in the upper 100 ft).

It is still my opinion that the CODE should to revised to limit abiguity. It seems to be very confusing and professionals have differing opinions.

 

[geotech2]

In deed the IBC 2000 causes a lot of confusions in the geotech community. The seismic design provisions of IBC 2000 is based on 1997 NEHRP, but not everyone will go check its commentary or NEHRP, as Damguy does.

Other misleading parts are:
1) SPT N values used are uncorrected directly from the field. We all know that hammer system efficiency has big impact on N. Donut hammer with R-P vs. auto safety hammer. The newer hammer may put you in a lower site class.
2) Soil with PI<20 is defined as cohensionless soil. You have to be careful when deal with CL. I have to order a lot of AL tests in such cases. I still don't know why they define this way.

Another issue is what if you have some borings in Site Class E while others at the same site being Class D. We had to call ICC and got some "personal" advice from the staff on this issue: You can determine the site class for individual building or structure, not for the entire site (since I am always involved in such big size projects). What if one big structure crosses different site classes, then you need to use your engineering judgement and discuss with the structural to evaluate the contribution from the weak layer.

Often structural engineers need site class in their analysis, but usually a geotech provides this. This also will impact the design loads considering seismic factors, and in turn affect foundation design which a geotech will get involved in.

More and more states adopted IBC. I thought IBC 2003 will make some changes on those issues, but it does not. I doubt there is a geotech in the code committee that writes the structural parts.