Seismic Soil Profile Type 1

[eastongeo]

I understand that the following are soil-structure interaction problems, but I'm trying to get some sort of an idea or general guideline regarding site soil profile for two typical scenarios. Do they require dynamic analysis?

1) For buildings which are supported on a deep foundation (ie. Piers or piles) it is appropriate to classify the site as an SE even though the piers/piles are drilled/driven through the “soft” soil and embedded into competent material?

2) For buildings with deeper basements like an underground parking garage and with footing foundations embedded into SC soils/bedrock, it is appropriate to classify the site as an SC soil type even though the soils against the basement walls are not Sc soils/bedrock (ie. SD or worse SE)?

 

[msucog]

i think i see what you're getting at however i don't usually see it written SE, SD, etc. i presume SE is site class E, etc.?

your question falls in to a gray area and i'm sure the response depends on who you ask on what day. i've asked the question to some seismic guru's and they gave me a wishy washy answer...pretty much said "well, that falls in to a gray area.". i will typically classify it based on the lowest level. so if the piles drive through D material, i don't consider it a B just because the piles are bearing on rock. however, if you construct a parking deck 5 levels through E material and bear directly on C material, i'd hesitate to call it a C (and likely would not call it a C). i suppose you'd probably need to look at the effects of the EQ against the wall if it goes that deep in the ground...maybe unless you're in C and some higher D material. i've never had to look at that scenario so i'm not completely sure what i'd do. i'm pretty sure code does not address this situation, however, i believe it does mention that the effects of an EQ on the lateral earth pressures should be evaluated...not completely sure if that means the geotech or the structural should look at it...i interpret it as meaning the structural (again since i've never run across this particular scenario and since i'm in a low seismic area).
was my answer wishy washy enough?

 

[Mccoy]

was my answer wishy washy enough?

As wishy-washy as the topic's wishywashiness allows!

As to the piers or piles, I'd classifiy the site according to the soil type at the pile caps (where the highest shear would occur). So it would be an SE, if such a soft soil goes down to the 100 ft depth.

As to the underground parking, I would ask myself which would be the significant thicknes with regard to soil amplification, I'd say the whole thickness against the lateral surface fo the structure, so I'd start from ground level down to the 100 ft to classify site profile.

This is today's (more precisely tonite's) opinion, as msucog says who knows if another day I might change my mind!.

 

[msucog]

here's one point that i think was originally asked but i'm not completely sure about the answer. to recap, let's say the parking deck extends 5 levels below grade and is founded on good C or even B material. the material it cuts through is low D or even E material. during a seismic even, the bottom of the foundation doesn't experience large displacement. however, 50 above that in E material, the displacement and/or associated seismic forces are jacked way up. so should the soil site class be classified as a B/C or a D/E material? i believe that is the ultimate question being asked. if i'm wrong, please correct me.

technically, i believe that under current code and asce7, it would have to be an E. i'm not sure that's absolutely correct and think it could be argued/interpreted other ways. however if you did perform a dynamic analysis, at least you could justify your particular engineering opinion one way or another. if there is some sort of "seperation" then maybe you could get a better site class since adjacent displacement would be allowed to move without interference. i recently ran across a scenario where one portion of the building (one area was hotel and other was parking deck below grade) was a D and the other a C...which site class to use? my response was that if the two particular portions of the building have "seperation" constructed in, then the displacement during a seismic event might not interfere with the adjacent structure so each structural area could have it's own site class. again, i feel that the whole subject has several gray areas that needs to be addressed...and perhaps by those much more "expert" in the subject matter than i am.
i'd be interested to hear any other thoughts.

 

[jdonville]

eastongeo,

Classify the site based on the top 100 ft of soil per the guidance in the IBC. Period.

Jeff

 

[msucog]

i respectfully disagree jdonville...to a certain degree.
if you refer to nehrp section 3.5.2 Steps for classifying a site(commentary), it says:
"The site class should reflect the soil conditions that will affect the ground motion input to the structure
or a significant portion of the structure. For structures receiving substantial ground motion input from shallow soils (e.g. structures with shallow spread footings, laterally flexible piles, or structures with basements where it is judged that substantial ground motion input to the structure may come through the side walls), it is reasonable to classify the site on the basis of the top 100 ft (30 m) of soils below the ground surface. Conversely, for structures with basements supported on firm soils or rock below soft soils, it is reasonable to classify the site on the basis of the soils or rock below the mat, if it can be justified that the soft soils contribute very little to the response of the structure."

 

[msucog]

here is the nehrp.gov publication site in case anyone interested. there's a new publication that appears to be very informative. it's the fema454

http://www.nehrp.gov/resources/index.htm#guidance

 

[Mccoy]

msucog,
the nehrp commentary you quote apparently answers clearly to the underground parking structure problem you mentioned in your initial post, leaving little room for discussion...
If it can be shown that shallow soils do not contribute significantly to the vibrational mode of the structure under seismic excitement, that that it is.

Maybe some room for discussion could be found in the case of rigid piles/shafts.

 

[msucog]

i think there's plenty of room for discussion since it says "where it is judged that substantial ground motion input to the structure may come through the side wall"...that leaves a ton of "gray matter" to decipher through. i do agree that is also leaves the discussion open for some deep foundation systems. i have not seen guidelines nor seen anyone put numbers to the statement...thus, it is left up to the engineering professional to decide that...again, gray matter. i don't know the answer to the question for the specific question at hand. where i'm at, the soils are usually not bad and are usually pretty darn good considering so i don't have a problem with using the lower level as the top of my 100'. also to note, the 100' is not the absolute standard. nehrp also leaves the topic open to be deeper under certain circumstances but that's a different discussion.

 

[jdonville]

msucog,

I haven't seen any changes to the IBC since the 2003 version, so my answer may have been overtaken by more recent developments.

There has been plenty of griping on this site at the lack of guidance on the reasonableness of taking the soil profile over the top 100 feet (do a search) to arrive at the seismic site class.

The simple answer remains: take the soil characteristics over the top 100 feet, with the prescriptive conditions for Site Classes E and F, and have done with it.

I have no idea of how the ICC came up with the site class determination method, but a couple of footnotes to the relevant research and/or guidance for specific foundation types would at least clear up a significant amount of the bellyaching over the "one-size-fits-all" approach.

Good luck finding your answer.

Jeff

 

[msucog]

i know that there is not definitive answer. and i don't believe there's been any real changes since 2000 except for the isobar maps. this all is sort of my point...lots of gray area open to interpretation and/or engineering judgement (or lack there of). i'm no seismic expert but i do think that the 100' should more or less from the bearing level for shallow foundations and the lowest level/top of pile cap for deep foundations. for E and F sites, much more care should be taken to evaluate the condition and i would lean toward taking it from the ground surface. as far as the belly aching about one size fits all, people gripe about such a thing for something so "simple" as how to come up with an allowable bearing pressure. everyone does it slightly differently...some with little experience...some with 50 years experience. it'll never be as easy as a plug and chug equation when dealing with geology and site specific conditions. nature and geology is much too complicated for such a trivial solution.
i would hope the seismic guru community can come up with a better solution to the needs of the industry. while i don't particularly agree with the simple table in the codes, i'm not sure how else to effectively do what they have done. hell, take a simple site with a building and mse wall...most just provide a site class based on the borings in the building area. how many provide a seperate site class for mse wall? also, the return period for the design parameters was determined by structural folks...not seismologists. mse folks say that a ~2500 yr return period is too conservative and so they fall back to aashto with the old 500yr return period (they should probably be using the 1000yr return in the new aashto requirements). either way, everyone has a different interpretation of what's needed. to be realistic, what really is needed? no one knows...it's all probablistic. the requirements will change if say the east coast all of sudden has several massive quakes and thousands of people are killed. that sort of is the way of the engineer...build it one way until something fails more than the public/lawyers can stand and then the requirements get bumped up.
so good luck to all in keeping up with the curve...