Seismic Site Classification

[StevenPumphrey]

With respect to seismic site classification,does it make any difference if your foundations are placed on piles which are driven to bedrock?

Let's say I did a site classification based upon SPT, and found the average N in the first 100 ft to be 25. There is soil for the first 50', and bedrock for the next 50', which the piles are driven into.

Basically, can I get away with classifying the site as Class B (or even A for that matter)?

Is the 10' of soil between the rock surface and bottom of spread footing include the pile?

 

[msucog]

it is not appropriate to call it a class b just because you're using piles (except perhaps under a very specific situation which is not your situation). i would go with the class d for your scenario. however i would entertain the idea of performing a shear wave velocity assessment to the client since it may be possible to achieve a class c depending on your site specific soils/rock.

 

[Murali27]

Seismic site classification reflects (defines) how the seismic waves (acceleration) amended (amplified or reduced) when it hits at base of structure. Soil classification basically amends the spectral accelerations.

Foundation type should not change seismic site classification. Having said that, by using piles, one can ascertain that the fixity to super structure can be achieved. Also superstructure will not be affected by local soil conditions.

Hope this helps.

 

[JAE]

Murali27 is correct that the soil classification adjusts the seismic demand since all the data, concepts, etc. are based upon a rock condition (Class B) and must be adjusted to reflect how the dynamic energy is delivered from the epicenter to the building site.

With piling driven deep to lower rock below, the lateral energy transfer to the building is not affected at all...the upper soil profile is still moving and driving the building.

Perhaps there's a consideration that the vertical seismic demand (0.2SDS(D)) might be based on the rock, but even then the code doesn't address this.

 

[msucog]

i disagree and it is specifically addressed in the codes and/or references of the codes. if you're going to use site class b for deep foundations, then you should perform an analysis to evaluate the site soil effects on the ground surface versus rock levels as well as along the piles/caissons. the ground surface will move differently than the rock levels so an eq event may overstress the foundations/connections. following that, the building will end up moving with the ground surface so the site class is taken from the pile cap level as long as you don't have below grade walls above the pile caps. if you have "short" deep foundations, then it may be appropriate to use site class b assuming you don't have liquefaction issues.

 

[msucog]

re-reading the previous posts, i initialy misread part of the comments. it looks like we're almost saying the same thing (now that the caffeine from my coffee has sunk in).

 

[JAE]

msucog - nothing like a triple shot of espresso right?

 

[msucog]

indeedy jae...love that java bean extra strong i do.