Sit Class per IBC 2003

[andi513]

The 2003 IBC reads that "Site classes A and B, shall not be used if there is more than 10 feet of soil between the rock surface and the bottom of the spread footing or mat foundation."

Makes sense so far, but what about deep footings? We have a building with about 80% of the structure supported by spread footings. In these cases there is less than 10ft of soil between the bottom of the ftg, and the rock.
The remaining is supported on drilled piers. The current geotech report states site class C due to the small area where more than 10ft is between B/Ftg and T/Rock. Is this accurate? Does the area, especially since relatively small really affect the overall response of the system? Second, what if I call my drilled piers "round and skinny spread footings"? then there is zero ft between T/Rock and B/footing?

Any enlightenment, maybe even possible literature links would be greatly appreciated.

 

[TDAA]

The soils adjacent to the piers will have an effect on the piers, which should be designed for. Do you have the expertise in seismic design to trump the code with your judgment? I am guessing your geotech does not, since he followed the classification outlined in the code. Also, does the geotech know if the shear wave velocity of the rock meets the requirement for site class B? Is the rock in the area "competent" enough to allow the use of an estimated value, or is it required that it be determined on site? Can you be sure that the 20% area is the only reason a Site Class C was provided?

Second: Why stop there; call those 4-foot diameter, 100 foot deep drilled piers "round and skinny spread footings". What’s in a name?

 

[andi513]

I am asking this since the IBC calls for "Site classes A and B, shall not be used if there is more than 10 feet of soil between the rock surface and the bottom of the spread footing or mat foundation."
It does not specify any condition for deep foundations.

Where do you draw the line is exactly my point.

 

[jdonville]

andi513,

There has been much discussion on this site with respect to Seismic Site Class. Unfortunately, the code developers have not seen fit to issue much (if any) guidance on the consistent determination of the SSC.

I would be pleased if someone 'official' said something to the effect of "whether or not there are deep foundations (rock-socketed or otherwise) makes no difference to the determination of the SSC" beacause that, at least, would be some sort of guidance. Unfortunately, there remains much confusion, and a Commentary on the code, containing worked examples and/or references to published works upon which the relevant section of the IBC is based, would do much to reduce it.

I am not, however, holding my breath.

Jeff

 

[TDAA]

According to Terzaghi’s bearing capacity theory, a foundation is shallow if the depth of footing is less than or equal to the width of footing. Others suggest that the depth of footing could be up to 3-4 times the width the width of footing and still be classified as a shallow foundation (taken from Das).

So, it depends on the depth and width you are looking at. However, most people would look at a drilled pier-type of foundation as a deep foundation, especially if you are looking to bear on bedrock. An exception to this would be a “drilled footing”, which would typically be in the 3 foot diameter range, and likely not set on the bedrock. I have recommended this approach to avoid removal of undocumented fill, as a way to construct the footing on native material.

 

[andi513]

So can I assume that for deep foundations, the limitation doesnt apply, since it clearly states " and the bottom of the spread footing or mat foundation."???

Andreas

 

[jdonville]

andi513,

I would act as though the criterion was " and the bottom of the foundation."

As I said, there has been much discussion on engtips. Do a search on 'seismic site class' or 'site class' and you should see most of the applicable threads.

Jeff