Sit Class per IBC 2003
Sit Class per IBC 2003
(OP)
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.
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.





RE: Sit Class per IBC 2003
Second: Why stop there; call those 4-foot diameter, 100 foot deep drilled piers "round and skinny spread footings". What’s in a name?
RE: Sit Class per IBC 2003
It does not specify any condition for deep foundations.
Where do you draw the line is exactly my point.
RE: Sit Class per IBC 2003
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
RE: Sit Class per IBC 2003
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.
RE: Sit Class per IBC 2003
Andreas
RE: Sit Class per IBC 2003
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