IBC 06 Table 1804.2 Bearing Capacities Allow. or Gross?

[waytsh]

Table 1804.2 in the IBC 2006 Building code gives presumptive soil values based on the class of material. Is the allowable foundation pressure they have listed a net or gorss value? In other words can I neglect the weight of the footing and soil overburden when using these values?

Thanks

 

[JAE]

It doesn't appear to say at all which it is.

 

[waytsh]

I thought I might have been missing something somewhere. Thanks for confirming that I am not losing it...at least not as far as this matter is concerned. I guess unless someone has anything to add I'll assume that this is a gross value.

 

[JLNJ]

It must be net. If it were gross and your footings were deep enough you'd be down to almost zero net.

Having said that I probably would not increase the allowable in an area such as a basement where I was removing the overburden without the recommendation from a geotech.

 

[msucog]

slightly away from your question, i would trust only the person that has set foot on the site (the project geotech). make sure to read all the caveats to using that table. also bearing capacity is more than just holding up the structure...you could have a soil capable of holding up 3000psf (net) but it might settle 3 inches which is likely not good for the performance of the structure. from what i vaguely remember from my concrete design classes in college, it's sort of like designing an elevated slab (and forgive me for my very rough analogy)...you might design a slab that has no problem not collapsing...but the darn thing might feel like you're walking on a water bed if the slab deflection under live load is not appropriately considered. if i used were to use that table, i'd use the most conversative interpretation of it. and again, the table cannot possibly foresee actual site conditions or variations...even if it does do a fair job at providing "minimums/maximums" some of the time. i don't have the code in front of me right now, but i suspect they likely provide a net number (assuming certain criteria)...i'll try to confirm that some time next week.

 

[BigH]

Robert Day's Foundation Enginering Handbook (McGraw-Hill and ASCE Press) discusses the IBC in Chapter 18. He states: [blue]"Settlement Considerations. Different structures may have different requirements in terms of the maximum amount of acceptable settlement. But the allowable bearing pressures indicated in Table 18.4 do not consider the amount of settlement that would occur under the aplied foundation loads.[/blue].
He further indicates that the presumptive allowable bearing pressure is also known as the allowable bearing capacity or allowable foundation pressure. This is where I have major problems. We are engineers and should be very clear in our choice of words. In my view and the view by most in the geotechnical community I would surmise is that we have two conditions to consider.
(1) shear conditions - this leads to the ultimate bearing capacity (based on shear) (i.e., about 6xSu for clays). The [blue]allowable bearing capacity[/blue] is the ultimate divided by the factor of safety to be provided (normally 3). This is almost always "gross bearing" as the embedment factor is included in the computation of the bearing capacity (the Df term).
(2) Settlement (or service) conditions - which, for practical purposes is based on settlement considerations (which might be structure mandated, but could be based on other considerations (like expected settlement under seismic loadings)). In such cases, in my view, we should carefully use the term [blue]allowable bearing pressure[/blue] as opposed to capacity (shear). This is almost always based on net bearing pressures.
In essense, [green]"capacity" implies shear or strength and "pressure" implies service or settlement[/green]. If we could all adopt this terminology, we would have a far simpler life. Countless hours are probably spent fretting over whether a quoted bearing value is "shear" or "settlement" governed. I would suggest that cleaning up our terminology would give us more time to enjoy other facets of life!

 

[msquared48]

JLNJ:

This would be highly unlikely as it is allowable to increase the capacity of the soil with depth.

I use the table Value X ( 1 + .2(H -1)) where "H" is the depth to the bottom of the footing. Soils report values would take precidence though and not be increased without the OK of the Geotech.

Mike McCann
McCann Engineering

 

[msucog]

i agree to some extent with bigh. however, shouldn't settlement always be considered? when i see capacity, i almost always think of settlement since it usually governs where i'm at. we always take a look at settlement potential. most people say bearing capacity (meaning pressure) so shouldn't shear and settlement be considered? i guess i oppose not considering settlement. is there any instance that you'd intentionally not consider settlement potential (whether estimated from experience in your geology or site specific testing)?

 

[BigH]

You always consider settlement of course - and as msucog indicates, there are very few cases (soft clays principally) where shear actually does govern. This is one reason why, in the many years I have practiced, I have done very few actually bearing capacity computations (other than 2xSu for a red light/green light for bearing in clays). If one looks at LRFD, etc., they make a big point of the shear strength reliability but then little shrift of settlement - which, we know governs most of the cases. Still, I think that we should get in the habit of using capacity for shear and pressure for settlement. Which, of course, is why 90% of the geotechnical reports say 'the allowable (net) bearing [blue]pressure[/blue]' can be taken as X kPa (or ksf). The other factor that always comes in and again, it is given very short shrift in codes and books is the influence of adjacent foundations on the allowable bearing pressure. Settlement is the primary concern of bearing pressures. The point is, sadly, that the building code doesn't given [brown]presumptive[/brown] capacities for a [brown]presumed[/brown] 25 mm, say, settlement of an [brown]isolated footing[/brown]. In such cases, such presumptive values would be far more useful and adequately annotated.

 

[msucog]

point taken. i personally would rather see the table not included in the codes (at least the commercial codes) but i suppose that'd require people to actually get a geotech involved. i think for the most part (where i'm at) a geotech report is required for commercial work. i'd argue it should be required for most residential work too to help lower the insurance claims but i won't hold my breath for that one...besides, the developer would then just tack on another $100k per house for spending an extra $30k to adequately drill a 100 acre development. i'll thumb through the section some time this week to see if any caveats jump out at me (i don't typically read that section regularly so i don't remember off the top of my head what it says).
bottom line: i suggest you get a geotech to check out the site and provide the necessary recommendations.

 

[waytsh]

I'm sure that we would all agree that it is best to have a geotech on site. Unfortunately I'm in an area that does not require a geotech on small commercial projects and trying to get a geotech involved is a battle I have been fighting for years. For some reason many contractors are not willing to make the expense even when I explain to them the benefits, and that they will most likely save money in the long run. I see many engineers making the statement on their drawings that the foundation designed is based on certain "assumed" vales and that a geotech should be consulted to verify the conditions in an attempt to put the onus back on the contractor. I am willing to bet the farm that 90% of the time a geotect is never consulted.

I apologize for this little aside. I have drifted slightly from my original question on gross versus net.

 

[msucog]

i often drift far away from the question at hand but eventually make my way back around. fortunately for me, the other professionals on this site put up with my long stories and admit me to stay around. i do find that sometimes the best answer is not the direct "obvious" response...however sometimes, i'm just conversing for the sake of trying to trying to learn from the other fine folks here.

and i've seen some rather funny "assumed" values in my relatively short engineering career...

 

[BigH]

msucog - yeah, like calling for a RE wall of 10 m height on soil with N=1 for 6m thick; and here in Indonesia, they assume 300 kPa bearing pressure even though we have soft clays at the surface. Some strange things, eh?