Increase in allowable pressure for depth of footing thread255-24941 1

[msquared48]

There is a statement in this string from 2002 by the OP that after the first twelve inches the allowable soil bearing can be increased 20% per foot of depth up to 300% of the initial soil bearing value.

Is there a reference section in the IBC or IRC to verify that? I have never seen it there... yet.



Mike McCann, PE, SE (WA)

 

[WARose]

I'm not sure I've seen it in the code either. Typically, I've seen this accounted for in adjustment factors in the bearing capacity equation. (It's how you can get some pretty high values for end bearing on piles.)

 

[BigInch]

Never heard of that 20% thing. I doubt you will find it in the BCs.

What I know is that, if you have a net allowable soil bearing strength, it is usually specified at a given starting depth, ie. 3000 psf at 3ft depth. If it is a "net bearing capacity", net never changes, so absolute soil bearing at a different depth can be increased by the weight of soil that overlayed that depth. Absolute bearing capacity at 6ft would be 3000_psf + overburden above ie. (6_ft-3_ft) * soil density_pcf
3000psf + 3ft * 110pcf = 3330 psf at 6ft.

 

[msquared48]

The use of this soil bearing increase would be to justify the end bearing pressure of augered concrete footings for pole structures due to the lower than normal placement depths for bearing.

I just cannot find it in the code - it may come from some other publication or an inventive Geotech. I know I have seen something like this used in the past, but could never justify it in writing.

I think it possibly could be justified by the passive restraining effect of the adjacent soil, preventing the rotational soil failure seen at shallower depths. At least that would make sense to me.

Mike McCann, PE, SE (WA)

 

[bookowski]

The NYC code has a footnote which allows a 5% increase per foot beyond 4ft up to 2x the basic value for certain soil types. I know that doesn't apply to you but it probably has origins in another code or reference which is maybe what you're thinking of.

 

[wannabeSE]

Blow the dust off your 97 UBC. There is a footnote in table 18-I-A

2 All values of allowable foundation pressure are for footings having a minimum width of 12 inches (305 mm) and a minimum depth of 12 inches (305 mm) into natural grade. Except as in Footnote 7, an increase of 20 percent shall be allowed for each additional foot (305 mm) of width or depth to a maximum value of three times the designated value. Additionally, an increase of one third shall be permitted when considering load combinations, including wind or earthquake loads, as permitted by Section 1612.3.2.

 

[msquared48]

Ok. I will. Now to see if it is still applicable.

Mike McCann, PE, SE (WA)

 

[BigH]

Thanks wannabeSE - now what happens when 6 inches below your footing you have a hidden layer of peat or soft sensitive clay? - or as they once found in Alberta a "dinosaur latrine"? and in msquared48's case, bearing using an auger - how does one muck out the disturbed (and/or dropped) soil that the auger flights will not remove? and what effect will this detritus have. I am sure all are aware of this - but . . . sometimes we need reminders. End of my day - now going to exercise this "old" body!

 

[msquared48]

That's the risk you take with any bottom bearing design where there is no Geotechnical investigation.

The situation becomes a very moving experience.

Mike McCann, PE, SE (WA)

 

[BigInch]

Once had a settlement problem with a gas compressor station. There were soil borings done in conjunction with the geotech investigation, but the borings missed the broken telephone poles and snow lenses that remained after someone did an uncontrolled fill of some of the area. The ice lense eventually got warm and melted. Not a good day that was.

 

[msquared48]

"Not a good day that was."

Was Yoda your teacher in school?






Mike McCann, PE, SE (WA)

 

[BigInch]

I'll rephrase.
That was not a good day.