Presumptive Load-Bearing Values

[henryf32308]

I need some guidance..

Section 1806 of the 2009 IBC addresses presumptive load bearing values (displayed in Table 1806.2).

Does anyone know if these Qult or Qall values? Anyone have a source for this info?

Thanks

 

[msquared48]

In the 1979 UBC, and I assume others, but this is the one I will address, Table 29B refers to "Allowable" loads for vertical and lateral loads for footings. OK

In Section 2906, it also refers to these values as "allowable", and not to be exceeded unless there is data available to suggest a higher value - soils test implied.

In section 2908 (b) and (c) for piling, there is a reference to "Method 1" where the "allowable" load for the pile is determined by a load test, the allowable not exceeding 50% of the yield point. OK for that too.

Although I could find no specific statement here relating to yield vs allowable for soil, I think the testing methodology for pile is a definite hint to the "allowable" vs "yield" values.

All the allowable values from the table are letting you do is forgoing the expense of a soils test in lieu of being conservative regarding the soil bearing values. So. yes, the soil can probably take more load if properly classified according to the tables. However, that is all relative, for it will also settle more to achieve that value.

Mike McCann
MMC Engineering

 

[henryf32308]

Thank you

Chapt 29 now concerns Plumbing Fixtures. All of the Soils and Foundations stuff is in Chapter 18.

Based on your response my feeling is that presumptive loads refer to allowable (not ultimate) loading.

This is what I needed to know.

Thanks again

 

[msquared48]

Right.

I was just referring to an older code verbage that would be closer to the original intent.

Cheers.

Mike McCann
MMC Engineering

 

[BigH]

I can't put my hands on it immediately, but I remember researching this several years ago for one of the forums here and presumptive presumes allowable - and this is for ensuring that a standard (I believe 25 mm) settlements are not exceeded. I'll try to find the references but I've just packed up about everything for a move.

 

[wannabeSE]

Read Section 1806.1
. . .shall be used with allowable stress . . .

 

[henryf32308]

Uh no...

1806.1 says to use the values in the table if you are designing with ASD

You can increase the values in the table by a third if you are designing with LFRD

It is not related Qall vs Qult

I agree with msquared48 and BigH that, from the context, this is a Qall number. I don't see this explicitly addressed in the IBC.

 

[BigH]

It may not be addressed directly in the IBC - but where did they get their numbers? Likely from the NYC Building Code or Chicago's - and I remember reading in the actual city codes (from many years ago) that the values are based on qall (deformation).

 

[Erdbau]

IBC 2006 had the column listed as "Allowable Foundation Pressure". IBC 2009 has "Vertical Foundation Pressure". The values are the same between the two versions.

 

[wannabeSE]

It seems as though I did not understand the notation. I assumed that Qult meant the ultimate bearing capacity to be used with LRFD (Load and Resistance Factor Design)and Qall meant allowable bearing capacity to be used with ASD (Allowable Stress Design). The 1/3 increase mentioned in section 1806.1 is used with the alternate allowable stress load combinations with wind and seismic.

After reading BigH's post, it seems as though the Qall is the allowable bearing pressure based on deformation/settlement. Whereas, the Qult might be the maximum bearing pressure based on soil failure (shear failure?). Unfortunately, I assumed they were values to be used in structural design.

I apologize for any impertinence on my part. My only experience has been using the design values and recomendations in soils reports provided by a geotech.