Allowable Bearing Press VS Structural Load 2

[geotech753]

Anybody know exactly how structural engineers interpret a allowable bearing capacity from a geotech?

It really is whatever the geotech report says, in my case, the allowable bearing pressure (for shallow footings) is dead load plus design live load (or: dead load as is + (design factor)x(live load) ).

It seems I am answering my question, however, on a project that I got feedback, I am seeing that the structural engineer is considering my allowable as only dead plus live. Is this just a judgement call on the structural engineer? - maybe because the difference between (dead + live) and (dead + design live) is small? So I am asking what the general consensus is prior to pointing it out.
Thanks.

 

[fattdad]

In my office the combined structural loading (i.e., live plus dead) is applied to the foundation and the foundation size is determined based on the design bearing pressure. There are some exceptions where the load is transient and we'll allow the bearing stresses to increase, but that's a case-by-case basis when the loading is short-term and not likely to influence the foundation settlement.

f-d

¡papá gordo ain’t no madre flaca!

 

[JAE]

The bearing pressures I have received from geotechnical engineers usually are an allowable design pressure intended to be used with service dead loads and service live loads combined into a total service load:

SDL + SLL = STL

No safety factors are on these loads. They are directly calculated loads and as fattdad states, we take the STL and divide by the allowable pressure and get a required area of footing (to put it in simple terms).

In some cases the allowable pressure is a net pressure, sometimes gross pressure.

Some cases, I have geotechs saying the pressure is for SDL plus 1/2 SLL. I don't see that too often and I'm not sure why it's done as we (the structural engineers) usually are already taking care of live load reductions in our calculations.

I've never ever added a factor to my live load and used it with a provided allowable design pressure.

 

[fattdad]

There can be some live load reductions. Take the case of wind loads. There's no doubt that this is a real structural load that the beams and girders must support. This structural support must be conveyed to the foundation and the foundation must have sufficient reinforcement to distribute that load throughout the bearing area.

The question becomes just how the bearing surface will deform under this transient load. Is it reasonable to allow the bearing pressure to go from the net allowable load (let's say 3,000 psf) to 4,000 psf as the foundation soils react against a wind gust? From this perspective there can be some live loads that are reduced. Live loads related to files, furnature, etc. are a different story.

f-d

¡papá gordo ain’t no madre flaca!

 

[BigH]

Philosophically, I'd like to go back to the original post - allowable bearing capacity. In my view, this is the allowable bearing capacity based on shear or strength considerations. We normally apply a factor of safety of 3 against the ultimate bearing capacity to arrive at the allowable bearing capacity. For wind loads (short transient loadings), we allow typically a 33% increase in the allowable bearing capacity (i.e., a decreased factor of safety to the transient nature of the loading). Note that this is not generally a net allowable bearing capacity - as the computation will have already taken into account the embedment depths.

The allowable bearing capacity must then be considered with respect to the service requirements of the foundation - such as limiting settlements or limiting differential settlement or for other reasons. In such cases, a different allowable pressure may govern - such as if one determines the settlement under the allowable bearing capacity to be 40 mm but the structural requirements limit the settlement to only 20 mm. Then the allowable pressure must be reduced in order to achieve more stringent maximum settlements than the allowable bearing capacity will cause. The reduced allowable pressures then become the allowable bearing pressures (contrast to allowable bearing capacity). The permitted bearing pressures are less than the actual capacity.

With respect to allowable bearing pressures, they are almost always (but not always) net allowable bearing pressures since the the settlement is caused by pressures exceeding the pressures already having been placed on the soil in the past. In the case of loading overconsolidated soil, though, this is not the case since the point of the existing overburden pressure to the "past" overburden pressure causes recompression settlement. Normally this is inconsequential but there are some structures where this might be more accutely considered.

I hope that this helps set out the basic difference between allowable bearing capacity and the allowable bearing pressure.

 

[Lion06]

BigH-
So you would typically allow a 33% increase for allowable bearing capacity, but not for allowable bearing pressure? Is that an accurate statement?
Also, one might assume all geotech's do not necessarily use the same terminology as you. That being the case, is this (in your opinion) something that should be discussed with the geotech for every project?

 

[jdonville]

Structural EIT,

The language in the geotechnical report should inform the structural engineer as to what the limiting factor is - settlement, stability or strength - regardless of the terminology used.

It is a fact that some of us (myself included) can be inconsistent (or even mistaken) about the terminology, and BigH has presented the subtle distinction between allowable bearing pressure and capacity well. If you have a question or concern related to the geotechnical report, you are well within your rights and responsibilities to query the geotechnical engineer to clarify your understanding of the content of the report.

If you work with a geotech on a regular or ongoing basis, you also can have the opportunity to influence the standard report language used, and help us poor geotechs stay on the path of righteousness and more consistent terminology.

Jeff

 

[COEngineeer]

I dont factor my live load either. Dont you think the LL IBC give us already have been factored? 40 psf for residential is pretty hard to achieve. You have to have a big party where the room is packed and you can hardly walk around (like a packed bar). Same thing with other live loads.

http://www.swijetty.com

Sea Water Intake and Jetty Construction

 

[csd72]

I would normally take it as the maximum design working pressure.

If wind is minor, then I would not check it in detail except for overturning and uplift. Bearing pressure would then be dictated by DL + LL.

If wind is a major design criteria (such as for a billboard sign) I would try and keep the total WL + DL within the allowable bearing pressure.

I would only go into the 33% increase if I had problems meeting the design intent.

csd

 

[Lion06]

for the OP-
I wouldn't factor the LL for check for bearing pressure conformance. I would factor the LL when designing the footing - but then I would factor the DL also.
Were you asking if the LL should be reduced or increased? It should not be reduced unless adding in wind (if required), and should not be increased unless doing strength design (but again, then you must increase DL also - not just LL).

COEngineer-
I think the 40psf is likely the max they would expect it to achieve, but is not factored. If you do strength design, you must still factor it. Also, I know I have some pretty heavy furniture in my bedroom, living room, and sunroom, but I agree that 40psf is relatively hard to achieve.

 

[BigH]

StructuralEIT - you make a fine distinction. I have no problem with allowable bearing capacity being factored upwards for the transient load (in fact, in practice, it seldoms governs except in soft to firm (medium stiff) clays). With respect to settlements, though, if you are in cohesive soils where the transient loadings are not sustained enough to cause additional consolidation settlement (the elastic settlement being small) you could, if needed, increase the allowable bearing pressures - but in these cases I wouldn't bother since it is a non-critical case - you know it doesn't govern so your 'check' should not be on it. For granular soils where the pore-pressures generated by the transient loading are quick, you could get addtional settlements due to the transient loading and I would check the settlements under the DL plus LL plus transient loading. This does not apply to seismic loading which could cause liquefaction (a whole new ballgame so to speak).

As for the terminology, I think that most standard texts use the same philosophy but perhaps worded slightly different although I have seen a few (Vargese, for instance (a good Indian text - and I reiterate a good one) where he mistakenly, in my view, does not make the distinction. This is a topic of terminology that creates so many problems - I hope that someone finally get geotechnical engineers - and especially book/paper writers on the same page.

 

[COEngineeer]

This is what I do, correct me if I am wrong.

Isolated footing: Area of footingLL+DL)/allowable bearing pressure

For continuous footing I do same method as above unless it is a basement wall. Usually the toe pressure control due to equivalent liquid pressure. I do not factor the equivalent liquid pressure nor the gravity live load.

When I design the reinforcement of the concrete however, I do factor the moment.

http://www.swijetty.com

Sea Water Intake and Jetty Construction

 

[civilperson]

Find the column with the largest Live Load percentage when compared with the (LL + DL). Size the footing for that case and then solve for DL soil pressures. Use that DL soil pressure to size all the rest of the footings, (ignoring live load in these other cases).

 

[Lion06]

I am not quite following you civilperson. Can you explain that a little more? What is the reason for ignoring live load on columns with less than the max % of LL to total load?

 

[civilperson]

An example: Column A has 70% LL and 30% Dl which totals 200 KIP. Other columns have 50% LL and DL. Allowable soil pressure is 3 TSF. 200 KIP/6 KSF = 33.33 SqFt. Use 6.0' square footing. The DL pressure is 60 Kips/ 36 SqFt= 1.67 KSF. Column B has 150 KIP total load, use 75 KIP/1.667 KSF=45 SqFt footing. The total load soil pressure of Column B footing is less than Column A soil pressure, thus OK, This gives equal DL pressures, (which are assumed to be sustained) and thus equal settlement.

 

[Lion06]

That is actually pretty slick.
What happens if Column B has a higher load than A (the one with the highest LL%)?
Doesn't this also lend itself to possibly truly having a different footing size at each column? Do you do any design smoothing to minimize the number of footing sizes?

 

[civilperson]

If Column B has total load of 300 KIP, (50% DL) use 90 SqFt footing. Footing sizes should reflect the loading: Yes different loads(+/- 10%) have different sizes of footings and usually the interior column, the side column, and the corner column are the classes of columns which have similar footings.

 

[COEngineeer]

hmm interesting civilperson, so you do this on any type non expansive soil or what? Maybe only silty soil? Is this a common practice? This will make most of the isolated pads bigger right? Do you do this on residential projects?

http://www.swijetty.com

Sea Water Intake and Jetty Construction

 

[msucog]

i would urge caution with the method described (at least in certain geologies) by assuming equal settlement simply because the contact pressures are similar.

 

[COEngineeer]

What do you mean by certain geologies msucog? expansive soil perhaps?

http://www.swijetty.com

Sea Water Intake and Jetty Construction