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Allowable Bearing Press VS Structural Load

Allowable Bearing Press VS Structural Load

Allowable Bearing Press VS Structural Load

(OP)
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.

RE: Allowable Bearing Press VS Structural Load

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!

RE: Allowable Bearing Press VS Structural Load

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.

RE: Allowable Bearing Press VS Structural Load

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!

RE: Allowable Bearing Press VS Structural Load

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.

RE: Allowable Bearing Press VS Structural Load

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?

RE: Allowable Bearing Press VS Structural Load

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

RE: Allowable Bearing Press VS Structural Load

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

RE: Allowable Bearing Press VS Structural Load

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

RE: Allowable Bearing Press VS Structural Load

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.

RE: Allowable Bearing Press VS Structural Load

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.
cheers

RE: Allowable Bearing Press VS Structural Load

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

Isolated footing:  Area of footing:(LL+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

RE: Allowable Bearing Press VS Structural Load

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).

RE: Allowable Bearing Press VS Structural Load

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?

RE: Allowable Bearing Press VS Structural Load

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.

RE: Allowable Bearing Press VS Structural Load

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?  

RE: Allowable Bearing Press VS Structural Load

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.

RE: Allowable Bearing Press VS Structural Load

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

RE: Allowable Bearing Press VS Structural Load

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

RE: Allowable Bearing Press VS Structural Load

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

http://www.swijetty.com
Sea Water Intake and Jetty Construction

RE: Allowable Bearing Press VS Structural Load

well, 3ksf for a 5x5 ftg is not necessarily the same as 3ksf for a 15x15 ftg. some soils and scenarios i've run across act very funky at higher loading conditions especially when the stresses are transmitted shallower/deeper. this is the reason why we start running swanky tests and settlement analysis when loads get above certain levels. i'm in the piedmont and there are some particularly nasty bands of soil that look like most other soils but act much differently under higher loads (highly compressible above certain stresses) or largely loaded areas (fill induced settlement).

as far as the mention of the live load reduction by 1/2 above, my philosophy on it is that the likelyhood of all the live loads acting for long periods of time is slim (most of the time) thus the settlement analysis should account for this to some degree to keep from over-compensating. keep all the dead and at least 1/2 of live. if the scenario happened to have live loads experienced for longer periods of time, the reduction would not happen. others may have different opinions.

RE: Allowable Bearing Press VS Structural Load

Just as a side info:

the recent Eurocodes (particularly Eurocode 7) have thoroughly reviewed the loads issues, building up a pretty interesting system.

There are many "load scenarios" in function of live loads, environmental loads, seismic loads, accident loads and so on.

There are coefficients which make up for the small likelyhood of two or more unfavourable conditions occurring. Many problems discussed above are worked out that way.

The system at first is not straightforward and takes soem time to get familiar with. Similar to LRFD.

RE: Allowable Bearing Press VS Structural Load

My DL category includes sustained live load, permanent equipment, etc.  If the minimum dimensions of differnt footings are more than 200% different, then a separate settlement analysis should be done for the different sizes. If similar, then the method seems appropriate.

RE: Allowable Bearing Press VS Structural Load

that seems reasonable to take out a few kinks. i have run across a few situations where the same size/load footings within 50-100 feet of each other had too much differential for the structural to handle. i guess it's impossible to have an easy method for every scenario out there. the main reason for this scenario was the residual soil i mention above under one footing and structural fill under the other. since the loads were not overly large (~100kips), it wasn't a completely impossible scenario (lab data suggested about 1-2" differential but was probably just slightly less--still enough that the owner/engineer wanted to fix) but was enough to require undercutting and replacement with densified aggregate for the footings in the area of the cut-fill line. we just spread the settlement out over several of the warehouse bays. if it had been something like a parking deck or several stories of building, the thing would've had serious issues if put on normal spread footings. and the slab area required remediation (removal and replacement of structural fill) over the upper 2-3 feet to reduce issues with the floor loads.

RE: Allowable Bearing Press VS Structural Load

And now we can look at the specific case of how to do this for a building addition where there is an immediately adjacent footing that's already realized its "1-in" of settlement and you are constructing a footing with a new load.

I find these second-generation foundation loads interesting to evaluate.

f-d

¡papá gordo ain’t no madre flaca!

RE: Allowable Bearing Press VS Structural Load

Do any structural guys out there do their own settlement analysis?  I don't know anyone at my firm that does or has.  
We just design for the allowable pressure given in the geotech - at least I have never had any of the project engineers I work with as me to do anything along those lines.  I have only been asked to size the footings not to exceed the allowable pressure.
I have designed some REALLY BIG footings.  I am talking about 45'long x 12'wide x 3'thick combined footings with shearwalls, and columns on them.  Should any additional analysis have been done on these types of footings?

RE: Allowable Bearing Press VS Structural Load

"Should any additional analysis have been done on these types of footings?"--i say it depends on the site specific conditions & loading. the larger the area of the footing (and larger loads) or the larger the area loaded (say for large fill areas), the deeper the stresses go.
i recently looked at a 30' tall CIP wall. the wall itself didn't scare me but the 30' of new fill going to be placed behind the wall after it would be built did. to make it more complicated, the ends of the wall stopped at the toe of the slope (the wall made sort of a Z shape with the end of the leg being at the toe of the slope). did some borings, lab testing, settlement analysis, etc...came out with 9" at the worst spot and <1" at the toe of the slope less than 100' away. the area was going to have to be surcharged. the architect kept telling me that he didn't have time to wait for surcharging. i told him to either follow our recommendations, don't follow our recommendations with possible severe consequences, design the thing to withstand bigtime movement, or put it on piles. so the owner made the wall go away all together since their schedule could not accomodate the surcharging/settlement monitoring program that was necessary...problem solved.
for "normal" circumstances: for a long continuous footing, we typically look at the stresses to 4B. for spread footings, we look to 2.5B. i'd call the 45'x12' closer to the spread footing or mass fill scenario. for large area mass fill scenarios, i usually end up look at the entire soil profile thickness.

RE: Allowable Bearing Press VS Structural Load

lets look at a very simplified scenario. 5x5 ftg vs a 12x12 footing with both have a contact pressure of 3ksf.
3ksfx5x5=75 kips
12x45x3ksf=1620 kips (i'm guessing it probably would not be this high for most cases since there would likely be other reasons for such a large footing size--resisting uplift, fuel tank, etc).
let's just say that the settlement analysis shows settlement on the order of 1% of the 2.5B depth.
5x2.5x.01x12=1.5"
12x2.5x.01x12=3.6"
say take 1/2 of each and you're looking at 3/4" to 1 3/4" settlement between the two scenarios. if you used the 200% size rule mentioned above, then a red flag would have gone up for this particular scenario if your footings were sized as mentioned.
there's a million different scenarios that could happen--so get the geotech to look at it.

RE: Allowable Bearing Press VS Structural Load

So the process you recommend is to size the footings not to exceed the allowable soil pressure given by the geotech in the report and then in addition to that either do your own settlement analysis or have the geotech look at your footing design?

RE: Allowable Bearing Press VS Structural Load

provide the geotech with all the details. column loads and any "non-typical" scenario". let them check it out. with the very large footing scenario, the actual loads are probably much smaller so that even though the stresses extend deeper in the ground, the actual settlement across those layers is less than for the regular sized footing.
for example, i recently provided a subsurface for a client to check a small building addition out in the parking lot. they provided maximum column loads of 60k. they (the architect and engineer) specified 10 borings to x-number feet deep. the depths seemed appropriate for the structure and nothing was suspect during the drilling. after completing the drilling, i was passing along preliminary information that everything looked ok for shallow foundation support with 3000psf bearing capacity. then he happened to mention the large generator (actually said "largest that CAT makes"). i told him i'd need to recheck my recommendations and asked for loaded. he also then mentioned the very large fuel tank that would accompany the generator. he made note of a meeting the following week and i invited myself because i had seen nothing of what he had mentioned and suspected i still did not know the big picture. since the structure was going to be critical to the 24/hr-365days/yr business, we began asking lots of questions. turns out, there are 5 of these generators and fuel tanks. the loading areas were large and heavy. so as it turns out, i didn't settlement analysis about eighteen different ways before he finally gave us all the information (as far as we know). we asked for this stuff up front but they provided only very minimal information. we were very close to having to re-mobilize the drill rig to do additional sampling for lab testing. luckily, we had a few samples from the project site from two previous projects. also, since this turned out to be a very critical structure to the company that also happens to have lots of money, they were going to put GAB in the upper 7 feet of the building anyway. with that, we felt comfortable with our recommendations without additional sampling.
moral of the story, help the geotech help you by providing as much information regarding the project as you can.

RE: Allowable Bearing Press VS Structural Load

If the geotechnical engineer has given you an allowable bearing pressure, then I wouldn't exceed it.  muscog is correct in saying that the allowable bearing pressure will really depend on the size of the footing so if you have larger than 'normal" footings - say 3m x 3m, then you should question your geotechnical engineer as to whether his recommendations are "size sensitive".  If they are, then the geotechnical engineer should have stated it - or given you a chart of allowable bearing pressures vs size of footing for a specified maximum settlement. (Note, you might have a couple of curves - one for 25 mm and another for 40 mm, say).  As a structural engineer, I don't think that it is your "business" to do settlement analysis - but as a matter of interest you might wish to do a rough estimation to confirm in your own mind the order of magnitude to be expected.

RE: Allowable Bearing Press VS Structural Load

BigH-
Thanks!!  That was a very helpful explanation.

RE: Allowable Bearing Press VS Structural Load

(OP)
What I end up doing is:
Ask or figure out the maximum and typical loads.
Run settlement analysis for the different scenarios.
Provide table with one of the columns being Max Load.
This limits the footing size.
(MaxLoad)/(All.Brng.Cap) = Max Ftng Area
Sqrt(MaxFtngArea)= B
Similar deal with continuous ftngs.

RE: Allowable Bearing Press VS Structural Load

It's not just about max gravity loads.  The buildings we design use almost every column as a lateral column - we do a lot of type II with wind connection buildings.  To top it off, this particular building had over (15) combined footings - most with (2) lateral columns, some with (2) lateral columns and a shearwall.  I have never done a settlement analysis, and I know if we went back to the geotech with these crazy footings asking him to do an additional settlement analysis that would certainly be "additional services".

RE: Allowable Bearing Press VS Structural Load

I'd like to add more, but you guys have covered it all.  Just to reinforce an earlier point.  If you give the geotechnical engineer the column loads then you will get a report that you can use to size your footings.  Somewhere behind the scenes, the geotechnical engineer should be looking at the range of likely footing sizes, the depths of influence and the consequence of these loadings on the soils.

There is a reason that geotechnical reports should state limitations to the effect that, "This report is intended for the _________ project and if plans change the recommendations should be reviewed" (or something like this).  If the original development is for a gas station and the proposed construction really turns out to be a multi-story office building, then the gas station recommendations may be just wrong.

f-d

¡papá gordo ain’t no madre flaca!

RE: Allowable Bearing Press VS Structural Load

For every building foundation that I designed while working for the COE, I used DL + 1/2LL as recommended by policy.  Maybe the given allowable bearing pressures had higher safety factors: I don't know.  For buildings it seems reasonable (because LL is transient) and I still do this to this day.

"Clever of me to use my spine to break my fall like that."

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