Rectangular vs. Triangular / trapezoidal bearing for Offset Footings 2

[efFeb]

Good Morning,
I have always used a trapezoidal / triangular soil bearing distribution when checking the maximum bearing on a gravity footing with eccentric loading.
I have been told by others at my office that I can use a rectangular distribution instead, with the average bearing pressure, instead of the maximum bearing pressure, used for bearing pressure checks.
Is there a code clause that allows me to do this? I have looked through my geotechnical textbooks and am seeing that, at least for SLS, I do need to take this triangular distribution. It is a huge difference in the values between these two approaches too.

Thanks so much in advance!

 

[phamENG]

In the states (or at least my area) geotechnical stuff is still largely allowable stress/service level type loading. So I use triangular distributions. A Canadian engineer or somebody with experience in limit state design for soil bearing will need to weigh in.

 

[human909]

Maybe I'm misunderstanding the question. But how does somebody arrive at a static solution if you have even (rectangular) load distribution and an eccentric load? Seems like the moment from the eccentricity isn't being resisted.

Sure a rectangular distribution will probably 'do the job' without anything falling over. But it is a shortcut.

 

[efFeb]

Hi!
I mean this:

vs. this

 

[WinelandV]

efFeb,

IIRC, you can use the uniform distribution if your soil is cohesive (and not cohesionless). Hopefully one of the geos who hang out here can weigh in.

Please note that is a "v" (as in Violin) not a "y".

 

[JLNJ]

Sometimes the geotech will let you spike the allowable bearing pressure in the triangular distribution, which has a similar effect to using the rectangular, lower, "allowable" pressure in your diagram.

 

[human909]

Ok I understand the question now. It should be evident to most that the trapezoidal is more 'correct' distribution of loads as you have a rectangular stress block for axial summed with a triangular for axial.

However for bearing checks I can see how checking the pressure spike against the geotechnical allowable bearing pressure as a threshold isn't really the ideal capacity check. In that context I can see the sense of the rectangular average pressure distribution.

I too would like to see codes or literature that, advocate this approach.

 

[KootK]

I have been told by others at my office that I can use a rectangular distribution instead, with the average bearing pressure, instead of the maximum bearing pressure, used for bearing pressure checks.

1) Yeah, I do that most of the time as does most everybody in Canada in my experience.

2) The method is thought to be applicable where settlement is your governing mode of geotechnical "failure". Settlement is usually a function of the average bearing stress and not the peak. If one is truly concerned about a true, soil shear failure mechanism, that would be a bit different.

3) For things like shear wall rafts with overturning and property line footing at basement walls, it can be tough to be competitive using the triangular distribution.

 

[phamENG]

KootK - have any articles or anything like that on its use? Your point number 2 nullifies my idea of it being somehow related to the use of limit states, so I'm curious...

 

[WinelandV]

See sheet 5 and the top of sheet 6 in the attached.

Please note that is a "v" (as in Violin) not a "y".

 

[phamENG]

Thanks, WinelandV. That's quite useful. And a whole lot easier than the triangular method.

 

[skeletron]

I've only ever applied the rectangular distribution when checking bearing under short-term seismic loads. In this condition I use the ULS bearing pressure provided by the engineer and check versus factored loading. Otherwise, for general bearing checks I've always used triangular/trapezoidal pressure using SLS bearing pressure and service loads.

 

[KootK]

KootK - have any articles or anything like that on its use?

The only place that I recall seeing it in print is in the design standards manual of a Vancouver based firm that I used to work for. I have that but am reluctant to post it wrt to copyright etc. And it's really no more official than just hearing it from any other engineer.

Your point number 2 nullifies my idea of it being somehow related to the use of limit states, so I'm curious...

Nope, your intuition is solid. In many cases, such as the one that skeleton described, it is a ULS technique used for overturning evaluation etc. The most common, non-ULS situation where I use this is shown below: property line footing under deep basement wall. It's impossible to keep the load within the kern of the footing and improbably that one really knows what any of the soil stresses look like with meaningful accuracy. So rectangles.

This is something that I do frequently but still only for special situations such as that shown below and ULS overturning. If I'm just doing an eccentric, isolated footing under long term loading then it's back to the triangles.

 

[canwesteng]

I'm not following how it makes a difference for overturning - isn't the resultant at the same place? For the case above it might make sense, but I think the real issue is the misunderstanding of geotechs of ULS capacity. I try and get a true ULS capacity from geotechs (i.e. not related to settlement, but pushing a chunk of soil up and out away from the footing) and use the elastic approach. Seems like most geotechs took the ASD bearing pressure, multiply by 3*0.4 and send you on your way, short almost a third of the strength you had before.

Wineland - what book does that come from? Much appreciated

 

[HTURKAK]

- This is an old book (Foundation Design ;By Wayne C. Teng, 1962) .



- Some Northern Europian countries were using that method. But now , they are using EC's. The basic assumption is the soil is plastic material. This approach gives some advantages for ULS Earthquake loading . Pls look for the worked example at NEHRP FOUNDATION DESIGN P-752_Unit5






Use it up, wear it out;
Make it do, or do without.

NEW ENGLAND MAXIM

 

[WinelandV]

canwesteng,

I believe it is from "Foundation Design" by Wayne C. Teng. I do not know which edition, as I grabbed that file from a poster here at eng-tips well over 10 years ago. It's proven to be very handy, which is why it has survived so many computer transitions.

Note: I possibly grabbed it from this thread thread507-4647 , but all files from very old threads are no longer there, so I'm not sure if DaveAtkins actually posted it in this one, or if he just referenced it there.

Edit - looks like HTURKAK beat me to the reference - well done, sir.

 

[HTURKAK]

Not at all.. By chance i have the copy of this old book. This was one of the reference books that i used in past. The other one was Foundations of Structures (CLARENCE W. DUNHAM, 1962)

Kind regards.





Use it up, wear it out;
Make it do, or do without.

NEW ENGLAND MAXIM

 

[Lomarandil]

Same as skeletron -- for ULS conditions, eccentric rectangular bearing pressures. For SLS, triangular pressures.

I'm pretty sure that approach is documented in AASHTO LRFD, but don't have time to hunt for the reference at the moment.

 

[JoshPlumSE]

Who knows what this looks like

I love that statement and drawing and I 100% agree with it!

To me, what's most important is that you satisfy statics. As long a the bearing you calculate sums up to the applied loads and moments, you've probably got a reasonable solution. Soil response is something of a SWAG (strategic wild a$$ guess) anyway. That's the reason why we include larger factors of safety on it.

Some general thoughts:
1) The triangular bearing pressure assumes a rigid foundation. Just like a triangular pressure under a base plate assumes a rigid base plate. It should be conservative when comparing it to an ALLOWABLE soil bearing pressure.
2) The rectangular bearing pressure is certainly a better way of looking at "ultimate bearing pressure" of the soil. Though, I personally still think it's ok to use it when comparing it to allowable pressures. Probably a little less conservative than the triangular pressure profile. But, more conservative for overturning checks.

 

[canwesteng]

Here is the DOT reference for anyone interested

https://www.fhwa.dot.gov/engineering/geotech/pubs/nhi06089.pdf

Figure 8-17 on p 8-26