Negative pressure at footing-soil interface

[Djidji444]

Hello,
Does anyone know if there is a limit according to the Eurocode in the sense of: what percentage of the foundation can be under 'tension' (i know that isn't real tension), but I have extremely eccentrically loaded foundation footing and I am interested in whether according to the Eurocode there is any limitation on how much of the foundation zone can be under negative pressure?
Thanks in advance for the answers.

 

[Celt83]

There is no such thing as negative pressure. The footing lifts off of the soil in that zone as it rotates into the soil mass.

Once your equivalent eccentricity exits the kern then you begin to fall outside of the normal geotechnical parameters so it's worth discussing the condition further with the projects geotechnical engineer.

Conditions like this can also be red flags that your system is becoming unstable, so make sure you are doing an overturning stability check.

 

[SE2607]

What controls this, at least in U.S. codes, is the 1.5 factor of safety against overturning.

 

[CDLD]

Djid are you asking if there is a minimum % of the footing to be in bearing?
I'm seen some rules of thumb floating around these forums, but I don't believe anything code mandated.
As others pointed out, overturning stability and bearing pressure need to be checked.

 

[JLNJ]

You cannot use P/A +- My/I for footings with parts apparently in "tension" and then use a "some tension is ok" rule of thumb.

 

[HTURKAK]

There is no any defined limit at Eurocode . However , tension cannot develop since soil tensile strength is assumed zero. When you write footing with large eccentricity and search the web , one of the outcomes;

EDIT;
having worked with EC-7 in past , AFAIK , it is not explicitly stated any specific limit on the degree of eccentricity .But my assumption is that , the resultant force with large eccentricities should be the ULS force. My opinion is , ‘middle third model’ or the resultant force in the kern area should be justified for SLS loading.
If the eccentric load lies outside the middle-third of the foundation, it is likely that the deformations of the frame will be underestimated.
..


He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

 

[JAE]

We've taken to doing this:

1. Under the ASD load cases where D is used we try to avoid ANY net uplift on the footing
2. Under the ASD load cases where 0.6D is used we try to only allow about 1/3 of the footing to have net zero pressure.

 

[SE2607]

We've taken to doing this:

1. Under the ASD load cases where D is used we try to avoid ANY net uplift on the footing
2. Under the ASD load cases where 0.6D is used we try to only allow about 1/3 of the footing to have net zero pressure.

That's the practice in your office, not based on a code criterion, right?

 

[JAE]

Correct.

 

[JAE]

Although I don't believe the IBC has anything in it that suggests you can't have any slight uplift on the tail of a shallow foundation. Correct me if I'm wrong.

 

[jhnblgr]

Wouldn’t uplift in the footing essentially mean the footing in that area serves no purpose? What gain is there from maintaining that section of footing. Wouldn’t the footing experience rotation causing failure in stability at this point and not meet code for overturning?

 

[Lomarandil]

Jhnblgr -- if the lateral load is bi-directional, the footing should by be symmetric. The heel lifting off in one direction is your toe in the other.

(Also in some cases like a retaining wall, the heel is longer to extend under more overburden soil)

 

[Djidji444]

Unfortunately, the communication with the geotechnical engineer is reduced to getting a geotechnical study of the area with the permissible soil bearing capacity from him, and it is up to me to design the construction and take everything into account.
The structure I'm designing (for the first time) is a very long and tall windbreak wall made of 40% porous panels supported by steel columns. Like, for example in the picture:

The porosity is taken into account when the wind force was calculated according to the Eurocode, but still there is a significant wind force that acts on the structure, and thus the moment on the foundation. And okay, I know I need to control the overturning and sliding stability of the foundation, and also that the pressure on the soil underneath the foundation does not exceed bearing capacity of the soil. But unfortunately, when getting the job from client, the construction is quite underestimated and smaller quantities of concrete were adopted and it is expected of me not to exceed the adopted quantities of concrete/foundation. But of course, there is tension under the foundation due to significant moment, which I know is not a real tension, there is no tension of the soil (it is zero pressure), but a negative pressure occurs on a 1/3-1/4 of the foundation surface, theoreticaly speaking, when the load combination is 1.0*dead load+1.5*wind. So my question was is there any limitation on how much of the foundation zone can be under 'negative pressure' in Eurocode. Because I saw that many people mention that in their country/company, based on national annexes or company rules, they limit, for example, 10% of the foundation to be under tension.

According to your answers, I see that no one knows any limitation in the Eurocode, therefore, I don't think it exists.
Does that mean that if I satisfy the following taska:
1. overturning stability
2. sliding stability
3. that the pressure stress (from the ULS combination of loads), which is recalculated as qmax according to the picture below, should be less than the maximum bearing capacity of the soil according to the geotechnical study.

If I satisfy these three tasks, does that mean that the foundation structure is okay even though, for example, 1/3-1/4 of it is under negative pressure (zero pressure) when the load combination is dead load + wind ?

 

[HTURKAK]

Apparently, the use of bigger foundation is not an option for you ..
I will suggest you , the use of helical pile /pier so that , the tension can be resisted . Write ( helical pile , pier ) and search the web .. One of the outcomes ,

https://www.piertech.com/products/helical-piers.html

....

He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

 

[Djidji444]

Unfortunately, helical piles would also increase the price of everything (I have about 250 foundations), which I am again forbidden to exceed, so now my question boils down to that if the previously mentioned 3 tasks are fulfilled, is it allowed to have negative pressure under the 1/3-1/4 of foundation?

 

[Smoulder]

Don't think eurocode has a limit. But I have hardly used the foundation section. I only check this using serviceability load which is similar to past practice that worked for a long time. I just check overturning and bearing for ultimate loads.

 

[HTURKAK]

- Negative pressure cannot develop under the ftg but you may discuss zero pressure . The resultant force location 1/3-1/4 of ftg could be OK . But in your case , the ULS loading is the wind and your calculations could not be exact (approx. wind speed,approx. perforated screen resistance to wind ...)

-I will suggest you to provide soil data , frost depth and foot print of proposed structure to get better responds.

...


He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

 

[jhnblgr]

Have you investigated drilled shaft foundations? I think that is your only option to create a cost effective foundation.

 

[JLNJ]

Depending on your loading, the mass of the footing can play a significant role in reducing your eccentricity. Make it as thick and wide as you can.

I'm working on a one-off footing right now where making the footing slightly wider (12'-6" square rather than 12' square) pulled my resultant back inside the kern and significantly lowered my peak pressure.