Tension stresses at footing-soil interface 1

[Serhiy]

Good day,

I'm designing a spread footing foundation for pre-eng steel building and some lateral load cases drive the dimensions of my pads up because I'm trying to avoid negative stresses at footing-soil interface. I do it only because most of the related literature tells so. One thing I'm not sure about is the rationale behind this requirement. Yes, I understand that negative stresses can't be resisted by soil and that's why we try to avoid them. On the other hand, what's the big deal if I neglect these negative stresses and check my positive ones only against my Qallowable. I guess one reason could be that in case of negative stresses there could be potential uplift of the footing portion and that's what needs to be avoided. Another sub-question is whether this is a code requirement or not to avoid negative stresses at footing-soil interface. I work in Canada and the only place I could think of checking is the Canadian Foundation Engineering Manual but my office doesn't have one. Thanks

 

[oldestguy]

If your negative stress is a compressive stress, why is the title of the post Tension stress? For protection of the steel, follow a suitable building code for cover between steel and the subgrade.http Or is your negative meaning zero compressive stress? Still no problem.

 

[Serhiy]

My negative meaning zero compression stress as per sketch below

 

[aeoliantexan]

The bearing pressure distribution reduces the effective size of the footing. The usual approach is to reduce the width by two times the eccentricity: B' = B-2e. The reduced size both increases the average contact pressure and reduces the ultimate bearing capacity of the footing (if your soil has friction).

 

[fattdad]

[ul]
[li]weight of soil above the footing.[/li]
[li]Passive resistance of the buried portion of the footing.[/li]
[li]I'm of the impression that the equivalent, "2/3rd" footing size only applies if you control the bearing to limit the uplift.[/li]
[li]Critical stresses on pre-engineered metal buildings is wind load with the doors open. They'll just fly away if you are not careful.[/li]
[/ul]

I'd lower the footings and consider the entire free-body diagram.

f-d

ípapß gordo ainÆt no madre flaca!

 

[Serhiy]

Thanks. As per aeoliantexan, I will try to reduce the width of the footing (I assume it would be term d in the equation S=bd^2/6 - please advise if that's not the case) and see if it helps. Lowering the footings is not an option - client wants them pretty much on the surface - I even have to rely on weight of main floor slab to provide uplift resistance.

My question was slightly different: many sources where eccentric footing load response conditions are explained they mention that footings need to be designed to maintain the eccentricity of less than L/6. This requirement ensures that all area under the footing is in compression (P/Afooting - M/S > 0). I'm wondering if this is a requirement that MUST be followed because in my case this requirement drives the footing dimensions up. My understanding is that in case I do have negative stresses (P/Afooting - M/S < 0), I can just ignore them and be ok with them as long as my P/Afooting + M/S < Qallowable (maximum compressive stresses < Qallowable). I would also ensure that there is no overturning happening. Would this be an acceptable solution?

 

[PEinc]

Make the footing wider so that you reduce the eccentricity. That will reduce or eliminate the negative bearing pressure.

http://www.PeirceEngineering.com

 

[Serhiy]

That's the problem. For example, for most of my load cases footing size 8'x8' works fine but for some of the load cases with large lateral force I get negative stresses on part of the footing and in order for me to eliminate them I have to increase footing width to 11'. My question was is whether it is a requirement or not to eliminate occurrence of negative stresses completely or I am allowed to have them given that my maximum positive stresses are smaller than my Qallowable.

 

[TEDstruc]

I THINK THIS CONDITION IS ACCEPTABLE AS LONG AS YOU DON'T END UP WITH A MAX BEARING PRESSURE ABOVE THE ALLOWABLE

 

[UcfSE]

You can't just ignore the negative pressure because that indicates you used the wrong equation. If your minimum pressure calculates as negative, then you are not predicting the correct positive pressure either.

Eccentricity less than B/6 or L/6 (which would be different by the way for a footing not rectangular shaped) means all the footing bearing is in compression as you have stated. Yes you can exceed B/6 or L/6 but you need a different equation; I don't prefer this personally because to have the footing lift off the soil means the footing has to tilt quite a bit more than I want. You cannot exceed B/2 or L/2 because the resultant reaction would no longer be on the footing.

 

[Serhiy]

UcfSE, when you say I need to use different equation, do you mean I need to use different section modulus to calculate my stresses below the footing? If yes, would this new section modulus be the section modulus of the portion of the footing which is in compression? If yes, would my neutral axis be still in the middle of my footing (assuming the footing is of square shape?

 

[UcfSE]

You can't use a section modulus approach because you can't develop negative stresses.

The neutral axis is only in the middle of a rectangle under pure bending. Once you introduce axial load, which your footing presumably has, the neutral axis is not in the middle.

 

[Serhiy]

What formulas or what approach you would then normally use in this situation?

 

[msquared48]

Look at these diagrams and formulas to get your pressures.

Mike McCann, PE, SE (WA)

 

[emmgjld]

For example, for most of my load cases footing size 8'x8' works fine but for some of the load cases with large lateral force I get negative stresses on part of the footing and in order for me to eliminate them I have to increase footing width to 11'.
What are the consequences of changing the pad geometry. Instead of 11'x 11' go to 9'x 13'-5"?

 

[Okiryu]

From the geotechnical side, this should be checked, because you want to avoid tension because soils cannot resist tension. From the structural side, as you mentioned, you want to avoid uplift.

Can you fill with granular soils on top of the footings to get more weight? or make the footings thicker? Can your Geotech give you higher bearing capacities (i.e. can he/she decrease the FOS) since the eccentricities are coming from transient loading?