hi all please help me...

[mvv]

Dear All

Is it correct if i take the advantage of soil weight while sizing the footing subjected to upward loads and moments? Please correct me and provide me the source if possible.


best regards
MVV

 

[csd72]

Yes, the weight of the soil above the footing can be used as part of your resisting weight.

dont forget that these loads need to be factored down by 0.6 to 0.9 depending on the code.

 

[dik]

and this can be increased (check with a geotekkie) depending on the soil type and moisture conditions.

Dik

 

[e104909]

Mvv,

Taking advantage of the soil may reduce the possiblity of stability failures but Adding weight to your footing will contribute to the increase of reinforcement to the footing itself so dont forget to check ultimate

You may also consider passive pressures.







Regards,
E104909

 

[hokie66]

e104909,
You would have to explain that one to me. Adding mass on top of a footing doesn't increase the reinforcement requirement. And passive pressure doesn't help with resisting uplift, as passive pressure results in a lateral force.

 

[msquared48]

A little clarification here:

If the footing is picking up soil loads to resist uplift, extra reinforcing, beyoud the normal bottom reinforcing, will be adviseable if not outright required in the top to develop the soil load. Otherwise, you're kidding yourself.

Other than that, I agree.

Mike McCann
MMC Engineering

 

[e104909]

Hokie,

I did not say checking of uplift. I just genereralized it. Overturning would be the concern. Adding mass to the top of footing means adding pressure to your footing. Slip is possible too. Now, structurally your footing should be checked..and geotechnical check to consider layers of soil is also needed for the realistic check of slip failure.

Now reinforcement is a differrent thing. As you go deeper the height of soil goes deeper and your reinfircement as msquared says will increase. Now depends on the level of water table that produces the uplift. Compare 9.81 with 18. Soil is still higher.

Everything will depend on the actual condition.







Regards,
E104909

 

[hokie66]

No doubt that uplift and overturning will impact the reinforcement. But simply adding soil to the top of a footing will not. The gravity load goes straight through the footing to the soil below, with no net affect on the concrete footing's bending.

 

[msquared48]

I agree Hokie.

Mike McCann
MMC Engineering

 

[dik]

Unless there are some unusual circumstances, I normally don't include the weight of soil over the footing to increase the bearing capacity, or the weight of the concrete.

The soil 'has always been there' and the strength measured already has it in place and the weight of the concrete is nearly equal to the weight of the soil displaced (unless foundation design is critical enough that this has to be considered and for caissons I often include the weight of the concrete due to the concentrated load).

Dik

 

[hokie66]

Fair enough, dik, but the OP wants to use the soil mass to resist uplift.

 

[ToadJones]

Hokie -
Is there a chance that the confusion here is whether the designer uses the net soil pressure or the gross soil pressure in designing the reinforcing?

 

[hokie66]

I'm not confused.

 

[ToadJones]

I wasn't saying you were

 

[e104909]

Toad jones,

Interesting!



Regards,
E104909

 

[ToadJones]

?

 

[dik]

Hokie... and I noted that the effect of the weight of soil can be increased if OK by geotekkie... by including the effect of shear on the failure plane.

Dik

 

[NS4U]

I typically do not consider the soil above the footing for uplift resistance. My reasoning is that for all I know, 10 years down the road they may decide to permanently excavate out 1, 2, or more ft of soil for some reason. I do use the concrete weight in calcing uplift resistance since that can't simply be removed.

 

[NS4U]

Also, interesting discussion on if additional reinforcement is required. Without running numbers, it seems to me that you would need to add top reinforcement to the footing for the uplift condition becuase of the non-uniform pressure distribution.

At one end of the footing you will have very low bearing pressures that will (likely) not be enough to counter the combined wieght of the concrete and soil above the footing in that area. You would therefore have negative bending at the the "heel" (for lack of a better term) and postive bending at the "toe" in the uplift condition.

For a concentric gravity load above the footing, I agree that you do not need to increase the reinforcement regardless of if there is 1 ft or 10 ft of soil above the ftg. becuase the soil load gets passed straight down through the footing.