Net soil pressure on underside of spread footing - question

[CopperGalaxy]

Hello

I was hoping somebody could point me in the right direction here. I have searched online for days and cannot get this straight in my head at all.

I'm designing a gravity based octagonal wind turbine foundation. I have isolated a 1m strip of the foundation to design the reinforcement for and I am idealising this strip as a cantilever beam fixed at one end to the foundation centre with the length set to the radius of the whole foundation.

I had applied the dead loads to ascertain the section forces (bending and shear) in the top of the beam. I understand that bottom reinforcment should also be provided to protect against tension in the underside however i'm finding it difficult to visualise how this would occur. Obviously upon tilting of the structure the bottom side will press down into the soil at one side causing upwards deflection due to the soil reaction pressure.

But how do I quantify this reaction? I found an equation online stating Mbottom = fL^2/2 + Mtop where f is the soil pressure. However I cannot understand how a net pressure upwards will occur. Surely the vertical upwards soil pressure is equal to the vertical downwards dead load and there would be no net upwards pressure.

If anyone could shed light on this I would be forever grateful and I apologise if it is just my confusion getting in the way of a simple exercise.

 

[JAE]

Provide a sketch please.

 

[Backcheckrage]

what is "upwards pressure" on soils?
is this a matt foundation?
are you designing the slab for lateral on the superstructure?
i agree with JAE you should provide a sketch to illustrate your situation.
Sounds like you need to do some modeling on the computer - conc foundation on compression-only soil springs then impose the vertical loads on your conc foundation. pull your slab moment and shear reactions from the computer analysis then use that as a starting point for proportioning reinforcement.

but if your loads are large you would need to go with a piled solution instead of just a simple matt. in which case you should be designing a pile cap on piles.

 

[Mccoy]

Surely the vertical upwards soil pressure is equal to the vertical downwards dead load and there would be no net upwards pressure.

Right now, the only example of an upward soil pressure which comes to my mind is that of swelling clays. Or even seismic waves for brief times.
In lack of the above specific conditions it sounds like the upward pressure is used as a synonim for upward reaction. Which would be a function of the spring stiffness as cited.
Also, as far as I know in suitable soils like stiff gravel and rocks of course a shallow foundation (rigid block), is standard procedure.Of course evaluation of dynamic soil properties is a must.

 

[CopperGalaxy]

Hello all

Thank you for your replies. Sorry i'm talking jibberish i'm still very inexperienced. I was analysing the problem by isolating one cantilever as part of the foundation and trying to obtain the bending moment in the top and bottom as the sketch suggests. I have the upper bending moment by analysing the effect of the dead loads which could cause a downwards deflection along the cantilever.

I'm now trying to get the bending moment values occuring on the underside by analysing the situation where it will deflect upwards (sketch 1). I have been advised that this will occur due to a positive net soil pressure acting vertically upwards causing an upwards deflection towards the end of the cantilever. I'm just finding it difficult to quantify the loads to apply to my finite element model.

In sketch 2 you can see that the actions upon the superstructure would cause deflections in the slab depending on the direction of loading (I've to design the foundation symmetrically due to the varying wind direction). I've been advised to think of the problem as a cantilever gravity wall where i'm trying to find the top and bottom bending moments in the toe and heel if that is of any use.

 

[RFreund]

I might be miss understanding the problem (especially since you mention FE Model) but...
The load applied to the underside of the footing (slab) is the bearing pressure which is usually trapezoidal or possibly uniform if you use Meyerhof approach. You may subtract out the weight on top of the footing (slab) but many times this is ignored or reduced by some amount. Then analyze the footing as cantilevered from the foundation wall.
You should be able to find some example in any foundations text book.

EIT

http://www.HowToEngineer.com

 

[FixedEarth]

With Octogonal foundations, do a methodical approach as follows:

1- Find your eccentrictity using extreme loadings of M/P, M is the moment & P is the axial load. (e is measured from the center of the Octagon.
2- Using DIN or other publications, get your effective ellipsoidal area and transform it into a rectangular area.
3- Check for overturning F.S.
4- Check for Bouyancy uplift if you have a G.W.T.
5- Calculate your rotational stiffnes using shear wave velocity & you must meet manufacturuer guidelines.
6 - Do your undrained and drained bearing capacity analysis using Hansen or Meyerhoff, etc.
7- Compute your total and differnetial settlement analysis.
8- Find your F.S. against sliding using passive earth resistance.

And then we come to your current step - determine your applied shears and moments using cantilever beams.(This is where the foundation designer starts & the geotechnical analysis has been completed). Think of it this way- you have a cantilever retaining wall with a toe and a heel. If you cut the heel off from the footing, it will be acted upon by a vertical backfill weight from the top. Since this cantilever beam is loaded from the top, you have tension at the top and compression in the bottom. If you were to cut off the toe, you will have upward soil pressure and now thing are reversed- tension in the bottom-Just like a footing. To see the graphics of cantilever beams explained very nicely, see chapter 12 of this link:

http://anc-tbquimby01.uaa.alaska.edu/courses/ce433/Quimby/ce43310.htm

The foundation designer can then check for one & two way shear, moment capacity and design the reinforcement. If your soils are compressible, loose, thin crust or you have a high G.W.T. condition, you may not get an octogonal shaped shallow foundation to work. Look at publications by DIN, Bowles & F.D.C. Henry's 1986 "Design and Construction of Engineering Foundations". Also Chapter 8 of "Theory & Practice of Foundation Engineering" 1968, by Goodman & Karol has an example on the analysis of octogonal shaped foundation. IBC 2009, Chapter 18 as you know has guidance on shear wave velocity values. Good luck.

 

[CopperGalaxy]

Hello all

Thank you so much for your thorough and informative help it has been extremely useful and I greatly appreciate it.