Resistance to uplift by Rectangular Grade Beam
Resistance to uplift by Rectangular Grade Beam
(OP)
I am faced with designing a rectangular grade beam, in fine to medium sand, to resist an uplift of 3500 #/ft (includes 1.5 safety factor). The soil is natural and has been found to have a bearing value of 2500 psf. How do I properly calculate the frictinal resistance of the concrete against the sand.
I could assume that I must shear 2 wedges of earth, one each side, having a 1 hor. to 2 vert. slope and use the weight of that earth, plus the weight of the concrete, as the resistance to the uplift. Is there a better method?
I could assume that I must shear 2 wedges of earth, one each side, having a 1 hor. to 2 vert. slope and use the weight of that earth, plus the weight of the concrete, as the resistance to the uplift. Is there a better method?





RE: Resistance to uplift by Rectangular Grade Beam
RE: Resistance to uplift by Rectangular Grade Beam
Instead, use a footing on the bottom and good control of the compaction. That ratio of 1 to 2 may not hold for your situation, depending on the backfill. Look at this as the footing is an anchor and your wall is just the structure attached for the uplift (with some added weight).
Don't forget to allow for saturation of that backfill, in which case it is much lighter (buoyed up by water).
I'd involve a geotech firm for these details.
Your mention of a bearing value of the soil. Well, that has nothing to do with your uplift situation.
RE: Resistance to uplift by Rectangular Grade Beam
If you need more resistance, then I would probably go with drilled piers or piles or such and get my uplift resitance for those from the Geotech.
RE: Resistance to uplift by Rectangular Grade Beam
Is there a geotech report? The firms we work with usually allow us to consider 500 psf of adhesion (frictional resistance) on the sides of grade beams or footings. This makes a huge difference, but I would want the geotechnical engineer to bless it before I relied to heavily on it.
Even when the geotech gives us the adhesion, I only consider it on the inside face of exterior grade beam, since I dont what could happen with the side grading along the exterior side. Maybe I will only consider a small portion of the exterior face of the grade beam, if I really need it. I also insist upon the grade beams being earth formed. I don't like relying on adhesion for plywood formed grade beams.
RE: Resistance to uplift by Rectangular Grade Beam
RE: Resistance to uplift by Rectangular Grade Beam
Also, your safety factor should be 1.67 (1/0.6).
RE: Resistance to uplift by Rectangular Grade Beam
The uplift is due to 120 mph 3 sec gust winds. It is a steel mill building composed of rigid frames and the Manuf.'s Engr. has calculated 46.3 k uplift for one reaction and 38.6 k for the other, based on this wind. Reactions due to loads and Seismic are very much lower.
Unless you all really disagree, I think there can be some wiggle room in the design criteria. If I have to, I will make the foundations massive. Piles are out.
The grade beam concrete will be poured into an original fine/ med sand earth trench....no plywood. I can go to a 20 deg. soil interface. I could tie the slab in such a way as to broaden the amount of soil shear on the inside face.
All help greatly appreciated.
RE: Resistance to uplift by Rectangular Grade Beam
RE: Resistance to uplift by Rectangular Grade Beam
Or whatever load combination is required by the applicable code that creates a level of safety against uplift.
RE: Resistance to uplift by Rectangular Grade Beam
Does the 0.6D does have to be applied to the foundation elements?
People have told me it is only to be applied to building framing dead weight, but I have never seen this written down in an authorative document.
We always consider the buoyant weight of the foundation. It seems absurd to take the 0.6 reducation on top of the reduction in weight you get from taking the buoyant weight.
RE: Resistance to uplift by Rectangular Grade Beam
If you are depending on the weight of your footing, not using the 0.6 factor on the footing weight doesn't make sense to me because you are simply reducing your level of safety against uplift.
The 0.6D isn't necessarily trying to suggest a safety against over-estimating dead load but I think it just provides an overall level of safety.