Resistance to uplift by Rectangular Grade Beam 3

[LJB7131]

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?

 

[bootlegend]

I would size the reinforcement/section depth to handle the load. Depending on the location, you never know when the soil could be removed for "some" reason.

 

[oldestguy]

I'd not depend on any frictional resistance of soil against concrete unless it is placed in an unformed, undisturbed trench. Even then it is questionable as to how good that is.

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.

 

[JoshPlumSE]

I would not count on any friction resistance for this type of grade beam. The uplift resistance, in my opion, should come from the soil overburden on top of the grade beam and the self weight of the 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.

 

[abusementpark]

LJB7131,

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.

 

[hokie66]

3500 lb/ft uplift is too large to realistically resist with just concrete mass, and relying on side wall friction near the surface is not wise. JoshPlum is correct...you need some type of ground anchors or tension piles to resist this loading.

 

[miecz]

If your planning to use the soil wedge to help resist uplift, a slope of 1 horz to 2 vert may not be safe. NAVFAC DM 7.2 puts this angle at 20 degrees for sandy soils. See thread 507-46927 for a discussion on this.

Also, your safety factor should be 1.67 (1/0.6).

 

[LJB7131]

Thanks to all of you for your helpful ideas.

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.

 

[BigH]

Piles might be out but, as Hokie666 put forth, I would think that installing a number of soil anchors would be preferrable and less expensive than "massive foundations." There's been a number of papers in the ASCE GT Journal (sorry, I still use the "old" name but not the oldest (SM)) on uplift of anchors in sand.

 

[JAE]

Don't forget to use 0.6D +/- W for the uplift check.

Or whatever load combination is required by the applicable code that creates a level of safety against uplift.

 

[abusementpark]

JAE,

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.

 

[JAE]

As has been suggested in other threads, the 0.6D + W combination provides you with an in-built general safety factor against uplift (similar to previous uses of 1.5).

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.