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cohesion/friction for footing uplift

cohesion/friction for footing uplift

cohesion/friction for footing uplift

(OP)
Hi all,

For those of you who work in high wind areas (Florida here) - have you ever relied on cohesion or friction for additional uplift resistance on footings - or perhaps some other factors that will help make foundation design more economical? I have never seen or heard of footings being ripped out of the ground due to strong winds ( except maybe overturning on fences or something similar) - failure seems to start with C&C, which once destroyed, relieves the foundation of much of the uplift forces. Thanks in advance!

RE: cohesion/friction for footing uplift

Isn't it a bit difficult to have frictional resistance when uplift is present? I typically allow partial uplift for wind conditions, but partial uplift is limited by the bearing pressure produced by the reduced footprint.

RE: cohesion/friction for footing uplift

(OP)
How would uplift negate friction?

RE: cohesion/friction for footing uplift

I've never relied on it because you typically wind up with a gap between the soil and the footings. You don't get much anyway.....it's not like a pile.

RE: cohesion/friction for footing uplift

I practice in Florida and I do not consider either for uplift resistance.
 

RE: cohesion/friction for footing uplift

From what I understand, you shouldn't use active earth pressure for any portion of the soil that may be disturbed from freeze/thaw shrinkage/swelling, etc, which means a portion of the top layer, typically determined by the geotechnical engineer (I usually see 10'). If you have soil above your footing, I believe you can use that dead weight but otherwise you are stuck with the dead weight of the footing plus a safety factor of 1.5.

RE: cohesion/friction for footing uplift

Here's some things to consider that our company uses to design buildings with cyclonic/hurricane loads. I know of at least a thousand or so have been built with this design criteria without any failures:
- Reduction in air density due to a cyclone/hurricane forming in a low pressure cell.
- Skin friction on piled footings. The skin friction increases with increasing soil cohesion.
- If the slab is integrated with the footings, as the slab lifts a vacuum will form under the slab, especially if the slab has an edge thickening which prevents air from accessing the gap between the slab and soil. This is the same as if you had a flat ruler under a sheet of paper and quickly tried to lift the paper like a momentary gust of wind. The longer the slab has been in place the stronger the vacuum due to air under the slab dispersing into the soil. This should be one of the larger contributing factors for residential construction.

RE: cohesion/friction for footing uplift

(OP)
so is there a way to quantify and design for this uplift from vacuum?

RE: cohesion/friction for footing uplift

nonplussed....using a vacuum as hold down? I would be very uneasy about that - probably due to my uncertainty about how to quantify it as well as the fear that somewhere, somehow, there is a pipe, hollowed out soil area, foundation drain, etc. that would negate any suction.

I always wondered, though, what that loud sucking sound was every time a large hurricane comes through the coastal areas.....or was that just the politicians?

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RE: cohesion/friction for footing uplift

I would never rely on a vacuum to hold down a foundation, they probably only weigh 20 lbs at the most 2thumbsup

RE: cohesion/friction for footing uplift

This has not been mentioned yet... with hurricanes comes, almost without exception, torrential rain. What (reliable) soil properties for friction or vacuum would apply for totally saturated soil? Don't do it. I'm in a hurricane region - coastal South Carolina.

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