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concentrated uplift on a perimeter foundation

concentrated uplift on a perimeter foundation

(OP)
For instance, say you have uplift on structure with 30' frame centers. Simplicity may want to just call out a constant dimensioned and reinforced perimeter footing. To check uplift control, could you not just use Enercalc concrete beam designer forinstance, calculate how much footing length you need to counter the uplift, and then apply that as an PLF on that span of beam. [---> done? (for uplift at least) missing anything here? please discuss.

RE: concentrated uplift on a perimeter foundation

This is how I interpret what you're saying:

Area of concern is the footing beneath a single column with an uplift load at the column to footing connection. Let's say your uplift load is W=-5kips. Load combination is 0.9D+0.9W, so to arrive at zero uplift between the footing and the soil, you would need 0.9D to equal 5kips. (5k/0.9)/0.15kcf concrete = 37 cubic feet of concrete. If your continuous footing is 3ft wide and 1ft thick, you'd need 12.33 feet of footing to resist the uplift. So, can you just design this as a double cantilever beam, support in the center with a UDL = foundation self weight, and call it good?

Sort of. I've used a similar approach to small PEMBs to justify eccentric footings where they're cast monolithically with the slab and an edge turn down. You have to be careful, though. There will be deflection in your "beam." A lot of simple calculation software doesn't check that in concrete beams - they just drive you to span/depth ratios. Make sure that movement is acceptable.

RE: concentrated uplift on a perimeter foundation

(OP)
PhamENG.. thanks for input

Ok, deflection is a concern....would a beam deflection (even with simple software) design deflect enough to create a problem as a foundation but NOT inherently as a beam as well? maybe so eh? always check.. good point

I wasnt thinking about the beam as a double cantilever but a simple span?... at some point isnt the adjacent slab beyond your required weight span acting as a pinned condition, or even fixed?

with slab contributory included lets just say about 750 lb/lf concrete spread footing weight. free cantilvered ends. deflection seems way ok. enercalc output attached.

RE: concentrated uplift on a perimeter foundation

I bring up the deflection because the prevailing assumption is that deflection is zero at the foundation - I've never met anyone who assumes any portion of their structure has actually lifted off of the ground. Because you're relying so much on the flexural behavior of the footing in this application, it becomes a real possibility. I think your question may stem from our differing approaches (double cantilever vs. simply supported). With my double cantilever, the deflection (measured at the ends of the cantilever) is actually the vertical displacement of the base of the column. Usually, when looking at beam deflection, the deflection is occuring away from the column and, as a result, has limited effect on the rest of the structure. Think of this as a transfer girder - the deflection will impose a joint displacement on upper floors.

Now the approach I suggested (double cantilever) is actually pretty conservative as long as your required foundation length doesn't overlap with the next column. That's because it ignores everything beyond the concrete absolutely required to bring your structure into equilibrium with bearing pressure = zero. As long as you have continuity across that plane, you'll be picking up that concrete as well. If you really wanted to sharpen your pencil (though I don't know why you would), you'd have to look at semi-infinite beam theory.

The simply supported model only works if you have uplift on two adjacent columns. But even then the simplified model starts to break down because, again, it's based on that Goldilocks point where Sum(F)=0 AND bearing pressure = 0. It's easy enough to use that simplification with a single column, but to hit it with two columns will either be impossible or force some strange detailing.

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