Floodplain Buoyancy Force Question (Last one)

[meamin]

I've looked at other threads and I am still not sure I am understanding what all needs to happen with the slab to OK a building for the buoyance force from flood loads. I'll create a fictitious scenario here.

100 ft total square area (10' x 10' sides)
3 ft flood protection grade

Buoyance Force = 62.4pcf * 100 ft^2 * 3 ft fpg
Buoyancy Force = 18,720 lbs TOTAL

Now, I'll have a 100ft^2 wood roof, 10'+10'+10'+10' = 40' linear feet of 10' tall, 8" solid grouted wall, 40 linear feet of a 12" thick, 12" wide strip ftg.

Roof DL = 12psf * 100ft^2 = 1,200 lbs
Wall DL = 84psf * 40ft * 10ft = 33,600 lbs
Strip Ftg. DL = 150pcf * 1ft * 1ft * 40ft = 6,000 lbs

Building Weight (Slab not considered yet) = 40,800 lbs. > 18,720 lbs. buoyance force

Therefore, my building weight is greater than the buoyance force..Great news.

Now I just have to design my slab to span 10 ft for (62.4pcf*3ft) 187.2 psf minus the self wt. of the slab correct?

I should be able to reinforce an 8" thick slab for 187.2psf buoyance minus 100psf self wt. = 87.2psf
As long as I reinforce my slab to resist the shears and moments from an 87.2psf uplift load I am OK right?

It seems that the other idea is sizing the slab so that the DL overcomes the buoyance which would mean a 15" slab is required (187.5psf self wt.). I think this is incorrect since the net buoyancy is already overcome when you consider the footing, grouted walls, and roof load. Add in the 8" slab I am proposing and that is another 10,000 lbs of weight to overcome the 18,720 lbs. of buoyance.

Thoughts?

Thanks,

AM

 

[1503-44]

I think you need to design your slab for maximum dead and live loads and pressures as case 1. Size bottom and or top steel as necessary.

Case 2. Now look at the slab as a beam with distributed loads, one from minimum dead load forces and pressures (maybe flood happens before walls are up and roof is on???)) and the other from upward buoyant pressures. Draw up your shear and moment diagram based on those concurrent loads and place bottom (and maybe top steel) as necessary.

Case 3. Design for max dead, live and flood.

Select the maximum amount of steel required from each case for both bottom and top steel, to result in a combined case solution.

Your minimum dead load total force down should be >= 1.5 x total buoyant force up as an uplift safety factor.

 

[Settingsun]

What you've described assumes that the building is dry inside. Is that what you're required to consider for design?

Your footings will also have a buoyant force on them if they're outside the 10'*10'*3' box you used to calculate the buoyancy force. It doesn't affect the slab structural design but would affect your overall factor of safety against flotation.

For structural design, you need to check that either you've factored the loads correctly for limit state design, or calculated your allowable stresses correctly for unfactored loads. Concrete design is mostly limit state design nowadays. I'm not familiar with 'flood protection grade'; is this a maximum flood level (eg probable maximum flood)? If it's the maximum conceivable flood level it would probably be considered an ultimate load already (but check your code). If it's a lower flood level like 100-year recurrence, you may have to factor the load (eg 1.5*187.2 psf), and also factor down the beneficial slab weight (eg 0.9*100 psf).

 

[1503-44]

Thanks. I forgot the 0.90 factor for the reduction of dead loads.

 

[meamin]

1503-44 : yes of course sorry, I am not forgetting about live loads, was just focusing on the flood vs dead load aspect.

I agree with your thoughts on reinforcement. Ideally, putting reinforcement near the center takes care of both negative and positive moments.

"Your minimum dead load total force down should be >= 1.5 x total buoyant force up as an uplift safety factor." Here you mean the overall building weight including slab correct?

steveh49: yes from the reading i've done on floodproof design, you consider a buoyancy load (hyrdrostatic, hydrodynamic on walls) when doing a dry floodproofing design. The flood protection grade is the base flood elevation + 1' or 2' of freeboard (safety factor essentially). The height of flood used in design is this fpg height so that it can be certified by insurance.

But yes you are definitely correct, need to factor my loads for designing the slab.

Thanks guys for the input

 

[1503-44]

Correct.

Placement of one layer of steel in the center minimizes rebar work, but requires more steel since the shorter moment arm means that the steel's tension stress will be higher. Pick the cost effective solution.

And sorry, but today I think I remember the uplift safety factor of 1.5 may be too high. I might be confusing that with the overturning SF. Please check that for possibly using 1.2 or 1.3 in lieu of that 1.5 SF. Or maybe better advice is forthcoming.