Sandwich Slab/ Split Slab with large uplift

[Galambos]

I have a new sandwich slab (concrete topping slab over waterproofing/insulation over elevated concrete slab) on the roof of an 8 story building on the coast in a 120 mph windzone.

I get some pretty large uplift pressures, and i was wondering if anyone has any experience with anchoring these down. I can provide vertical hairpin bars at a regular spacing, but this would penetrate the membrane. Is there a simple solution that I am overlooking, or is that it?

thanks,
Ted

 

[hokie66]

What are you worried about blowing away? The topping slab?

 

[JAE]

I'd be very reluctant to penetrate the waterproofing layer with a bunch of ties. Seems like a wet problem waiting to happen.

I would first try to simply add the dead weight as required to provide the necessary hold-down capacity.

 

[Galambos]

yes, i am concerned about the topping slab. Yes, it is monolithic, but these are LARGE uplift forces on the order of >100 psf.

 

[hokie66]

I suppose you could get 100 psf, or 5 kPa, at the edges of a roof. But part of that is the internal pressure, which is resisted by the structure. The topping only has to resist suction. I would be surprised if the suction is greater than 50 psf, or 2.5 kPa, so about a 4" (100mm) topping slab. My comment is based on experience, not on how your code reads.

 

[JedClampett]

What's your EWA? Can you increase it?

 

[JAE]

Galambos - I would agree with hokie66 that you could ignore internal pressure for the topping. That would help a bit (about 8 psf).

I plugged in an imaginary 100' x 100' x 107 ft. tall structure using ASCE 7-05, with V = 120 mph, Exposure D, I = 1.15, G = 0.85, effective area = 50 s.f.
and I got the following uplift values on the roof (which does not include internal pressure):

Zone 1 - 50 psf (interior areas)
Zone 2 - 70 psf (edge areas)
Zone 3 - 86 psf (corner areas)

The 86 psf equates to a 11.5" thick slab (based on 0.6D+W) so that is pretty thick. Without the internal pressure perhaps a bit thinner.
For 100 s.f. effective area I got Zone 3 at 59 psf. That would be a 8" slab. Again - at the corners only.

If you have a 3 ft. or greater parapet you can ignore zone 3 and replace with 2. That would get you down to a 9 1/2" slab (at 50 sf. area) or 8" slab at 100 sf.

The commentary for wind - chapter C6 - doesn't specifically address topping slabs but the idea is that you determine an effective area that would be applied over a contiguous slab.

The only way to really check this would be to actually model the whole slab with the various zone pressures applied and see what effect adjacent slab connectivity has on helping to hold it down.

 

[RFreund]

I'm trying to wrap my head around the 'physics' of this. Since there is no such thing as 'suction' (physics never sucks) only a lack of pressure or a low pressure area where higher pressure is trying to get to. Therefore you would need air in the space between the topping and the elevated slab, if there was not the air below the elevated slab would be pushing up on the slab and topping, right?

EIT

http://www.HowToEngineer.com

 

[hokie66]

Looks like that 0.6D is killing you. In Australia, we would use 0.9D for that case.

 

[JAE]

Nature (slab on roof) abhors a vacuum (wind uplift).
There IS air under the slab - a little bit - but there is. I suppose you could argue that any "sticky" contact between the slab and the waterproofing below it might create a means of holding the slab down....much like some roof systems I've seen where they have multiple tube chimneys sticking up through the roof membrane that create suction below the roofing and help hold it down.

 

[asixth]

I reckon a midrange category cyclone or hurricane could blow a thin topping slab off the top of a building. Hurricane winds are damaging.

 

[slickdeals]

Do you have a parapet?

It’s no trick to get the answers when you have all the data. The trick is to get the answers when you only have half the data and half that is wrong and you don’t know which half - LORD KELVIN