Sandwich Slab/ Split Slab with large uplift
Sandwich Slab/ Split Slab with large uplift
(OP)
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
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






RE: Sandwich Slab/ Split Slab with large uplift
RE: Sandwich Slab/ Split Slab with large uplift
I would first try to simply add the dead weight as required to provide the necessary hold-down capacity.
RE: Sandwich Slab/ Split Slab with large uplift
RE: Sandwich Slab/ Split Slab with large uplift
RE: Sandwich Slab/ Split Slab with large uplift
RE: Sandwich Slab/ Split Slab with large uplift
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.
RE: Sandwich Slab/ Split Slab with large uplift
EIT
www.HowToEngineer.com
RE: Sandwich Slab/ Split Slab with large uplift
RE: Sandwich Slab/ Split Slab with large 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.
RE: Sandwich Slab/ Split Slab with large uplift
RE: Sandwich Slab/ Split Slab with large uplift
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