Roof Topping Slab in High Wind

[bookowski]

I've got some concrete projects in a high wind area (~170mph and exp D). The roofs were supposed to be monolithically pitched to drains. The contractor built them flat and wants to pour a topping slab to achieve the slope. Unless I'm missing something this does not appear to work w/out a silly amount of concrete. Uplift pressures are 100 to 200psf depending on the zone. I have of course gotten the response of 'we always do this' but I'm not seeing how it works short of drilling/epoxying dowels in to tie it down. Am I missing something and there's a rational way to make this work, short of tying it down?

 

[ANE91]

Wind Engineering 101, as understood by admittedly a non-aviation person:

Roof uplift is idealized as the net effect of two mechanisms, as described above.

Wind flowing over the roof reduces the above-roof pressure. Thus, there is a differential of pressure between the inside and outside, with the force vector pointing upwards and acting from within the building, where the pressure is relatively higher.

Additionally, wind leaking through jambs and other imperfect seals inflates (or deflates) the building, increasing (or decreasing) the below-roof pressure. The result is an increase (or a decrease) in the pressure differential across the roof assembly.

Air only pushes. It cannot pull. Kopp’s partial chapter, that I provided above as a screenshot, addresses how air can get under and push up on layers of roofing.

Open buildings still experience pressure differentials. The eaves aren’t airfoils. Air is incident upon the eaves and rolls over and under the roof, not in equal volume. Vortices are also generated and shed.

 

[HDStructural]

Thank you for that explanation. So it is possible that there is no net uplift on roofing materials if there would be a perfect seal in all elements. All of the uplift would be a pushing force from underneath the roof?

 

[ANE91]

Thank you for that explanation. So it is possible that there is no net uplift on roofing materials if there would be a perfect seal in all elements. All of the uplift would be a pushing force from underneath the roof?

Exactly.

I encourage you to visit the concept of a solid sphere in a wind flow. Wind does not rip the sphere apart, laterally; it is solid, so there’s no fluid “inside” that wants to get “outside.” You’ll find these concepts explored in text about bluff body aerodynamics. Simiu, Kopp, Flay, Holmes, Davenport…lots of writing on the subject.

There’s no such thing as a perfect seal, in the real world.

 

[EngDM]

There’s no such thing as a perfect seal, in the real world.

except when I'm drinking

 

[human909]

Thank you for that explanation. So it is possible that there is no net uplift on roofing materials if there would be a perfect seal in all elements. All of the uplift would be a pushing force from underneath the roof?

Exactly. And in the case of a topping for the purposes of this discussion there wouldn't be an air later between it and the substrate. Sure there might be some trapped bubbles but nothing of consequence.

 

[dkrekelberg]

Using two separate pours on roof decks to create the slope is not uncommon, ideally it would be monolithic. A few things to consider - 1. the topping/sloped pour dead load is high enough to resist the net wind uplift forces, your roofing system will be adhered to the concrete deck so it must resist the wind uplift & 2. ensure there is no additional dead load (designed monolithic sloped pour vs. installed flat pour and sloped topping). There are also other issued with a cold joint between the two pours, the topping slab will not be able to properly outgas/cure once the roofing system is installed - it will be bound by the primary slab below and the roof system above. Allowing the topping slab to cure properly prior to roofing will be important.

I had a project where the deck was poured monolithically but surface finish was poor, large depressions and irregularities. The contrcator wanted to come back through with a self-leveling concrete material to fill the low spots. This is tricky as the roof system had to be adhered to the surface of the concrete. The contractor performed the leveling on their own before the RFI could be resolved and the self-leveling material ended up delaminiating in many spots, caused quite a mess and delayed construction.