Ballasted Roof (uplift)

[DTS419]

How much does the typical (say 4") ballasted roof weigh?

I've been reading about the phenomina of a ballasted roof resisting extremely high uplift forces without the gravel blowing off the roof. Very interesting? Does anyone know any more about this?

I'm very curious what makes the gravel perform as a monolithic component. If you have a 20 psf uplift, each individual stone weighs much less than the force of the uplift, so what keeps the stones from being sucked off like popcorn?

 

[apsix]

I'll have a stab at it.
If the pebbles are at all rounded a portion of the underside will also be subject to the suction therefore total uplift is reduced.
I don't know anything about ballasted roofs.

 

[youngstructural]

They do not act like a mass... I'm not entirely certain of the physics involved, but this is about boundry layer phenomina, and an individual stone would be just as likely to sit on the roof (IF the roof was an irregular concrete surface that matches the surface irregularity of the equivalent gravel).

This behaviour of air is well understood, and I believe our Mechanical & Aerospace friends take advantage of it.... Essentially the layer of air next to an object "sticks" to it and forms a sort of "grease" keeping other air from contacting the surface proper...

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[msquared48]

It's the same physics as a stream bed where gravel of certain sizes are stable up to a certain flow rate, then they move. I suspect the same is true here, just a different fluid.

Mike McCann
MMC Engineering