Wind load on Structural slab overhang.

[Rafael31896]

I am designing an 11 story tower with approximately 120 ft in height and a wind speed of 175 mph. The tower is rectangular in size with around 90 ft in width and 250 ft in length. We are designing balconies on the width side of the building with a max of 13 ft cantilever. At the roof level, architects want to have a 20 ft over hang as the ceiling of top balcony. The height to height elevation is around 11ft.
I was discussing with my colleague as to what wind loads and directions to use. For the roof section (not overhanging) we used partially enclosed building asce 7-10 MWFRS with a 0 degree slop roof. This yielded us uplift forces and an overhang upward force in addition. However we discussed the approach of treating the 20ft cantilever as an open building design which yielded us upwards and downward pressures. The overhang slab is a structural concrete slab debating over RC or PT. There is a 3ft parapet right at the edge of the 20ft roof overhang. I wanted to know if any code treats overhang of that magnitude as an open building. I know in canopy design they account for upward and downward forces at base levels but not sure about downward forces at the highest elevation in the building. The Slab would be 0 degrees (no accounting deflection) tapered in the bottom face so as to reduce dead weight. I also wanted to know if there is downward load on the cantilever overhang, would I run a load combination of the upward load on the main roof and downwards load on the roof overhang simultaneously ? Wanted to see if any code accounted for excessive roof overhangs and or if there is a cut off length.

Any an all help would be appreciated.

 

[JAE]

For a building of that height and size, and an overhang of that magnitude, I would be very tempted to have it analyzed via wind tunnel vs. ASCE 7 wind parameters.

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[struct_eeyore]

You are creating a wing, at 120ft , that's supposed to function in a cat 5 storm. JAE is right - you will have, possibly massive, second-order effects effects that will not be captured by ASCE - vibrations and fatigue might come into play as well.

 

[Yao1989]

Agree with JAE, for a 20ft overhang on a 120ft, ASCE 7-10 does not cover anything like that. Definitely hire a wind consultant and run tunnel test. The results are often multiple times higher.

I have used open building design in ASCE before, but the issue is that it assumes completely open, and no interaction with the solid wall face on one side, which sometimes makes things worse. Pardon my SI units, but I recently worked on a tower where I had gotten 1 kPa from roof uplift using ASCE 7-10 open building during concept design, only to get wind tunnel test results with 2.5 kPa later. I suggest not learning it the hard way I did.