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(OP)
Hi,

I have a question regarding wind loads. Suppose I have a 400 ft tall building, but I am only analyzing something between floor heights of 20 ft to 30 ft. I understand, qz can be taken at 30 ft. However, why is negative wind load (leeward wall) calculated using qh (h=400 ft)? It does not make sense that negative wind load is constant along entire height of building. Is this accurate?

Might help to mention what code you are working to?

It is IBC.

I couldn't find the reason why the leeward wind pressure is assumed constant, I guess it is somehow determined through wind tunnel studies. For a direct and concise explanation, you should post your question to ASCE for the answer.

Wind pressure is positioned along the “windward” and “leeward” sides of a structure or building.
Understand that the windward pressure depends on the wind force that is alongside the height of a structure or building, and the leeward pressure is presumed to be constant.
The leeward pressure is negative wind pressure that is in essence pulling (not pushing) on the structure or building.
When both the windward and leeward pressures are in the same direction, then the combined wind load pressure is determined by subtracting the negative leeward wind pressure from the positive windward wind pressures.

(OP)
Thanks for the reference. Yes code is IBC.

However, the building being 400 ft, 300 ft , etc should not effect negative wind load at the bottom the building. It is strange that this is the requirement, especially with such an extreme height different (30 ft vs 400 ft). Therefore, all cladding would need to be designed as if it was at the top of the skyscraper!

Also, is the exposure type allowed to change along the facade of the building? For example, the base of the building at street level is not as exposed and the top of the tower. Therefore, the bottom half is similar to exposure B while the top is exposure D. ASCE 7 states exposure to be determine by mean roof height, not height of the area/opening under consideration.

Not sure if you found the answer to this or not.

It's a bit of a weird quirk of wind loading in the ASCE (which is the basis of the IBC). Actually, there is some explanation given in the ASCE 7-10 commentary (C27.6.1).

Unlike the windward external pressure coefficient, the leeward pressure coefficient is allowed to be reduced based on the building height aspect ratio. As the building becomes taller, the leeward coefficient is allowed to be reduced to a minimum of -0.3. The idea is that, if the aspect ratio is very small, the leeward pressures become strongly correlated so you have to use a larger leeward coefficient of up to -0.5.

It seems like the reason why it is set linear/constant with height is for conservatism and it's also done so that you get similar base overturning moments as if you had used the structural loading approach.

This approach is a bit different to Eurocode, which I BELIEVE might allow you to consider a varying leeward pressure coefficient with height. The language of the Eurocode is a bit less explicit and it's been a while since I revisited it, but that was my impression when using it.

-------------
Ilyas

Eurocodes recommend to base leeward pressures based on qz at the top of the building (same as US code).
I think that it depends on the aspect ratio of the structure, and how the wind moves around/over the building.
For a (tall) slender building the majority of the wind will pass around the sides, however for a more stocky shaped building
(even if its a tall building), then a lot more of the wind will travel upwards over the building. Therefore it is reasonable to use
qz based on the building height for the leeward suction. How far down the leeward face that the suction should be based on the
qz value of the building height, or if lower qz values can be used for slender structures (where the majority of the wind passes around the sides
and not over the building) is another question. Not sure what the US or Euro codes say about this.

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