ASCE7-10 Wind C&C Kh & Kz factors (Table30.3-1)

[imagine18]

Hi,
We have a project that consists of an exterior renovation of the lower 70ft of a 700ft tall building. Our scope is just the cladding on the lower 70 ft. To determine my Kh and/or Kz factor, do I reference the 70 ft for the "height above ground level, z" or do I have to reference the 700ft?? If i have to go with the 700, the wind speeds appear to be extremely conservative. Thoughts? Thanks

 

[Leftwow]

Would you need to figure what type of deflection you have at the base to account for cladding?

 

[mike20793]

The cladding pressures are dependent on how the wind flows around the building, so it is based on the building dimensions. You need to use the full height of the building. The wind speed should be the same regardless of the height, but the pressures can vary greatly. A 700 ft tall building will experience fairly large pressures relative to a 70 ft building so I don't think they are conservative at all. You can reduce the wind storm MRI for deflection calculations.

 

[mike20793]

Also, note that you can MWFRS pressures if you have a large tributary area (>700 square feet) for your cladding. Those will be lower than the component and cladding pressures.

 

[imagine18]

Mike - ah yes, the wind speed will be the same. I'm trying to figure the pressures out for the bottom 70 ft of the building (sorry I should have used the word pressure vs speed in the orig post). If i use the full height of the building, I come up with about 115psf for the bottom 70 ft. If I only use the the 70ft elevation as my reference, I end up with closer to 60psf (strength).

 

[Hokie93]

You need to use the elevation of the component and cladding item of interest for the windward wall (z = 40'-0, z = 50'-0, z = 60'-0, etc.) and the mean roof height (h = 700'-0, presumably) for leeward walls, side walls, and the roof.

 

[imagine18]

I was afraid you would say that hokie...thanks for the input. That is how i read the code as well.

 

[SAIL3]

I have never designed a structure that high....using the full height(700ft) to obtain the leeward pressure without any regard to the width of the bldg seems to me to be extreme especially in the lower portions of the bldg, even though that is how the code is setup.....I would do some further research into this ...there are many skyscrapers built that have had wind tunnel tests done and the test results may be available....