Expssure Category

[abkTEC]

Hi,

I am designing a 75 foot tall building. A fellow engineer told me that the exposure category of the building can change as the building height increases. For example the first few floors are designed for exposure B and the top floor could be designed for exposure category C. I have never heard of this. Does any one know if this is true? Or where in the code it would explain that? Our building is an exposure category B, in NY state.

Thanks for the help!!!

 

[UcfSE]

This is not true.

 

[abkTEC]

UcfSE,

I would design the entire building with the same exposure category?

 

[AELLC]

I agree, but what if the building was 1000' high, surrounded by low-rise structures or trees? Wouldn't the effects of the surrounding buildings and/or trees slowing down wind velocity at low altitudes above ground have a nil effect of the wind velocities at say, 200' above ground?

 

[UcfSE]

abkTEC,

Yes. The exposure does not vary with building height. Exposure accounts for the terrain around the structure, and makes adjustments to the power-law equation and the atmospheric boundary layer height, based on the terrain. Height effects are accounted for in K_z.

 

[TDIengineer]

I have not seen provisions within ASCE 7-10 that would allow a different exposure based on elevation. Section 26.7 of ASCE 7-10 does indicate you could have a different Exposure for each upwind direction.

26.7 EXPOSURE
"For each wind direction, the upwind exposure shall be based on ground surface roughness, that is determined from natural topography, vegetation, and constructed facilities."

 

[abkTEC]

Thanks for all the help!!

 

[dcarr82775]

I have used different exposures for different directions, but you can't mix different exposures in the same direction.

 

[dik]

In these environs, there is an increasing exposure coefficient that is predicated on building height and for tall buildings is generally calculated at each floor level.

Dik