ASCE Wind Topographic Effect at Valleys

[cdhpsu]

In the continuation of Figure 6-4 in ASCE 7-05 or Figure 26.8-1 in 7-10, the chart "Parameters for Speed-Up Over Hills and Escarpments", the first Hill Shape has a note under it that says, "or valleys with negative H..." I cannot find anywhere else in the code that says this factor should be applied to a site near a valley. Nor do I understand how one would apply a negative height to the calculation of the K1 variable.
Does anyone have any insight to what this note is referencing or an opinion on if a topographic factor should be applied to a site near a large valley?
Thanks

 

[woodman88]

I assume that H starts at the main flat ground level. So a valley that is below the main flat ground level would always be negative. The calculation would be for the wind at the top of the valley. I would apply the factor to any buildings at the top of the valley.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[msquared48]

I understand where the are going with this, effectively to reduce the pressure at the base of the valley for winds normal to a valley. But it does not seem to consider the funneling effect of increasing the wind speed for winds parallel to the valley floor and impacting the buildings in this area. I would consider this the far more critical direction.

Mike McCann, PE, SE (WA)

 

[cdhpsu]

Thanks for you help guys,
I suppose I see where you are going Woodman treating a valley as a 2-D hill that's inside out, and if my site is in the bottom half it protects me from wind normal to the valley's length. M2, I think I agree with you that the funneling effects are really the biggest concern so I suppose this does not help me with them, and I guess that is a question for the local jurisdiction.
I guess I figured since this note does not seem consistent with the rest of this section in ASCE, that perhaps someone on here with a longer memory than mine might remember something from a previous edition of the code that has been removed, or if it was a hold over from some other reference. What are you going to do though, talk to the local code official I suppose.