Kzt Factor at multi-tiered site

[dozer]

I tried this on the ASCE code forum but got no response so I'll give it a try here.

I'm working on a site that has multiple tiers. Each tier is from 250m to 500m wide then rises 10 or sometimes 15 meters to the next tier. There are 5 tiers. The slope between tiers is 1 on 3.

My question is should I look at each tier as an escarpment in which case you will get a pretty significant wind speed-up at the windward edge of each tier, or should I consider the overall site one big escarpment in which case the H/Lh ratio is less than 0.2 so topographic effects need not be considered.

My reason for going with the latter would be criteria no. 2 in section 6.5.7.1 of ASCE 7-05 (7-10 is the same but I don't have the section numbers handy). To illustrate, let's say my first tier is 10m above the baseline, then the next tier rises another 10m. Since this is not at least twice the first tier, wind speed-up effects need not be considered.

If the second tier had been 20m above the first tier than I would consider wind speed-up effects since 20m is twice 10m.

Does this make sense or am I missing something? Is there any literature on this specific case? I've searched the internet and haven't found anything.

Thanks for the help.

 

[Bagman2524]

I would think you have a pretty good case of considering it as a single escarpment. The wind speeds would differ based on the height of each level.

 

[dozer]

So are you saying you would determine Kh and Kz based on the height above the lowest level?

 

[ishvaaag]

I think a single escarpent is a reasonable assumption; forfeiting the effect that the buildings may introduce, a horizontal wind most likely will show the biggest speed at the top edge by the compression of the layer to meet the escarpment; and it is from opposing the kynetic energy of this wind that the pressures on our buildings are derived; it is unlikely intermediate levels experience bigger wind pressures than atop except being induced by the buildings themselves.