Wind design: Converting Air Permeability to % void of debris fabric

[anchorengineer]

I'm calculating a wind force on some debris netting. I need to convert astm D737-96 Air permeability to a % void.

The Air permeability is 4,373.5 CFM/SF. I need to convert this into a percent void. I know the answer is around 50%. I also know that it involves a portion of the wind pressure formula 0.00256*V^2. I haven't had enough coffee yet today.
Thanks!

http://www.anchorengineer.com

 

[ishvaaag]

It maybe in the kind of info you already have but I post part of one study I made on urban microclimate here in the early eighties; you will find mainly collected info on the reduction of wind speeds by windbreakers/wind barriers of different porosity, from which to derive the pressure mechanical effects. In spanish.

 

[JedClampett]

Another way to go at this is if you have the size of the strands and spacing you can use the ASHRAE Design manual formulae (at the ends of the volume) to get the fluid losses. From there, it's a direct solution to the wind pressure, using the velocity pressure calculation.

 

[ishvaaag]

And per ASCE 7-05

?
6.4.3 Air Permeable Cladding. Design wind loads determine
from Fig. 6.3 shall be used for all air permeable cladding unles approved test data or the recognized literature demonstrate lowe loads for the type of air permeable cladding being considered.

Approved ... by whom?
Technical literature, different opinions.

Really then the safe assumption is load for impermeable surface.