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Obstructed Wind Flow
5

Obstructed Wind Flow

Obstructed Wind Flow

(OP)
Question: what constitutes "obstructed" wind flow as per ASCE 7? For example, on Figure 6-18D note 2 explains that "Obstructed wind flow denotes objects below roof inhibiting wind flow (>50% blockage)"

I'm kind of thrown for a loop with that "below" the roof statement. Does that mean decking does not count? Does that mean just supporting framing members? Also does this mean obstructed in the direction of wind flow for each particular case? In other words if it was a shelter roof that the truss supporting the decking was open on its side (no decking covering it), does that constitute open [for that case] if the area it was blowing against was 50+% blocked by the truss member themselves? Thanks in advance.
 

RE: Obstructed Wind Flow

3
The way I always interpret it is that if the materials under the deck, like pipes, tanks, vehicles, etc. cross the 50% threshold, it's obstructed. The deck and trusses have nothing to do with it. So picnic shade; unobstructed, tank shade; obstructed.
But unless I'm pretty sure that it's never going to change purposes, I always pick the one with the highest coefficients between obstructed and unobstructed.

RE: Obstructed Wind Flow

2
If the "obstruction" can be moved, I usually do not consider it.

Such an obstruction could also be an earth berm against the outside concrete wall.  

Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
 

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