Wind Loads Parallel to Ridge - Definition/Application?

[DexterR]

Hi All!

We had a long discussion about this at our firm, but could not come to a concensus.

First:
What exactly are wind loads parallel to the ridge?
We had the following ideas:
*the wind "drags" the roof along as to blows parallel to the ridge (similar to how drag is generated in fluid mechanics).
*or, the wind generates some lift away from the interior of the structure.
*or, the parallel to ridge loads are compared against standard roof/wall wind loads and the worst case is taken.
Are any of these on the right track?

Second:
When would wind loads parallel to the ridge be applied?

Thanks in advance for any light you can shed for us here!

 

[Lomarandil]

I resonate best with your second explanation -- that wind generates lift (or negative lift) perpendicular to the roof planes (e.g. away from or toward the interior) as it blows past. There's no substantial component along the direction of wind corresponding to "dragging" the roof.

As for when they are applied -- anytime a roof ridge is potentially exposed to wind in that direction (which is almost always). It seems to govern design of structural elements more often for low roof angles.