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C &C wind load on parapets - really around 45 psf?

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My office has recently studied the components and cladding (C & C) wind loads on parapets in order to design the parapets on four story wood framed buildings.  I would like to present our conclusions and request everyone’s input as to whether you believe we have interpreted the code correctly.  I would also like to know, if our analysis is correct:  Do you all believe that the C & C wind loads required by ASCE for use on parapets are reasonable?

 

Here is our analysis:

 

Using 6.5.12.4.4, the code requires GCp factors as follows (for h less than 60’, not near corners, assuming impervious finish on the parapet, and same structure (stud) takes load on both sides of parapet):

 

For Load Case A (windward parapet):

 

Windward side of the windward parapet:  per Figure 6-11A, about 1.0 (zone 4), with a 10% reduction allowed if the roof is flat.

 

Leeward side of the windward parapet:  per Figure 6-11B, about 1.8 (zone 2, roof element, it is our understanding that the negative roof wind pressure is to be applied to the back of the parapet)

 

Total GCp factor:  2.7 (assuming flat roof)

 

Basic wind pressure, qp is 17 psf (85 mph, h=50’, exposure C).

 

Wind load on parapet:  46 psf.

 

This is greater than a floor live load.  Does anyone really think that we need to design parapets for this much wind load?

 

If you assume exposure B, which is reasonable almost anywhere in southern California, the load would be 34 psf.

 

To us, it seems that the GCp factor of 1.8 required on the leeward side of the windward parapet is very high.  If there was a wind storm and I was on the roof, this is where I would want to be.  And I don’t think that I would experience any suction if I was huddled at the corner where the roof meets the parapet.  And how about the case of a very tall parapet – say 10’ or so.  Does the GCp of 1.8 apply to the entire height of this parapet?

 

For load Case B, which is essentially the leeward parapet, the code requires GCp factors of 1.0 on the back of the parapet, and 1.1 on the front of the parapet, for a total of 2.1  ASCE-7 seems to be saying that, since there is no roof on the leeward side of the leeward parapet, the GCp is 1.1 instead of 1.8.

 

All of your input will be appreciated!

 

 

Thanks,

 

Sharad T. Patel, SE

Patel Burica & Associates, Inc.