Per ASCE 7, what is the mean roof height of a BLDG?

[dcredskins]

Per ASCE 7, what is the mean roof height (h) of a bldg as follows?
1. Office BLDG, flat roof (40'-0") with 2'-6" parapet.
2. Office BLDG, flat roof (40'-0") with 2'-6" parapet, and 12'-0" penthouse (penthouse covers only 25% of the roof plan).
3. Office BLDG, flat roof (40'-0") with 2'-6" parapet, and 10'-0" screen wall around Roof Top Units.

 

[faromic80]

From what I understand, mean roof height applies to a sloped roof. For example, if you have a sloped roof, the mean height of the roof is 2/3 the total roof height (centroid of triangle). Someone correct me if i've been understanding this incorrectly.

 

[WillisV]

The poster is asking if the parapet, penthouse, etc. affects the value of qh. I would use 40' for all three conditions.

No. 1 the answer is definitely 40'
No. 2 is a bit of a judgement call but with 25% or less being taken up by the penthouse I would still go with 40'.
No. 3 again if its around a few units I would still stick 40'

 

[dcredskins]

WillisV,

You got my point. In ASCE, I did not find any specific section dealing with this issue, if we need to consider parapet/penthouse/secreen wall as a part of building height, to my knowledge. I understand the "engineering judgement call" part. However, I am looking for any documented proof.

 

[csd72]

look at the diagrams with the charts.

 

[dcredskins]

csd72, in ASCE 7-05, pp. 37-74, only thing I see is h = mean roof height, in feet (meters), except that eave height shall be used for theta less or equal to 10 degree. I did not find any section talking about parapets/penthouses/screen walls. Am I missing anything?

 

[msquared48]

I think that the intent of the code here is to establish an average height of the building, both considering the ground topo at the foundation, and the roof structure.

That being said, I would consider parapets and penthouses as adding to the "height". I would use 42.5, 45.5 and 52.5.

Mike McCann
MMC Engineering

 

[jnadeau]

I would take h=40 ft for all three cases but then calculate pressures for the parapet from ASCE 7-05 6.5.12.2.4 and the pressures on the penthouse and screen from 6.5.15.

 

[WillisV]

jnadeau said what I was trying to convey. H is to the top of the main roof plain for qh calculations - calculations on the actual parapets, screens, etc. would use those heights.

All examples I have ever seen with parapets still use the heigh to the roof level (less parapet) for "h". For example all of the "Guide to the Wind Load Provisions" books. For an online example, see the following:

http://www.gostructural.com/userfiles/File/Problem1_7-StoryBuilding.pdf

which again uses the height less the parapet as "h".