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RE: metal roof decking

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Dave,

I have run the numbers also, and the deck can generally carry the out of plane compression / tension loads.  But I feel a bit nervous about the deck as a compression element.  As a practice, if my nominal joist to joist spacing is 5 or 6 ft I will cut the spacing of the joist to the wall by about half so that there is only about 2'-6" or less from the joist to the end wall with the wall carrying the end of the deck with an angle.  

The joists will be cambered, and the angle will not quite be level with the joists.   The deck bearing elevation will definately be different with the application of live / snow load.  The deck will also flex with application of the live load.  You really need to be careful about running the numbers.

Harold Sprague, PE
Krawinkler, Luth & Assoc.


-----Original Message-----
From:	Mike Brown [SMTP:mike.brown(--nospam--at)cshqa.com]
Sent:	Thursday, September 03, 1998 10:20 AM
To:	seaint(--nospam--at)seaint.org
Subject:	Re: metal roof decking

Dave:

We have had to do this exercise in  the past per building official requests.
Our conditions were using CMU walls and joists were spaced at 6' on center
(pretty much the same thing).

The deck needed to be analyzed where the joists were parallel to the wall.
The axial load came from the deck supporting the walls against out-of-plane
loading due to seismic/wind loads.  And of course the bending loads were
from gravity.  The gravity load used depended on the snow loads, if any.
The steel deck manufacturer provided the section properties and we just
followed the light gauge steel procedures or cold formed steel design
recommendations for the interaction equations.  So far we have never had to
stiffen the deck.  Overall, this is just another step in the analysis of the
subdiaphragm.

For joists perpendicular to the wall, the axial loads from the wall are
resisted by the joists.  A horizontal bond beam at the diaphragm level spans
horizontally to transfer the loads to the joists.  The deck is parallel to
the wall at this location and has no axial load carrying capacity.

I'm sure there are many other ways to analyze your situation.  I did not go
into much detail for the above analysis, but this should get you pointed in
the right direction.

Mike Brown, P.E.

-----Original Message-----
From: Dave Anderson <dnae(--nospam--at)home.com>
To: seaoc <seaint(--nospam--at)seaint.org>
Date: Wednesday, September 02, 1998 10:21 PM
Subject: metal roof decking


>Hi
>
>Is anyone design 18 gage metal decking as roof diaphragm
>no concrete and to support tilt up wall 34 feet long
>if yes . did you check it for axial and bending loads?
>that in CA . zone 4 ! by 96 UBC revision
>the joist spaced 8 feet on center
>
>thanks in advance
>
>
>D.A. P.E.

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