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Re: sloped metal deck diaphragms

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At 07:43 PM 01/11/2000 -0500, you wrote:
>If a roof system is to be a sloped roof with a steel deck how is it treated
>as a diaphragm. The outside of the roof is a typical gable roof with a
>slope of 7 or 8 to 12. The structure of the building is a steel frame with
>a Hambro type floor system. It is three stories. My previous employer did a
>similar building that used a system of horizontal steel member x-bracing at
>the underside of the roof with the steel joists sloping with the deck on
>them. The deck was thus not used for the lateral load resistance but only
>vertical. It would seem to be the way to do it as loading the deck on angle
>would be like loading the top of a slender deep beam with it leaning out of
>vertical. Any thoughts?? What about the similar situation with sloped wood
>roof trusses and plywood roof sheathing?? 
>
>David Handy, P.Eng
>Ontario, Canada
---------------------
Thoughts:  Assuming the steel deck product is accredited for diaphragm use
in general, the fact of the roof surfaces being pitched (rather than being
essentially parallel to idealized horizontal loading) is not a major
complication.

The whole matter should reveal its "mysteries" if subjected to some
free-body diagram modeling, complete with chasing down of all the resulting
boundary reactions with due attention to principles of statics and strength
of materials. This is a low-tech exercise, but offers a tool of discovery
that disallows (or detects) the usual mistaken jumps to conclusions that we
are tempted in mid-career to make.

The fact of a shear web (the plane of roof diaphragm) not being remotely
parallel to the external applied load, but still working, is uncontroversial
where corrugated metal roofing supports vertical loading: nowhere is the
metal actually sloped parallel to the direction gravity pulls, yet the shear
gets carried. 

Charles O. Greenlaw  SE   Sacramento CA