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steel (purlins and sloped roofs)

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Hi,

        This is my question for the week.  I am presently designing a one
storey elementary school which has a variety of roof framing schemes.  For
one portion of the school I am using a truss to span over a mechanical room.
My question is this, does anyone have any bad experiences using cfc purlins
to span over distances greater than 3 meters.  Some engineers in my office
have told me that using cfc purlins will lead to sag problems as they are
too spindly.  The loads are aproximately 1.1 kpa dead and 2.32 kpa
live(snow), the slope of the roof is 32 degrees.  What I have done is to
replace the purlins with a thicker metal deck 76x.76 spanning perpendicular
to the trusses, and decreased the spacing between trusses by adding another
truss member.

        Another question while still on the subject of sloped roofs, is it a
valid assumption to assume that the the tangential component of the dead and
live loads is resisted by the metal deck and that the purlins can be
designed solely for strong axis bending.  I have always thought that this
was the classic case of Mx and My bending.  How about the design of the
eaves member?


Any input or discussion on these subjects would be much appreciated.


Jon Turner