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Re: steel (purlins and sloped roofs)

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If the metal deck to purlin connection is capable to resist the tangential force
component and the metal deck can transfer the force to the frames or perimeter 
walls, I think you can assume that the purlin is braced in that direction. If 
you can design a positive deck connection at the ridge of a symmetrical roof, 
then the force from one side of the roof will cancel each other. Otherwise, you 
can use tie rods connecting all the purlins and tied the rod at the ridge to 
cancel the forces from each side of the ridge. 

Wira Tjong

______________________________ Reply Separator _________________________________
Subject: steel (purlins and sloped roofs)
Author:  <seaoc(--nospam--at)seaoc.org> at internet-mail
Date:    9/7/97 12:33 PM


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