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Re: Collar Ties

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Well, by relying on the diaphragm, you have to remeber that when you do
your sesimic design, your diaphragm already is stressed to some level. 
You must add that to your sesimic load, and check the capacity.

When checking an existing structure to see if it is in immediate danager
of collapse, I think your analysis is proper.  However, I would never
actually design a new collar tie roof this way.  We use collar ties
regularly, and design is as a simple truss, checking combined bending in
the top chord below the collar tie.  Sometimes we have to double up the
collar ties and use double shear bolt values connecting the tie tension
force to the rafter or top chord.

Good luck.

scott.horn(--nospam--at) wrote:

>      I had occasion to evaluate the performance of collar ties installed in
>      a house with a roof that spanned approximately 45'.  My initial
>      evaluation using statics principals resulted in failure of the rafter
>      from bending moment.  The failure was far enough from allowable to
>      cause concern with out additional investigation, and not enough to
>      justify rework with out sharpening my pencil.
>      I modeled the collar tie in a 2D finite element analysis program with
>      an X and Y restraint at one end and a Y (vertical) only on the other
>      end.  The answer was the same I found with based on simple statics.
>      My next approach was to consider contribution by the wall and roof
>      diaphragm system in resisting the thrust.  I modeled a spring to
>      represent the amount of deflection I felt would be conservative for
>      the diaphragm and wall elements (double top plate with paneling on the
>      outside).  I found that by allowing the small amount of deflection (<
>      1/2") redistribution of loads occurred and the rafter was adequate.  I
>      further convinced myself that this was adequate by considering that
>      the live load was not highly likely to be seen again during the life
>      of the building as the roof slope was steep and the roofing system had
>      already been installed.
>      I believe that the largest contribution to the stiffness of this system is
>      the roof diaphragm.  Whenever I've been called in the fix a roof of this
>      type that has pushed out the bearing walls, the failure has occurred prior
>      to installation of the roof diaphragm.
>      What do you think about this approach?
>      Scott
>      Bechtel