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Re: Shear Transfer from roof diaphragms....

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> .....a little follow up from yesterdays email.  I stamp everything I
design
> and no senior engineer is present in this office to review my designs.
This
> is the hand I'm currently dealt and know that I need a second set of eyes
> reviewing everything that is structurally related.  The design code is IBC
> 2000 and the roof pitch is 5:12.  The man who stated that I didn't need to
> look at shear walls was unfortunately my boss who is a PE but not a
> practicing engineer who is familiar with lateral design.

I've had this argument with bosses before.  They say you "don't need to
worry about" something.  I either don't know enough about the system to
write it off or do know enough to realize that I *do* need to worry about
it.  Basically your options are to 1) convince your boss that this is
critical, 2) do the design on your own time, 3) do the design on company
time and try to justify the "waste" of time when you turn in your time card,
4) blow it off and pray that this building doesn't experience design-level
loads, or 4) find a new job.  Hopefully, you can do the design and then
justify to your boss the need for the design.

> The Simpson A35 and LTP4 connectors look good if end blocking is present,
> but I don't believe that continuous end blocking will work due to attic
> ventilation requirements.  If the distance above the top plate is 18" and
> the eave height is 11", what Simpson connector is available to transfer
> loads out of the roof diaphragm perpendicular to the roof truss?

Here is what I've done in a similar situation:
First, you need a new roof diaphragm chord because the top plate isn't
attached to the roof.  I installed 2x4's between the trusses perimeter
nailed flat to the roof deck with a continuous metal strap for tension.  To
transfer the shear from the roof to the top plate I detailed mini-shear
walls between some of the trusses.  This was basically a box made from 2x4's
with a 3/8" plywood panel.  The number of these panels depends on your
lateral load.  Design the nailing into the top plate to transfer the shear,
and the truss hold-down to handle the uplift.  Specify a vertical truss web
member at this location for nailing.  These pieces can be pre-fabricated and
installed between the trusses before the roof diaphragm.


> Where is the IBC 2000 specific about minimum requirements for lateral
> support?

If you mean overturning support of the truss, you might have to look in TPI
publications.  I don't have any references on hand.


> Also, is it common to have roof trusses at 24" o.c. and wall studs at 16"
> o.c.?  How is the double top plate analyzed for this situation?

Very common.  A double 2x4 top plate can easily handle traditional rafter
loads, but maybe not long-span truss loads.  If you use 16" stud and 24"
truss spacings, you have to assume the truss bears in the middle of a stud
span.  You get a strength increase for the flat bending application, for
roof loading duration, and the fact that at least one of the plate pieces is
continuous.  You must take into account a break in either the top of bottom
of a double plate.  You cannot assume the two plates act as a single unit
unless you design it for the horizontal shear transfer.

If the vertical load is high enough, you will have to either specify a
triple top plate, specify high-strength wood for the plates, or specify a
reinforcing detail for when a truss misses a stud near a plate splice.

----
Jason Kilgore
Leigh & O'Kane, L.L.C.
Kansas City, Missouri



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