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RE: Shear Transfer from roof diaphragms....

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Dan:

The easy part is the blocking. If you put the blocking in every other truss space, there should be plenty of area for attic ventilation. I have seen others notch the blocking or drill a hole in it. 

The hard part will be educating your boss as to the new code requirements. Show him all of the applicable sections of the code which require shear walls, holdowns, diaphragms and talk about continuous load path. Stress to him that times have changed and these are new requirements that must be complied with.

I called Simpson to ask if their connectors like H1, H9 and H10 could be used to bring down the diaphragm shear forces from the roof into each truss and down to the shear wall. Their answer was emphatically "no". As someone else said, the truss people do not want their trusses used for that purpose either. If you decide to use the connectors and the lateral bending in the trusses to bring down your shear force, you are on your own and the manfacturers (connectors and trusses) will not stand behind you.

I do not believe slip would occur between the plates if they are nailed together every 16". Under those assumptions, the double top plate could be analyzed as a continuous composite member. Since they are layed flat you can use the Flat Use Factor, Cfu for bending since it is loaded on the wide face and Ch for shear.  

Jim K.  

-----Original Message-----
From: Dan Boltz [mailto:dboltz(--nospam--at)mckinleyassoc.com]
Sent: Thursday, November 13, 2003 9:08 AM
To: SEAINT
Subject: Shear Transfer from roof diaphragms....


.....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 shearwalls was unfortunately my boss who is a PE but not a
practicing engineer who is familiar with lateral 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?

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

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?

Thanks.


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