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Re: Plywood Roof Diaphragms

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On Mon, 21 May 2001 17:56:18 -0400 CDaniels <ced(--nospam--at)larsondesigngroup.com>
writes:
> I seem to be continually running into problems on projects with 
> regard to
> the interaction of the roof diaphragms and shear walls.  Most of the
> projects I work on are  single story wood framed buildings or at 
> least truss
> framed roofs and CMU bearing walls.  There are no seismic loads and 
> most of
> the time code specified nailing of roof and wall sheathing is 
> adequate but
> lately wall heights and square footage keeps growing and I find 
> myself
> stretching for solutions.  My experience with other engineers in my 
> area and
> even my co-workers is that they don't do a lateral analysis or they 
> make
> exorbitant assumptions which nullify the need to do a lateral 
> analysis.  How
> does one explain or justify this to a client that only knows the 
> other
> "engineer" says that he/she doesn't need blocking, drag struts, 
> bracing etc.
> Below is a list of dilemmas I seem to be continually involved in 
> arguing
> against or for.    
>   
> * How do you transfer diaphragm forces from the roof sheathing to 
> the wall
> plates without blocking between the trusses or rafters?  The 
> blocking is the
> first thing that the architect wants to eliminate because of roof 
> venting
> and here in the East no drawings I've seen ever shows blocking at 
> this
You need blocking between the trusses or rafters. It can be eliminated at
say 8 ft. centers to accomodate vents.

> location.  If you use hurricane ties at the trusses it is possible 
> to
> develop enough strength to justify the design but if you have a 
> truss with a
> raised heel is roll over of the trusses a problem?
> * On a roof with pre-engineered trusses, how is an interior shear 
> wall
> handled?  Can the bottom chord of the truss be used as a drag strut 
> or

Yes, bottom chord can be used as a drag. Shear walls parallel to the
trusses can be fastened to the bottom chord with Simpson TP or similar.
For shear walls which are perpendicular to the trusses, X bracing is used
sometimes, if the shear cannot be carried to the exterior walls.


> collector if the shear wall is only a fraction of the building width 
> and
> does not extend to the underside of the roof deck?  This would 
> assume that
> the pre-engineered truss(es) above the wall would have to behave as 
> a rigid
> element.
> * On a roof with pre-engineered trusses, how is an interior shear 
> wall
> perpendicular to the truss span handled? Can this even be done 
> without an
> exhaustive design of cross bracing within the pre-engineered truss 
> system to
> transfer forces form the roof sheathing down to the shear wall?   

For what it is worth, wind governs over seismic in about half the cases.
I have never understood how many of the houses in the east stand up
without much lateral analysis/resistance.

Stan Scholl, P.E.
Laguna Beach, CA> 
>  
> Chris Daniels
> 570-323-6603
> ced(--nospam--at)larsondesigngroup.com
> 
> 
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