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Plywood Roof Diaphragms

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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
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
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?   

 
Chris Daniels
570-323-6603
ced(--nospam--at)larsondesigngroup.com


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