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RE: Plywood Roof Diaphragms

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Chris,
I forgot to add a couple of things:

1. Why is it that most people think that if you are not in a seismic region
that you are free from lateral force requirements. Most of the conventional
wood framed homes are governed by wind, not seismic. In high seismic zones,
most rectangular shaped home will have wind govern in at least one
direction.

2. Even if you have a masonry building, if you have any overhang on the
roof, you will have an uplift on the eave that will want to move the roof
off the building both laterally and vertically. I might be wrong on this
one, but the code specifically states (at least the 97 UBC does to my
recollection) that all roof and floor framing shall be continuously blocked.
This is the justification - it is in the code including the Conventional
Construction provisions (which does not concern itself with CMU buildings
that I am aware of).

3. Interior shear can be transferred to interior walls by the use of a drag
truss or drag strut. Most of our work involves the design of plated trusses.
I calculated the load I need to get the shear from the roof diaphragm to the
wall and specify the force one the plans for the truss company to provide a
drag truss. I also make sure that it is specified that the truss is
concentric to the wall as most contractors follow uniform spacing of trusses
and if it happens to fall off to the side of the wall so be it. Make sure
you specify it directly over the wall to simplify the transfer to the wall
through the lower chord.

Where the trusses are perpendicular to the wall, this poses a somewhat more
difficult and tedious issue as in most cases, you end up building cripples
up between the framing. However, another solution is to provide a girder
truss at this location which can transfer shear to the all parallel and
flush frame the perpendicular trusses on both sides. Still another solution
is to design the wall as bearing and hang the trusses off the double plate
of the wall where the boundary nailing of the sheathing creates the shear
transfer down into the full height wall below.

4. You can provide a drag strut and you may wish to make this a 4x member
because you may need to nail the diaphragm closely or provide two rows of
nails. In either case, you are most likely to build your wall up to the flat
plate line and cripple up to the drag member (for a sloped roof). If you do
this, and you make the pony wall continuous, you can reduce your H/b ratio
of the shear wall to the height where the force occurs at the top of wall
rather than at the top of roof. The pony (cripple) wall needs to be sheathed
to accommodate the shear transfer, but it is an easy detail in most cases.

5. Re-entrant corners can be handled in either of the manners above, but
don't forget to account for the shear (wind or seismic) coming into the
collector from both sides of the diaphragm.

You can't eliminate wind from anywhere in the world and it can be brutal.
Once you consider that wind is a lateral force, you simply can't get away
from doing a lateral analysis anywhere in the world.

Dennis

> -----Original Message-----
> From: CDaniels [mailto:ced(--nospam--at)larsondesigngroup.com]
> Sent: Monday, May 21, 2001 2:56 PM
> To: 'seaint(--nospam--at)seaint.org'
> Subject: Plywood Roof Diaphragms
>
>
> 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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