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RE: More questions about rigid plywood Diaphragms

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Let's remove the question of roof with peaks in them and deal only with
roofs that have a monoslope of 1/4" per foot.  Secondly let's assume a
150' x 500' warehouse with 8" concrete tilt panels 25' high. (limited
penetrations in the 150' length).   The shear walls would definitely be
considered rigid items.  Now explain to me how you can consider this
wood diaphram as rigid.  If I were to design this with steel bar joists,
metal deck, and 3" of concrete you would definitely have a rigid deck.
Comparing diaphram deflections, the relative story drift for the wood
diaphram would greatly increase the P-Delta design of the panels (or CMU
if used).  

Now let's assume a nice 50' x 50' building with CMU shear walls
splitting the building into four 25' x 25' tennant spaces.  The
deflections would be almost negligible as a result of the geometry of
the building and diaphram.

It is not expectable to use broad sweeping comments about the rigidity
or lack of rigidity of wood diaphrams.  I am not one who will classify a
wood diaphram as rigid or non-rigid.  A wood roof diaphram is a
nonhomogenous construction system (thus creating the design
difficulties).  We as engineers use the best model suitable for
calculating design requirements.  I personally do not want to create a
FEA model for design of a residential roof diaphram.  Assumptions must
be made and applied with sound engineering judgement.  Otherwise we end
up back at the other topic on this list of "what can we do in order not
to loose money on these small jobs".  If I see a roof penetration in the
diaphram that I am not comfortable with, install blocking, strapping,
and increased nailing to overcome the discontinuity.  This will increase
the local stiffness of the structure but will still not result in the
entire roof being considered rigid.  

I have spent many years in South Florida designing wood roof diaphrams
after learning diaphram design on the west coast.  The forces that are
transferred into diaphrams as a result of hurricane winds are usually
much higher than those applied as a result of seismic forces or strong
inland winds.  The Standard Building Code requires serious attention to
uplift but is very inadequate in diaphram design.  Hurricane Andrew did
so much damage but according to studies a majority of the damage could
be attributed to shoddy workmanship; not inadequate design.  

My point is that I sit here and read this discussion on rigid versus
flexible wood diaphrams and see no need to modify design methods or code
requirements.  I do feel that more attention should be given to diaphram
discontinuities and irregularities.  I have seen many instances of
diaphram tears in both high wind and seismic regions as a result of
inadequate reinforcement in areas such as these.  Other areas that could
also use some attention would be possibly where lateral loads are highly
concentrated such as drag struts, anchorage's, or subdiaphrams.
 

-----Original Message-----
From: Paul_Reilly(--nospam--at)dot.ca.gov [mailto:Paul_Reilly(--nospam--at)dot.ca.gov]
Sent: Tuesday, June 29, 1999 10:24 AM
To: seaint(--nospam--at)seaint.org
Subject: Re: More questions about rigid plywood Diaphragms




Devils advocate.  I have reservations about the wholesale classification
of
light frame roofing systems as flexible.

      May we recognize that roofing systems typical to light frame
construction
are composed of ductile frame elements:   tied, sheeted, braced, and
pinned
about a level boundary perimeter (perfect world).  The individual
components do
not possess the properties of classical rigid elements, however, as a
system,
the components certainly perform as a framed entity.  The model is best
represented by a trussed roof system with multiple and evenly spaced
intermediate points of bearing with a simple perimeter.

     On cut-roofing applications, the model becomes respectable frame as
straps,
collar ties, ridge ties , purlin struts, kickers, braces, blocking
(stepped
applications), straps, ceiling ties and chords (more?) are added.
Inherent
redundancies make it all work.  The top is sheeted, the bottom is
sheeted,
intermediate supports are numerous and it has historically performed as
a
laterally rigid diaphram.

 Got a calc for that?  When pigs fly.  Have some extra recreational time
for a
FEA?

     I do not believe that the code references the consideration of
rigid
horizontal diaphrams because we have a tidy FBD for every element in a
roofing
system, especially light frame construction.  The historical performance
of
these systems and the investigation of failures has provided the present
basis
for the recognition of these systems to be considered as rigid
diaphrams.  I
would like to support the continued recognition of these systems as
rigid
diaphrams.

Paul Reilly