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Re: rigid diaphragm chord forces?

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Traditionally this is an envelope solution.  The actual relative rigidity of
the diaphragm is a function of the plan geometry and the vertical elements.
A square footprint with ductile moment frames will be more in line with the
infinitely rigid diaphragm than a long rectangular building with shear
walls.  The break point for modeling is when the lateral deformation of the
diaphragm is twice the average story drift, the diaphragm should be assumed
as flexible.  The approach is that since we really don't know (did I say
that?) we look at the diaphragm both ways and provide chord reinforcing.
How you arrive at the chord reinforcement is also a simplification.  Just as
shear wall design has moved away from the traditional shear web and boundary
jamb analogy to account for the participation of distributed bars, the same
should be true for diaphragm design.  For most cases the shear web and
boundary chord analogy is still used for ease of calculation.

The SEAOC seismic design manuals have some fair examples.  For slab on metal
deck the deck manufacturer will have a flexibility factor for the various
configurations and the old Tri-Services manual has some good procedures.
Most of the deck manufacturers will assist you in obtaining copies of the
relevant publications and information.  I have always been partial to Verco,
and they provide excellent support.

Paul Feather PE, SE
pfeather(--nospam--at)SE-Solutions.net
www.SE-Solutions.net
----- Original Message ----- 
From: <dadie(--nospam--at)sbcglobal.net>
To: <seaint(--nospam--at)seaint.org>
Sent: Tuesday, August 26, 2003 9:27 PM
Subject: rigid diaphragm chord forces?


> in flexible diaphragm design, i can see the deep beam analogy and the
> resulting chord forces
>
> however, in rigid diaphragm design, i thought the whole point was that the
> diaphragm (concrete slab over metal deck) was assumed rigid, has a center
of
> mass, has a center of rigidity and is controlled by rotation.  this
rotation
> transmits forces to the lateral resisting elements below based on their
> relative rigidities in shear only.
>
> have i missed the point here?  if i did can you provide a design reference
/
> document that shows how rigid diaphragms develop chord forces?
>
> tia
> da
>
>
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