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RE: rigid diaphragm chord forces?

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Check the seismic chapter (21) in ACI 318.  There are specific provisions
for checking these forces.  And yes, you are partially
orrect.  -Deformations- are assumed negligible because concrete diaphragms
are so rigid in shear and bending.  But if you have no bending capacity, it
won't matter what your shear capacity is.

HTH,
Jake Watson, P.E.
Salt Lake City, UT

-----Original Message-----
From: dadie(--nospam--at)sbcglobal.net [mailto:dadie(--nospam--at)sbcglobal.net]
Sent: Tuesday, August 26, 2003 10:28 PM
To: seaint(--nospam--at)seaint.org
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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