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plywood diaphrams - flexible or rigid

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I have not yet seen a reference to a book that I purchased
last year, so I am going to quote a few portions of it as it
applies to rigid and flexible diaphrams. Much of this has
been stated already as peoples opinions. This book
reinforces those opinions.

The book is titles "Seismic Design of Buildings and
Bridges", 2nd edition. It is based on the 1997 UBC and was
purchased from ICBO. Its author is Alan Williams, Ph.D.,
S.E., C.Eng. His title is Principal, Structural Safety
Policy, Division of the State Architect, State of

from Section 2.4.1 Basic Components pg 91...92

"A diaphragm is considered flexible, in accordance with UBC
Section 1630.6, when the midpoint displacement, under
lateral load, exceeds twice the average displacement of the
end supports. This is illustrated in Figure 2-17. The
diaphragm may then be modeled as a simple beam between end
supports, and the distribution fo loading to the supports is
independent of their relative stiffnesses and is
proportional to the tributary areas supported. Flexible
diaphragms consist of diagonally sheathed wood diaphragms,
plywood sheathed diaphragms, and steel-deck diaphragms.

When the midpoint displacement of the diaphragm does not
exceed twice the average story drift, the diaphragm is
considered rigid. Allowance must then be made for the
additional forces created by torsional effects with the
diaphragm and supports assumed to undergo rigid body
rotation. The distriburion of loading to the supports is
proportional to their relative stiffnesses and is
independent of the tributary areas supported. Rigid
diaphragms consist of reinforced concrete diaphragms,
precast concrete diaphragms, and composite steel deck.
Figure 2-18 provides a comparison between flexible and rigid

from Section 2.4.2 Plywood diaphragms pg 92

"A plywood diaphragm is a flexible diaphragm which acts as a
horizontal deep beam." ........

Ken Reed, PE
Redding, CA