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Re: rigid plywood diaphragm??????

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I start to wonder if the issue not in the apparent flexibility or ductility
in timber shear walls, but rather in the code prescription of handling it
one way or the other.  I received a copy of the UBC prescription for
reference and I note that

"Section 1930.6 Horizontal Distribution of Shear.  The design story shear,
Vx, in any story is the sum of the forces Ft and Fx above that story.  Vx
shall be distributed to the various elements of the vertical
lateral-force-resisting system in proportion to their rigidities,
considering the rigidity of the diaphragm.  See Section 1633.2 for rigid
elements that are not intended to be part of the lateral-force-resisting

Where diaphragms are not flexible, the mass at each level shall be assumed
to be displaced from the calculated center of mass in each direction a
distance equal to 5 percent of the building dimension at that level
perpendicular to the direction of the force under consideration.  The effect
of this displacement on the story shear distribution shall be considered.

Diaphragms shall be considered flexible for the purposes of distribution of
story shear and torsional moment when the maximum lateral deformation is
more than two times the average story drift of the associated story. This
may be determined by comparing the midpoint in-plane deflection of the
diaphragm itself under lateral load with the story drift of adjoining
vertical-resisting elements under equivalent tributary lateral load."

In other words isn't the code telling us to consider the diaphragm flexible
or rigid depending on the calculation being done?  Alternatively it seems to
be saying that WE should ascertain the "rigidity" and use that in our
boundary conditions assessments.  I may be misunderstanding it but I don't
see the code telling me how to determine whether the diaphragm element  is
rigid or not.  Hence the on-going discussion.  The same seems to be here in
Canada too.

Thor Tandy  P.Eng  MCSCE
Victoria BC

-----Original Message-----
From: merrick group <merrickgroup(--nospam--at)>
To: SEAonc seaint <seaint(--nospam--at)>
Date: Tuesday, May 04, 1999 11:22 AM
Subject: rigid plywood diaphragm??????

Rigid plywood diaphragms are of code and physics.

Maybe something is wrong with the design if there is a radical difference
between analysis for a rigid or flexible horizontal diaphragm.

Plywood stiffness is nearly directly related to its nailing shear capacity.
The force is balanced by being delivered to where it can be resisted.
Exception would be large h/t walls with flexible hold-downs. Replace the HD.

Tributary area methods for the flexible diaphragms places the rigid
diaphragm shear center near to the mass center.

A test of rigidity is a global concern. Maybe the deflection test of
rigidity should be from outside to outside wall where extreme affects of
torsion occur.

APA publishes force deflection curves of wood shear walls. They are flat
near their capacity and continue for a very large deflection. The walls will
eventually share the load relative to their strength. Compare the results to
the ductility factors of steel and see which is superior!

The above are issues for discussion only and are not to be considered as

David B. Merrick, SE