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# RE: Flexible diaphragm

• To: <seaint(--nospam--at)seaint.org>
• Subject: RE: Flexible diaphragm
• From: "Dennis Wish" <dennis.wish(--nospam--at)gte.net>
• Date: Wed, 5 Dec 2001 21:56:15 -0800

```Stan,
Point of Clarification: If this is a masonry building (exterior walls
and elevator core) then the diaphragm may just be flexible. The
deflection of the diaphragm is enhanced by the lateral weight from the
mass of the walls. Generally, the lighter the walls, the less affect it
will have on the diaphragm deflection. The code specifies the designer
to evaluate the diaphragm by calculating its deflection. Depending on
the span between shear resisting elements (the diaphragms aspect ratio),
the diaphragms capacity and the mass of the walls, it is very likely
that the deflection will exceed what the code defines as rigid.

While floors have higher shear capacity than roof diaphragms, the mass
of the walls tributary to the floor is also much greater. Section 1630.6
Horizontal Distribution defines the guidelines for diaphragm deflection:

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

Now, I don't know about you, but this section of the code sounds like it
was designed by Rube Goldberg. For what it's worth, this section of the
code requires the engineer to check the diaphragm's deflection and if
the calculated deflection exceeds 2 times the average story drift of the
associated story (but it does not reference the section of code that
tells you how to calculate the story drift which I still assume is
0.005h) then the diaphragm is flexible. Usually this will occur in
tilt-up concrete buildings, large diaphragm masonry buildings,
Unreinforced masonry building, lighter weight buildings with large
aspect ratios (approaching 4:1) etc.

I raise the issue with you (and others on the list) only because it is
one of the areas that I criticize as not being clearly defined to the
point where a computer program can be written (none of the commercially
available programs except our spreadsheet will calculate diaphragm
deflection and the diaphragm is simply assumed to be "non-flexible" but
not necessarily rigid). I know that Keylat does not calculate diaphragm
deflection and I believe Woodworks 2002 does not either.

Regards.

Dennis S. Wish, PE
California Professional Engineer
Website:
http://www.structuralist.net

Professional Forum:
http://www.structuralist.net/cgi-local/yabb/YaBB.cgi

-----Original Message-----
From: sscholl2(--nospam--at)juno.com [mailto:sscholl2(--nospam--at)juno.com]
Sent: Wednesday, December 05, 2001 8:15 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: Flexible diaphragm

There is no problem with this, but some will consider the diaphragm to
be
not flexible and in my opinion it probably is pretty rigid and therefore
subject to requirement to do rigidity analysis.

Stan Scholl, P.E.
Laguna Beach, CA

On Wed, 5 Dec 2001 16:19:14 -0800 "Bill Marczewski"
<bmarczewski(--nospam--at)pndast.com> writes:
> I have a condition in an existing historic building where we may add
> concrete or masonry core walls for an elevator system.  The building
> has a
> wood floor system, which I am assuming to be a flexible diaphragm.
> This
> question may be a "no brainer" for many of you but here it is:  Is
> there any
> problem with a flexible diaphragm transferring load into a rigid
> wall
> system.  I only have previous experience with rigid floor systems
> transferring load into concrete/masonry shear walls.  Thanks in
>
> Bill S. Marczewski
> Peratrovich, Nottingham & Drage, Inc.
> Engineering Consultants
> Scow Bay Trading Building, 399, 31st Street, Suite A
> Astoria, Oregon  97103
> 503-325-1250 Main; 503-325-9789 Fax
> Office Locations: Anchorage, Seattle, Juneau, Astoria, San Francisco
> www.pnd-anc.com/; bmarczewski(--nospam--at)pndast.com
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