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# RE: Diaphragm Chords

• To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
• Subject: RE: Diaphragm Chords
• From: Robert Rollo <rrollo(--nospam--at)TEAM-PSC.com>
• Date: Wed, 2 Dec 1998 17:02:15 -0600

Title: RE: Diaphragm Chords

-----Original Message-----
From:   Jim Kestner [SMTP:jkestner(--nospam--at)somervilleinc.com]
Sent:   Wednesday, December 02, 1998 4:31 PM
To:     Structural Engineers Association International
Subject:        Diaphragm Chords

Here is a question that I hope might generate some discussion........

Have any of you ever wondered if a diaphragm can work without having a
specific edge member, etc. assumed to resist the calculated chord force.
Can a portion of the diaphragm along the edge actually work as the
"chord"? It obviously has to be fastened together to resist those
forces.

This is similar to a large steel plate or concrete wall working as a
shear panel  with no edge members. The bending stresses are simply
resisted by the deep rectangular section with a  triangular stress
distribution which is maximum at the edge (assuming deep member theory
does not control).

I could see this not working for fluted metal deck in one direction
where the tensile chord forces would tend to open up the flutes.

To talk in practical terms, let's say we have a building with steel
joists, metal deck and load bearing masonry walls. The joists span to
the long walls (span parallel to the short walls).
The bond beams and the reinforcing are cut at each masonry control joint
(so that it cannot act as a chord).  [Robert Rollo]  I was always taught, and COE prohibits interuptionof your diaphragm level BBs ? How does the building work to

transfer the lateral forces. I believe the deck works as the chord
parallel to the long wall.  [Robert Rollo]  I believe you are right.  The first joist in from the short wall
(parallel to the short wall) probably works as the chord  in the other
direction. [Robert Rollo]   as a matter of fact, i have had respected engineers tell me that they used to place the first joist as colse as practical (6") from that wall for this exact reason, not to induce much horzontal bending into the top of the wall !

I have never tried this but, I know there are buildings out there like
this that were never detailed properly but nevertheless still work. [Robert Rollo]   Isn't that always the case, buildings have been sitting out there in 80/90 mph wind regions for 50+ years with not a problem, yet we couldn't make them calculate if we killed ourselves trying.  how about the old 4 bay stick framed steel buildings with no apparent lateral force system whatsoever, yet the veneer brick shows no cracking at all ! Must be partially restrained connections eh?

Jim Kestner, P.E.
Green Bay, Wi

I

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