# RE: Rigid or Flexible Diaphragm Analysis?

• To: <seaint(--nospam--at)seaint.org>
• Subject: RE: Rigid or Flexible Diaphragm Analysis?
• From: "Matthew Stuart" <m.stuart(--nospam--at)aespj.com>
• Date: Thu, 14 Jun 2001 14:16:50 -0400
```ALL METAL DECK ROOFS ACT AS FLEXIBLE DIAPHRAGMS, IF YOU NEED MORE RIGIDITY
TO GET THINGS TO WORK TO YOUR LIKING INSTALL HORIZONTAL DIAGONAL BRACING IN
THE FORM OF STEEL ANGLES DIRECTLY UNDER THE DECK. MODEL YOUR STRUCTURAL ON
STAAD WITH THE DECK AS FE'S AND THE ANGLES AS "TRUSS" MEMBERS. YOU WILL GET
A PRETTY GOOD IDEA OF THE SHEAR DISTRIBUTION IN THE DECK AND THE DISTORTION
LIMITATIONS OF THE ANGLES.

-----Original Message-----
From: 	Bill  Marczewski [mailto:bmarczewski(--nospam--at)martind.com]
Sent:	Thursday, June 14, 2001 1:37 PM
To:	Structural Engineering Forum (E-mail)
Subject:	Rigid or Flexible Diaphragm Analysis?

I have a complex roof structure I am designing for a high end resort
condominium building.  The plan configuration is irregular, being described
as a "U-Shape" with 45 degree bends at the intersection of the horizontal
and vertical "legs" of the building plan.  The roof consists of structural
steel wide flange sections to carry gravity loads, and I am proposing to use
moment frames to resist lateral forces (unfortunately no walls are available
to use braced frames).  Metal deck will be used on top of the structural
steel to form the diaphragm.  Because of the buildings odd plan geometry,
and moment frame orientation to the orthogonal planes, I am reluctant to
regard this structure as a flexible diaphragm.  I am concerned with twisting
(torsion) at the 45 degree bends, and do not feel that the loads will be
distributed as assumed under flexible diaphragm analysis using tributary
and I hope to receive some comments from those of you who have designed
similar structures.  Computer analysis is not possible for verification of
hand calculations, as most of our programs will not perform a diaphragm
(rigid or flexible) analysis on a sloped roof system.  RAM Structural claims
to have this capability next year, but I will be complete with this project
greatly appreciated.  Thanks.

Bill S. Marczewski
Martin Design, Inc.
1360 S. Clarkson St.
Denver, CO 80210
bmarczewski(--nospam--at)martind.com

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