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RE: Rigid or Flexible Diaphragm Analysis?

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Better be careful........
Just because you are dealing with a metal deck roof does not guarantee that
it will behave as a flexible diaphragm.  The diaphragm behavior is
controlled by many variables.  Geometry of the diaphragm, quantity of
lateral resisting elements, and placement or location of lateral resisting
elements are just a few variable that will affect the actual lateral
behavior of the roof diaphragm.  If you are uncertain about its behavior you
should analyze the diaphragm for both types of behavior, and design your
lateral system for the worst conditions.  Some of your lateral resisting
elements will be controlled by a rigid diaphragm behavior, and some will be
controlled by a flexible diaphragm behavior.

-----Original Message-----
From: Matthew Stuart [mailto:m.stuart(--nospam--at)aespj.com]
Sent: Thursday, June 14, 2001 11:17 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: Rigid or Flexible Diaphragm Analysis?

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
area loading.  This project requires a lot of solid engineering judgement,
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
next month.  Comments and suggestions or leads to references would be
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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