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Re: Rigid or Flexible Diaphragm Analysis?

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Depending on the geometry and span to depth ratio of the diaphragm, metal deck roof systems will definitely vary somewhere between rigid and flexible.  As a point of reference, flexible diaphragms are defined in the UBC as diaphragms with twice the deflection of the vertical elements of the lateral system.  The tri-services manual provides numerous examples of estimating diaphragm flexibility, and most of the deck companies have derived their published factors from it.  If you do not have the tri-services manual, contact the deck company you intend to spec and they should have additional information and excerpts they can give you.  Jeff Martin at Verco is always a good source of information.
The first approach is to look at the system both ways, rigid and flexible, and evaluate whether a full envelope design approach is within the realm of economy.  If the frames are designed for worst case distribution, it will not matter how the diaphragm behaves as long as the diaphragm is designed to accommodate either behavior.  Face it, most rigid diaphragms are "detail designed" with chords, drags, and simple beam analogies the same as flexible diaphragms.
What is the roof slope?  If you look at one side of the roof slope, use the flexibility factor analysis of the one side, combine it with the opposite side, and then start looking at the amount of reduction in stiffness you must assume is contributed by the lack of planarity to go from one behavior to the other and you will start to get a reasonable feel for the range of realistic behavior for your conditions.
Paul Feather
----- Original Message -----
Sent: Thursday, June 14, 2001 10:37 AM
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