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.
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
* This email was sent to you via Structural Engineers
* Association of Southern California (SEAOSC) server. To
* subscribe (no fee) or UnSubscribe, please go to:
* Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you
* send to the list is public domain and may be re-posted
* without your permission. Make sure you visit our web
* site at: http://www.seaint.org