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Rho in Diaphragms[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
- Subject: Rho in Diaphragms
- From: Gerard Madden <GerardM(--nospam--at)CRJARCH.com>
- Date: Tue, 2 Nov 1999 09:19:00 -0800
I am starting a new thread for redundancy issues in diaphragms instead of the 10/lw topic from which it evolved. Darry H wrote: The diaphragm load is just what I've been confused about. In my case I've got a 1-1/2 story residential where the upper walls aren't in-line with the bottom walls. I'm trying to show that the lateral forces are being transfered to the lower exterior shear walls via the floor diaphragm and roof diaphragm. 16126.96.36.199 says to use 3*Ca/R for Fpx in equation 33-1. After this, I believe 1633.2.6 allows me to get 1.7 times more allowable design strength and 1612.3 equ 12-9 says I get to divide it by 1.4 again to calculate what exactly is the design strength needed for the floor and roof of this 1-1/2 story. I'm still trying to get E for equ 30-1 so it looks to me that I have to use rho. (To avoid further confusion I'm simply using 1.5 as the maximum default value for rho so I don't have to worry about it.) DH I think you have a many issues here. 1) You are using the SIMPLIFIED method, requiring the 3*Ca/R value for diaphragms. 2) You are describing a transfer diaphragm, where the vertical resisting elements are parallel but have an offset. That is a Plan structural irregularity per table 16-M item 4. You need to look at those sections as well. 3) Rho does not apply to collector design (Section 1633.2.6). You need to determine the force demand to the shear walls and multiply that by omega or determine the limiting force parameter to your shearwall (or brace) that may allow you to use a force significantly lower than that obtained by using the omega. Examples of limiting factors are overturning, diaphragm yield, shearwall or brace capacity. 4) I don't think you can divide by 1.4 even in ASD. There were previous posts about this and after examining it further, I believe this to be true. 5) NO WHERE in the code does it say that E applies to diaphragms, Fp does. For the diaphragm itself, you do not have to consider RHO. Look at it this way, How can you improve the redundancy of a diaphragm? I don't know either. 6) The code needs refinement in its wording, we need the Seismic Design Handbook Volumes II & III to be published immediately, and we need a 1"997 UBC for Dummies" to be published or "1997 UBC Cliff Notes" that says the intent of the code in plain english. I know these really helped me learning DOS and Shakespear respectively. I guess I better add a disclaimer to my posts, It seems to be the right thing to do and also a funny anecdote. Gerard Madden, P.E. Civil Engineer, Associate CRJ Associates, Inc. email: gerardm(--nospam--at)crjarch.com tel: 650.324.0691 fax: 650.324.0927 web: www.crjarch.com DISCLAIMER: 1st the Funny one (At least to me): The following post is property of the National Football League. Any Broadcast or re-transmission without the expressed written consent of the NFL and the San Francisco 49ers is Strictly prohibited. Now the Real One: These views are mine alone. These opinions should not be construed to be policy at or the opinion of CRJ Associates. AND FINALLY THE QUOTE: "It's looks like one huge shit sandwich, and we are all gonna have to take a bite" From the movie Full Metal jacket (Scene where the reporters from Stars and Stripes are meeting to discuss the turn of events in the Vietnam War at the TET Cease Fire) . p.s.: Sorry for the foul language, but it kind of sums up some of the frustrations interpreting these changes.
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