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RE: Rigid frame deflections (again)
[Subject Prev][Subject Next][Thread Prev][Thread Next]- To: "'seaoc(--nospam--at)seaoc.org'" <seaoc(--nospam--at)seaoc.org>
- Subject: RE: Rigid frame deflections (again)
- From: "Bill Allen, S.E." <BAllenSE(--nospam--at)pacbell.net>
- Date: Mon, 26 Jan 1998 06:27:23 -0800
Once you had the frame forces, couldn't you solve the frame drift by virtual work by placing a unit load at the top of the frame and solving in terms of L, h, Icol and Ibeam? Regards, Bill Allen -----Original Message----- From: James F Fulton [SMTP:James_F_Fulton(--nospam--at)rohmhaas.com] Sent: Monday, January 26, 1998 5:45 AM To: seaoc(--nospam--at)seaoc.org Subject: Re: Rigid frame deflections (again) re-send ______________________________ Forward Header __________________________________ Subject: Re: Rigid frame deflections (again) Author: James F Fulton at CCMCED01 Date: 1/22/98 3:37 PM Kate, when you asked the questions earlier this month, some responses indicated that you could refer to Kleinlogel's book and find formulas for rigid frame DEFLECTIONS. I've got a copy of the 2nd American Edtion, Second Printing (1964), and in that edition at least, there are only equations for frame forces and moments--no deflections. I looked in a couple of other references on my shelf (incl Roark) and can find no DEFLECTION formulas for rigid frames. I would be interested in knowing if and where others find deflection formulas for rigid frames. I've got a copy of the Interchange article, and I used slope-deflection theory to derive the equation given there for the pinned=end frame. However, based on my derivation, the equation in Interchange is incorrect in that the factor in the denominator times Ibm should be "1" , not "2", which incidentally is what you quoted in your original note. (You remember, the one where you did not want to use a "fancy shmancy" $$ computer program for these "small and uncomplicated" frames) Well, it looks to me that defermining frame deflections, as opposed to forces and moments, for even simple rigid frames is not trivial and there does not appear to much information (equations) for these structures in the texts either. So a computer program appears to be the way to go, even for a statically determinant frame, if one's interest include deflections. ______________________________ Reply Separator _________________________________ Subject: Rigid frame deflections (again) Author: seaoc(--nospam--at)seaoc.org at Internet Date: 1/21/98 7:35 PM Guys and Dolls: Now that I have the Steel Interchange sheet from Modern Steel Construction (4/93) which shows the quick hand-calc for rigid frame defelction, I have started using it. HOWEVER, I have noticed something very odd. Extremeley odd. The formulas for pinned and fixed bases follow (and please excuse the crummy formatting): drift = P*H(squared)/6E *(H/Icol + L/2Ibm) where P = load H= height of frame L=span of frame Icol= I of column Ibm= I of beam E=29000ksi For fixed-at-base frames: drfit=P*H(cubed)/12EIcol * (3K+2/6K+1) where K= Ibm*H/Icol*L everything else as above Now here is the strange part. I have two frames along line D in my project. One has a 17 foot span and the other a 7 foot span (L). Both of them are 10.5 feet high (H). And I am assuming: P = 1 kip for each frame (just getting relative rigidities at this point). Relative rigidity R is the inverse of the drift; i.e. 1/drift For both fixed and pinned at base calcs, my 7 foot long frame is stiffer than my 17 foot one. I have hunted HARD for errors in my calcs and I can't find any. I have done the calc both by hand and with a spreadsheet and I still get the same answer: the tall skinny frame is stiffer than the short fat one. I do not buy this. Has anyone else had this problem? Is it me? Is it the calcualtion?? Kate O'Brien Simi Valley, CA RFC822 message headers.txt >>
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