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# RE: Rigid frame deflections (again)

• To: "'seaoc(--nospam--at)seaoc.org'" <seaoc(--nospam--at)seaoc.org>
• Subject: RE: Rigid frame deflections (again)
• From: "Horning, Dick/CVO" <dhorning(--nospam--at)CH2M.com>
• Date: Thu, 22 Jan 1998 09:48:29 -0700

```I don't think your description ("tall, skinny frame") is accurate if
they are both the same height and cross-section.  The frame stiffness
would be influenced by beam distortion, so intuitively I would expect
the frame with the shorter beam to be stiffer, all other things being
the same.

> ----------
> From: 	Kathleen  A. O'Brien[SMTP:wildwoman1(--nospam--at)compuserve.com]
> Sent: 	Wednesday, January 21, 1998 4:35 PM
> To: 	SEAOC Forum
> Subject: 	Rigid frame deflections (again)
>
> 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
>
>
>

```