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# Stiffness calcs for Moment Frames

• To: SEAOC Forum <seaoc(--nospam--at)seaoc.org>
• Subject: Stiffness calcs for Moment Frames
• From: "Kathleen A. O'Brien" <wildwoman1(--nospam--at)compuserve.com>
• Date: Mon, 5 Jan 1998 03:18:35 -0500

```I have in my possession  the following calculation for preliminary designs
of moment frames when we want to determine the drfit of a particular frame:

For pinned-at-base frames (and please excuse the crummy formatting):

drift = P*H(squared)/6E  *(H/Icol + L/Ibm)

H= height of column
L= length of beam
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= Icol*H/Ibm*L
everything else as above

Now here are my questions:

1.      Does anyone know where this comes from? It was something we used at
an office I worked in LONG ago. I feel a lot better using something when I
know how it was generated (this one looks as though it might have been
derived from the stiffness calc in the Amrhein masonry handbook).

2.      The K value - the document I have shows it as Ibm*H/Icol*L. I think
I (or possibly the person who gave me the formula) miscopied it, because it
wouldn't make sense to multiply the length of the column by the I of the
beam or vice versa(?) that would be like multiplying apples and oranges.

3.      Does anyone know of a more recent stiffness formula for moment
frames? We usually do moment frame calcs by hand (with some help from
Microsoft Excel) because:

a) we are a small office and don't do that many moment frames.
b) the ones we DO do are small and uncomplicated.

and it doesn't make sense to shell out beaucoup \$\$ for fancy shmancy
software.

Kate O'Brien, P.E.
Simi Valley, CA

```