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# Re: Rigid frame deflections (again)

• To: <seaoc(--nospam--at)seaoc.org>
• Subject: Re: Rigid frame deflections (again)
• From: "T" <vicpeng(--nospam--at)vtcg.com>
• Date: Wed, 21 Jan 1998 18:42:30 -0800

```Yes I agree

Thor Tandy
Victoria  BC
-----Original Message-----
From: Powers, Tony <tpowers(--nospam--at)hdrinc.com>
To: seaoc(--nospam--at)seaoc.org <seaoc(--nospam--at)seaoc.org>
Date: Wednesday, January 21, 1998 6:00 PM
Subject: RE: Rigid frame deflections (again)

>Kathleen:
>
>I may be missing something, and I have done no calculations, but (no
>offense) I think it might be you.  You said:
>
>>  the tall skinny frame is stiffer than the short fat one.
>>
>If both frame are 10.5 ft. high, then it's not a question of a "tall
>skinny" vs. "short fat" frame.  The one with the 7 ft span is stiffer
>because, for the same height frame, it has a shorter, and therefore
>stiffer, beam which makes the entire frame stiffer.
>
>Anybody agree?
>
>Tony Powers
>
>
>> ----------
>> From: Kathleen  A. O'Brien
>> 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)
>>                 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
>>
>>
>>
>
>
>

```