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RE: seaint Digest for 27 Jul 2004

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Mark,
You are correct that ACI does indeed address torsion in R/C spandrel beams. This is because you can often times not avoid torsion and torsion cracking in the spandrel beam. It is a function of the aspect ratio of the spandrel beam, and the fact that it is cast monolithically.

Plan checkers often push us to evaluate load paths and mechanisms that we often times ignore, or take for granted. I had a plan checker have me assess the stability of a beam. He did not accept Yura's 2% approximation rule. So I dusted off my Timoshenko, and showed that the beam was stable by a factor of 32.

Steel design practice has traditionally not checked torsion in spandrel beams because (as you pointed out), torsion in a spandrel beam is "self limiting". To address the problem posed by the plan checker, I would suggest the following: 1. Wide flange shapes are poor in resisting torsion. Even if the wide flange shape would resist torsion your end connections are probably not designed to transfer torsion into the column. 2. I would suggest that you evaluate the shear tab connection. I would suspect that you could assume zero moment at the shear tab connection to the web, you could then calculate the maximum moment at the center of bolts connecting the shear tab to the beam. You should have ample moment capacity at that point and throughout the shear tab.

Ultimately you will show that you have no torsion demand in the spandrel girder.

I hope that helps.

Regards,
Harold Sprague

From: Mark Gilligan <MarkKGilligan(--nospam--at)compuserve.com>
Reply-To: <seaint(--nospam--at)seaint.org>
To: "INTERNET:seaint(--nospam--at)seaint.org" <seaint(--nospam--at)seaint.org>
Subject: seaint Digest for 27 Jul 2004
Date: Wed, 28 Jul 2004 03:56:51 -0400

I have a steel structure with a simply supported wide flange girder
adjacent to a stair opening.  The loads from the beams come primarily from
one side and are transferred using a standard AISC shear tab connection.
There is metal deck with concrete fill on top of this girder.

The plan checker is asking me to address the torsion on this beam.  My
practice has been to not explicitly address torsion in these cases since
whatever torsion there may be will be self limiting.  While the beam may
initially apply a torsion to the girder, if the girder attempts to continue
to rotate, the beam to girder connection and the beam stiffness will act to
resist any torsion.  ACI specifically addresses this situation in ACI 318
but I do not believe that AISC does.

I am trying to figure out a polite answer other than "NO", that will not
take too much time.  Probably the best answer would be to reference a
position by AISC.   Input would be appreciated.


Mark Gilligan

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