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Re: Lateral restraint of steel beams at support locations

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----- Original Message -----
From: "Casano, Karen" <Karen.Casano(--nospam--at)dgs.ca.gov>
To: <seaint(--nospam--at)seaint.org>
Sent: Thursday, July 27, 2000 11:02 AM
Subject: Lateral restraint of steel beams at support locations


> I would like to get some opinions from those of you with structural steel
> design experience.   AISC ASD B6 (Specifications Section) states that "At
> points of support, beams, girders, and trusses shall be restrained against
> rotation about their longitudinal axis."
>
> Hypothetical situation:  A line of simple span WF roof girders is
supported
> vertically by interior TS columns.  At the support locations the
> perpendicular beams do not occur directly at the columns, but
approximately
> 4 feet away on either side of the columns.  The beams and girders are
> supported on the top flanges by metal decking (no concrete fill).  The
> connection of the girder to column is a standard bolted knife-plate
through
> a slot in the TS, or shear tab plate welded to the side of the TS column.
>
> 1)  Could these girders be considered adequately restrained against
rotation
> at the columns by virtue of the connections?

Technically Yes.  If you refer to the commentary for section B-6, this
section is intended for beams or girders sitting on top of the column.  With
a continuous column and knife plate as given the girders may be considered
adequately braced.  With the beam top flanges attached to the decking, and
the intersecting beams at 4' each side of the column, the entire assembly is
braced by the diaphragm.  (As you stated this is general, specific instances
require judgement).  If possible, I always prefer to place the intersecting
beams directly at the column.
>
> 2)  If a girder was a continuous or cantilevered member sitting on top of
> the column, could you consider the beams 4 feet away as providing lateral
> restraint against rotation at the column?

This is different.  with continuity or a canilever, the compression flange
is at the bottom.  The beam column connection would need to be rigid enough
to prevent rotation at the interface, thereby providing "continuity" of the
column through the connection.  Possible but not preferred.
>
> 3)  If this line of girders is also a lateral load collector line, and one
> of the girders is the top beam of a braced frame, could you consider the
> column connections to be adequately braced against lateral displacement
> under lateral loads?

I personally don't like this one.  The effective compression element in the
drag line would be braced 4' either side of a connection.  If you think of
this as an axial element similar to a column, the connection would need to
be considered similar to a column splice in the horizontal plane.  Possible
with continuity as in case two above, but I would need to be convinced of
the necessity of the framing layout.
>
> I realize that some of your answers may depend on the depths of the
members
> and/or the magnitude of the forces involved, but I'm not looking at one
> specific project.  I'm trying to get a general feel.
>
> Also, I know some engineers who use a "rule of thumb" that lateral bracing
> should be designed for 2 to 5% of the compression force.  Do many of you
use
> this?  I would like to get a feel about whether this is a generally
accepted
> practice.
>
This is the way I was taught.


> Karen Casano, P.E.
> San Diego, CA
>


Paul Feather



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