Return to index: [Subject] [Thread] [Date] [Author]

RE: Lateral restraint of steel beams at support locations

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Title: RE: Lateral restraint of steel beams at support locations

>line of simple span WF roof girders is supported
>vertically by interior HSS 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. 

It may be better to run the girder over top of the column. that knife-plate/slot connection is quite expensive.

>1)  Could these girders be considered adequately restrained against rotation
>at the columns by virtue of the connections?

Maybe, maybe not. It depends upon the torsional rigidity of the assembly. By assembly, I mean the torsional rigidity of the beam after you account for the increased flexibility at the connection. Essentially, you're depending upon the warping restraint of the beam to restrain the rotation.

>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?

It's essentially the same situation...depends upon the warping restraint. However, if the girder is stiffened over the column or a kicker is placed to restrain the column top, yes. Either of those details should always be used in cantilevered roof construction. Otherwise, the column stability may be unacceptably affected.

>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? 

Similar case again. Maybe there is also a flexural component of out-of-plabne strength resisting in this case too.

>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.

2 percent of the compressive force is an old rule of thumb for bracing member design. However, its shortcoming is that the brace needs also to be stiff enough, not just strong enough. Provisions that address both strength and stiffness have been provided in the new AISC LRFD Specification, which is being typeset now. The information added is consistent with that presented by Joe Yura and Todd Helwig in the AISC/SSRC seminar series on stability bracing. Once the new LRFD Spec is available, I will post the link to the *.pdf that gets posted on our web site.

Hope this helps.

Charlie