Braced cantilevered beam above the bottom flange

[laneng]

I was recently performing a structural survey of an existing building in order to find the capacity of the roof. In one of the middle spans, a W24 beam cantilevered both ways over the columns to support the beams in the adjacent bays. At the beam column connection, an 18" deep bar joist had a bottom chord extension that was intended to brace the bottom flange of the beam. However, the bottom chord extends straight and braces the beam 6" above the bottom chord. Is the cantilevered beam considered braced at the column?

 

[XR250]

Probably

 

[laneng]

My mistake, since the joist has a 2 1/2" tall seat, the beam is braced 8 1/2" above the bottom flange.

 

[BAretired]

Doesn't sound like much of a brace.

BA

 

[structSU10]

Attached is a paper describing the effectiveness of such a system. Besides an analysis with an advanced FEM program, analysis per AISC appendix 6 should be able to give you an idea. I would this its inadequate to be considered a brace point.

 

[msquared48]

There should be an end plate or full depth stifffener plate each side of the steel beam to which the angle brace from the joist can be angled down and bolted to the stiffener within 2 inches of the top of the bottom flange. I would accept nothing less than this arrangement.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[laneng]

It should be noted that this is an existing building, built in the early '70s. The backspan is 50' and the beam is cantilevered 10' on each side. If I assume a that there is no bracing whatsoever, the beam should have collapsed under only the dead load long ago yet site has seen its share of snow over the years. I feel the compression flange is braced at some level, if only because it hasn't fallen down, just not fully braced. Does anyone have a resource that would give some indication on the level of which the beam is currently braced?

 

[Hokie93]

What C[sub]b[/sub] factor are you using? The 14th edition AISC Steel Manual provides an equation (C-F1-5) for C[sub]b[/sub] in the Commentary to Chapter F that is applicable to your situation/geometry. The same equation is also provided in Yura's bracing seminar notes. With this equation, your C[sub]b[/sub] factor could be considerably greater than 1.0.

 

[laneng]

I found an article in Modern Steel Construction that states that bracing the top flange is better than bracing the bottom flange for a cantilevered beam. The top flange of this beam is supported every 6'-3" by the joist.

[link

http://www.modernsteel.com/Uploads/Issues/May_2010/052010_May_10_si_web.pdf

]

http://www.modernsteel.com/Uploads/Issues/May_2010/052010_May_10_si_web.pdf

[/url]

 

[structSU10]

If you look at my earlier attachment you will see in a lot of cases its the column-beam interface that starts to become an issue, with local buckling problems at the column within the web of the beam. Basically without a stiffener at the column, or proper bracing, capacity can be much lower than anticipated. This notion is also discussed in AISC design guide 24, section 4.2

 

[BAretired]

I found an article in Modern Steel Construction that states that bracing the top flange is better than bracing the bottom flange for a cantilevered beam. The top flange of this beam is supported every 6'-3" by the joist.

http://www.modernsteel.com/Uploads/Issues/May_2010/052010_May_10_si_web.pdf

I don't think that Modern Steel Construction is suggesting that compression flange bracing may be omitted because the tension flange is braced. That would be wrong.

BA