Lateral Bracing of Compression Flange

[ironmon]

I have a W24x68 spanning 20 feet.
At the 1/3 points are W6x15's perpendicular at -4" below the W24's top flange on each side of the W24.

What is the unbraced length of the W24 for simple span bending?
20ft or 6'-8"?

Is there any AISC guideline for this type of framing?

In a framing scheme using simple connections the perpendicular members are usually framed to the web, just below the top flange, which would mean the compression flange of the main member is braced at that point.

However, can that connection be at any height in the web?

At what height does it no longer brace the compression flange for Lateral Buckling?

Thanks

 

[271828]

AISC 13th Edition Manual Appendix 6.

 

[civilperson]

Assuming that the connection plate is welded to the top flange, (or both flanges near an inflection point), and the web; then the W6x15's might be sufficient for a compression flange brace if the formulas (A-6-5) and (A-6-6) are satisfied. If factored moment is 7990 kip-in, then brace strength required is 2.8 kips for single curvature and brace stiffness of 5.8 kip/in.

 

[ironmon]

A friend of mine found a reference in AISC's Steel Interchange from Modern Steel Construction Magazine November 1994.

"A rule of thumb is that the connection for a brace should extend at least into the upper 1/3 of the beam depth to consider is as lateral support for the top compression flange"

Of course this is a judgement, in my case the W6 is well within the upper 1/3.

In a follow-up December 1994 MSC

"The sub-framing need not attach directly to the compression flange."

 

[msquared48]

The thing you have going for you here is the two bolt connection of the W6 to the W24. This will impart a small resisting moment in the force couple of the two bolts to resist the rollover of the top flange. Also, when attached to the web, usually there is a full depth shear plate or web stiffener (on the W24) that would also help distribute the moment and support the top and bottom flanges.

Mike McCann
McCann Engineering

 

[MarcbSE]

Mike:
The problem is that it is pretty hard to fit a two bolt connection in a W6. It just isn't deep enough. A W6x15 only has a "T" dimension of 4 1/2".

 

[civilperson]

T is 4 3/4". Gage of less than three inches is common on W6s. Use 5/8" dia bolts and 2 1/4" spacing or connect to the flanges with angles and then to the shear tab.

 

[271828]

Trying to use the W6 as a torsional brace will almost certainly not work because of the web distortional term in the required stiffness. Recommend to check that first before anybody strains something trying to figure out how to get 2 bolts in a W6 LOL.

 

[civilperson]

Beam bracing must prevent twist of the section. If the brace is attached near the compression flange, (-4" TOS W6 is in the top 1/3 as noted by ironman), then the brace is effective. The two bolt connection could be in a row parallel to the long axis of the W6 as well as at right angles.

 

[miecz]

Trying to use the W6 as a torsional brace will almost certainly not work. It would help if the OP would clarify if this is a lateral type brace (6.3.1), or a torsional type brace (6.3.2).

 

[271828]

I was typing about the idea started with Mike's post--using it as a torsional brace.

I believe the OP was originally typing about a lateral brace, which might work.

 

[miecz]

Found this on AISC's web site:

http://www.aisc.org/Template.cfm?Se...tDisplay.cfm&InterestCategoryID=402&FAQID=904

 

[ironmon]

Thanks jimiec, that's what I was looking for.

 

[ironmon]

Wow, I atually have a copy of the hand out notes from the lecture that is referenced in that link "bracing for stability" by Joseph Yura.

It's all in there....never thought to look.

I just have a copy given to me by anothe engineer, never attended the lectue.

Thanks again, that was a homerun.

 

[slickdeals]

Is there a way you could PDF those and upload them for public use unless there are copyright issues? I am sure we would all benefit