Lateral Torsional Buckling

[brane23]

I am designing a beam supporting wood joists. A wood plate runs the top of the beam and is bolted through the top flange. The span is 25 ft. Are the wood joists considered a bracing point? This project is an upgrade and the existing beam was not braced and would be considered inadequate if the joists are not considered bracing. Thanks!

 

[SlideRuleEra]

Probably ok. See "Designing For Lateral-Torsional Stabiltiy In Wood Members" (free .pdf download) at this link

http://www.awc.org/pdf/tr14.pdf

http://www.SlideRuleEra.net

 

[cap4000]

Probably Fine. I normally have 1/2 inch diameter threaded nelson studs or anchor bolts welded to the beam top flange staggered at 2 foot on center. The key is the 10d nailing securely attaching your wood joists to the 2 x bolted plate. Typically a 2% lateral bracing force is normally the industry standard. What is your max bending moment in the steel beam?? I have alot of old research on beam and column bracing experiments including it done successfully with only rigid cardboard material.

 

[brane23]

Thanks! The max moment is 58.75 kip-ft

 

[UcfSE]

I wouldn't recommend using the beams as bracing unless you analyze the connection to be sure. Make sure the bracing force has somewhere to go once it gets into the wood joists also.

What some engineers do for this case is assume an unbraced length larger than the joist, such as assuming a brace only in the middle. It helps the beam design but is more conservative than hoping an existing connection using wood and probably toe nails will have the strength and stiffness to brace a steel beam. The AISC 3rd ed. lrfd manual has bracing design requirements. You should check those, imo.

 

[brane23]

I did and designed for a braced length of the beam span which is the most conservative. However, the existing beams would nowhere near meet the Lp requirement for unbraced length. So... it's a judgement call as to whether the joists give enough lateral support. Probably best to be conservative.

 

[SmithJ]

This is an issue that I have struggled with in my career. Is the wood plate on top of the beam bracing the beam or is the beam bracing the wood plate. Since we know we can't have it both ways, I prefer to err on the side of conservatism. If the wood plate is being entirely supported by the beam, how can we say it is bracing the beam. Bracing against what?

Unless I misunderstood the original configuration, I don't think the wood plate is bracing the top flange of the steel beam. Also, depending on your load (gravity vs. uplift) you might need to have the lower flange braced as well?

Thanks,
JS.

 

[brane23]

The wood plate is attached to the top of the beam. The joists are then attached to the top of the plate and staggered on 16" centers. Keep in mind I'm replacing an existing wood beam and extending the span. I would NOT use wood joists in this configuration. I'm not doubting the wood joists' ability to handle the axial load from the lateral force but instead the ability of the connection (nails) of the wood joists to the plate.