When to consider laterally supported

[JohnnnyBoy]

I'm trying to design a 40'-0" dropped beam supporting a mezzanine with an overhang. I believe in this case I would be able to consider the beam laterally supported if the joists are properly fastened to the beam although there is would be only top flange support and no bottom flange support. Would this still be adequate to prevent the effects of lateral torsional buckling or just prevent it by a factor and LTB should still be considered in the design. Obviously if it needs both top and bottom flange I would be able to design the bracing to be able to support the bottom flange but I'm just not sure if its necessary.

Any feedback would be much appreciated.

 

[KootK]

Yup, you should be good to go unless your joists are widely spaced or you bottom flange goes into compression.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[JohnnnyBoy]

Joists are spaced at 16" so should be good there. In the case that they were to be spaced at let's say 4'-0" I would just take the 4'-0" spacing in my calculations for LTB correct.

 

[KootK]

Exactly.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[theshearstud]

Not trying to get caught in the technical weeds, but aren't those buckling equations derived assuming torsional fixity at the supports? So all this is assuming that you have beams framing into the ends of your dropped beam (which would provide some level of torsional restraint), correct?.

 

[mike20793]

What kind of joists are they, cold-formed, wood? I'm assuming wood since you used the term dropped beam; are they are bearing on the dropped beam below? I don't think the connection will brace the bottom flange at your cantilever, but it certainly would brace the top flange at KootK mentioned.

 

[humanengr]

Is this a steel beam / AISC design? Take note of AISC requirements for lateral braces - must have adequate strength and stiffness as defined in the AISC specs.

 

[JohnnnyBoy]

In this case the beam would be connected to steel HSS members using angle iron and bolted to the column. This would assume torsional fixity if I am not mistaken as the columns would resist the induced torsion.

The joists are 14" TJI, yes they would be bearing on the dropped beam below. You are correct this would not brace the bottom flange but would still resist the beam from LTB from my understanding.

 

[JohnnnyBoy]

In this case the design is in Canada therefore it is ASTM.

Appreciate all the imput.

 

[KootK]

With regard to the bracing strength and stiffness requirements which can be tough to satisfy, you'll find this an interesting discussion: Link. I won't bullshit you however. In this scenario I'd just call the beam braced via judgement and carry on.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jayrod12]

And since the design is in Canada, I assume you mean it is CSA?

 

[JohnnnyBoy]

Great!

Thanks