Cantilevered Beam Backspan- Unbraced Beam Length

[chief45]

- Have several cantilevered steel beams which naturally have negative moment over a portion of its backspan.

- The top flange is continuously braced by a composite slab. However, the bottom flange (currently) is unbraced.

- Trying to determine what unbraced length should be used when checking if the beam has the capacity to resist the negative moment.

- Attached you will find a partial framing plan, bending moment diagram, and a blow up detail which illustrate the condition and the unbraced length I’m trying to determine.

Any help you could provide on this would be greatly appreciated.

Thanks

 

[BAretired]

Agreed. The code is a minimum standard which must be satisfied.

BA

 

[JoshPlumSE]

I've gotten into this discussion with many folks over the years.... most of whom thought that the RISA program should have automatically recognize the inflection point as a brace point.

The code provisions are now pretty clear that inflection points cannot be considered a brace point. Hence the "proper" design would be to use the entire backspan length as the unbraced length.

That being said it was a common design practice for years and years to consider the inflection point as a brace.... But, only if you also used a Cb of 1.0. That is an important caveat.

Since Cb is fairly large for beams with a moment diagram like this, the old design practice is not necessarily un-conservative.

 

[w12x26]

I am curious to see the load diagram for this problem. Obviously this a building structure and I really do not understand the relatively high loads.

 

[miecz]

JAE-

Without the beam size, beam spacing, deck thickness, deck, reinforcing, and concrete strength, I can only say it "might" work. I'm not even sure there are shear studs on this beam.

chief45-

It's still not clear to me. Are there shear studs connecting the beam to the slab?

 

[BAretired]

If it is a concrete slab and composite deck, the deck could be simply spot welded to the beam flange.

BA

 

[miecz]

Even if the intent was not to design this beam as composite, the designer could detail studs for the top flange. Those studs might be cheaper than adding bracing to the system. The attached sheet shows how the slab could then provide a continuous torsional brace, according to Appendix 6.

 

[BAretired]

miecz,

Interesting idea. I would not have thought it would have been possible to get enough stiffness out of the web of the beam.

I am impressed with your calculation but still prefer a brace at the inflection point and at the end of the cantilever.

BA

 

[Lion06]

If you can provide shear studs into the slab, just add a full depth stiffener at the inflection point and the bracing is taken care of.

 

[miecz]

StructuralEIT,

The op stated that typical beams on the project are composite with the slab. I agree that a full depth stiffener can work, but that sidesteps my assertion that the system can meet Appendix 6 without a stiffener.