RESOLVE UNBRACED LENGTH (LTB) ON THE STEEL BEAM WITH METAL DECK

[Anrio]

Hi all,
I'm confusing about the ratio unbraced length for LTB, MAJOR & MINOR on the steel beam. There is the deck above the steel beam which connected with stud. The length of the beam is 9000 mm and the spacing of the stud is 250 mm
So my question is ,How much the ratio of the unbraced length for LTB, MAJOR & MINOR should I apply with that condition?
Pls kinda help me to solved it

 

[JedClampett]

Two things, Anrio:
[ol 1.]Your question is garbled. I think I know what you're asking (what do I use for an unbraced length for a beam supported by metal deck with studs, probably with concrete?), but I'm not sure. And from the massive response, neither does anyone else.
[li][/li]
[/ol]
[ol 2.] You double posted. That's against the rules. I kind of don't blame you, but still.
[li][/li]
[/ol]
[ol 3.] My answer would be zero. Usually the spacing of the studs is so close that zero unbraced length is acceptable.
[li][/li]
[/ol]

 

[BAretired]

1.

The top flange of the beam is continuously braced by the studs. If the beam is a simple span, it is adequately braced against lateral torsional buckling. The bottom flange is not considered to be laterally braced, so if the beam is continuous, it may need to be braced between the supports and the inflection points, i.e. where the moment is negative.​

2.

On the double posting, you can simply go back to the other forum and delete your post.​

BA

 

[Anrio]

Sorry for the mistake with double post, I have deleted the previous post.

So do you mean I have to braced the continuos beam on the negative moment.
If I braced the the bottom flange with web stiffener on both side,so could it be braced the negative moment? If not, have u any suggestion to solved the lateral torsional buckling on the negative moment?

 

[BAretired]

I think you need to provide a little more information. The beam is braced against lateral torsional buckling at locations where torsional rotation is prevented or at least restrained. It is conceivable that web stiffeners attached to both flanges would be sufficient provided that the top flange is restrained against rotation.

BA

 

[Anrio]

I have attached the beam condition on my ETABS.

The continuos beam that I was mention is beam 356x203x71.4.
So the matter is the beam ratio is high about 1.8. It's beacause the beam was determined by the program as simple steel ( not the composite beam). So the stud of the deck to beam is being ignored.
I have been check the report that the unbraced length of the beam was calculated by the program is 0.9xxx (although in the real condition the top flange of the beam is braced by the metal deck).

So can I reduced the ratio of the unbraced length ratio (LTB)?

 

[BAretired]

I have attached the beam condition on my ETABS.

I know nothing about ETABS.

The continuos beam that I was mention is beam 356x203x71.4.

That beam does not appear to be continuous; it appears to be a simple span.

So the matter is the beam ratio is high about 1.8.

What do you mean by beam ratio?

It's because the beam was determined by the program as simple steel ( not the composite beam). So the stud of the deck to beam is being ignored.

I don't know what the program is doing. Maybe someone else here knows. I assume the beam is composite. What thickness of concrete slab will you have acting compositely with the beam?

I have been check the report that the unbraced length of the beam was calculated by the program is 0.9xxx (although in the real condition the top flange of the beam is braced by the metal deck).

What do you mean by 0.9xxx? I thought you said the beam spans 9000mm.

So can I reduced the ratio of the unbraced length ratio (LTB)?

If I am not mistaken, your 356x203x71.4 beam is laterally braced against LTB, initially by steel deck and later by a composite concrete slab but your question is not clear; I can only guess what you mean by "the ratio of the unbraced length ratio (LTB)". I recognize that English is not your native tongue and I sympathize, but engineering requires precise wording in order for the meaning to be clear.

The beams shown in red, i.e. the 305x127x37 may be a different story. They each appear to be supported on two columns approximately 6000mm apart. Those beams are continuous and will presumably have negative moment over a portion of their length. In those beams, the bottom flange cannot be considered braced against LTB and will require lateral bracing in the negative moment regions.

BA