Steel beam and metal deck unbraced length 4

[Elindio1]

I have a concrete slab supported on a secondary steel beam. The structural drawings that I have are not the final construction drawings. They do not show any studs or how the metal deck is attached to the steel beam. Now, as per " Fundamentals of Beam Bracing" by Joseph Yura "a cross member merely resting (not positevely attached) on the top flange can significantly increase the lateral buckling capacity. The restoring solution is sensitive to the initial shape of the cross section and location of the load point on the flange. Because of these difficulties, it is recommended that the restoring effect not be considered in design". My question is the following, Can deck puddle welds be assumed as the possitively attachment points and by doing so, can I develop the continuos lateral support of the beam to develope the full capacity of the section?

 

[haynewp]

First, I did not pose the question that way at all!!

Second, the whole previous justification in the replies above was that the diaphragm takes the load, now you are saying it is ok as long as it is not a spandrel beam?

 

[271828]

No, LOLOL.

I wouldn't use the deck to brace the beam, but I think the forces might be small enough that one could get away with it at interior beams.

Somebody else brought up the diaphragm stuff. Right or wrong, I don't think of this as a diaphragm strength and stiffness issue. I can see how the deck would bend/buckle locally enough to let the top flange translate a little. As with any stability bracing problem, once the member starts buckling, the required brace force shoots way up.

 

[haynewp]

Oh those crazy flutes {sighs}.

 

[haynewp]

I just got another response on this one. This is from a different professor who is also heavily involved in research with AISC. I explained the whole diaphragm idea with the flutes parallel and his response:

"I would not want to count on the deck for lateral bracing in the direction you are considering."

 

[haynewp]

I would like to open this one back up again for comments or any references on bracing girders with deck where the flutes are running parallel. I still have an unsettling feeling about this one.

I will admit I always used to assume a beam is braced whether the deck was parallel or perpendicular to the beam or girder as does the other engineers that I know. It seems to me that if you can assume a cmu or tilt wall is braced continuously by a roof deck with flutes running parallel along the top of the wall, then you should be able to assume a beam is braced also. Would you consider the same wall braced at the top if there were no joists or beams framing into the wall and the wall had to rely solely on the strength of the parallel flutes along its entire segmented length? Or would you put in additional cross bridging as I have seen done many times to help brace the top of the wall?

However, I am having a problem finding a research document that justifies this and addresses any local buckling of the deck that might occur in the immediate area of the beam. I think for 99% of the beams that go up, the force to brace the beam is low enough to assume the parallel deck is adequate for bracing, but I am only about 85% sure of this.

I have a situation now where there are roof beams along the edge with 3" long spanning deck running parallel

 

[haynewp]

For anybody that is still interested in this topic, I found the following in the AISC manual examples page III-25 regarding a composite beam under construction:

"During concrete placement, because the deck is parallel to the beam, the beam will not have continuous lateral support. It will be braced, at 10ft on center by the intermediate beams."

I am not saying this is all encompassing. The floor deck in the example he speaking of is 3" composite metal deck.