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Steel deck as lateral bracing

Steel deck as lateral bracing

Steel deck as lateral bracing

(OP)
Hi,

When you have beam parallel to joists, there is no lateral support between the columns except the steel deck.

Do you use steel deck as effective lateral support to the top flange of the beam?

Which equation do you use?

Thanks

RE: Steel deck as lateral bracing

yes, the steel deck acts as lateral support for the top flange (assuming its connected of course)
I believe you'll need eqs A-6-7 and A-6-8 of AISC 13

RE: Steel deck as lateral bracing

also assuming the deck is spanning to the beam, not parallel to it.  I would not use it as bracing if it is spanning parallel to the beam in question.

RE: Steel deck as lateral bracing

Very true StructuralEIT. I just assumed deck was perpendicular since he said the beam was parallel to the joists.

RE: Steel deck as lateral bracing

Some further thoughts here -

If the deck has a concrete infill - a typical floor beam - then, once the concrete hardens, the direction of the deck would become immaterial to the lateral support of the top flange.  The flange would be continouously laterally supported, assuming composite action.

If it is a typical roof beam with no concrete infill, then I agree with the comments above, assuming you are not using decking less than 1.5 " in thickness and 20 or 22 gage.  I would not rely on 3/4" Formdeck.

Mike McCann
McCann Engineering

RE: Steel deck as lateral bracing

(OP)
Thanks everyone,

In a X bracing under seismic loads, the horizontal beam is in compression or in tension.  Do you still use the steel deck (without concrete infill) as a lateral support on the top flange?  In this case, the bottom flange of the beam is only laterally supported at his supports.

RE: Steel deck as lateral bracing

Top flange, yes since the deck is there.  

Bottom flange - could brace with purlins, but don't forget the effect of the moment inflection point that could be considered a lateral brace point for the bottom flange too.

Mike McCann
McCann Engineering

RE: Steel deck as lateral bracing

AISC 13th specifically states "In members subjected to double curvature bending, the inflection point shall not be considered a brace point." (Appendix 6 section 3)
Though if you brace near the inflection point you can substantially reduce the moment that must be carried by the much longer unbraced length.

RE: Steel deck as lateral bracing

I think he is asking about the whole beam being braced as a compression member in the braced frame.  I would say yes for weak-axis buckling (assuming your beam is oriented such that it is bending in strong axis) and no for strong axis buckling (the deck just doesn't have any bending stregth in the direction necessary to brace the beam as a compression member).

RE: Steel deck as lateral bracing

I disagree that steel deck will not brace a beam if it is parallel to the beam.  Steel deck can take load parallel OR perpendicular to the flutes--that is why it can function as a diaphragm.

DaveAtkins

RE: Steel deck as lateral bracing

I remember having this exact discussion some time ago and I don't remember the outcome, but I don't feel comfortable saying that metal deck (without concrete poured in it) will brace a beam if the flutes are oriented parallel to the beam it is supposed to brace.

RE: Steel deck as lateral bracing

I think there is a difference between a diaphragm shear force perpendicular to the flutes and a true compressive force (a brace force) perpendicular to the flutes.
Unless you can show (using AISC 13th Ed. Appendix 6) that the deck can take the brace force in compression, I would say forget it. Even if you can show it does, I wouldn't count on it.

RE: Steel deck as lateral bracing

I am not sure, but I have seen that some jurisdictions do not allow you to assume the deck acts as a continuous tie/strut for a roof diaphragm resisting seismic forces from the exterior walls in the direction perpendicular to the flutes. This may also be under the IBC seismic section now.

The point is they recognize some lack of stiffness of the diaphragm in that direction for functioning as a tie/strut, I do not know how this equates to bracing beams with the same parallel deck. It likely works for most all cases but I am sure there are some that it wouldn't. For instance, a large composite girder with wet concrete and 22 gauge form deck running parallel to it.

RE: Steel deck as lateral bracing

Tying the top of a wall to the roof diaphragm on the walls parallel to the deck flutes isn't done directly through the deck. it is taken through the edge angle or other edge member which transfers it to the joists (member perpendicular to the deck flutes) which then redistributes it (through bending) across the deck diaphragm.
The accordian shape of the deck is not going to resist a substantial amount of lateral force applied perpendicular to the flutes. Like the accordian, press on the sides and it collapses.

RE: Steel deck as lateral bracing

I know, that is what I was trying to say. And I believe you have to detail the joists to carry the tie forces across the entire diaphragm. But in the girder direction, the steel deck is adequate as a strut/tie itself, last I heard. I am saying I agree with you at to some extent. But I bet my life on it there are plenty of engineers out there that will argue with you all day that the deck does brace ANY beam once the it is welded down, whether it is running parallel or perpendicular to the beam.

Local buckling of the deck in the area of the beam is what I would worry about, whether it is part of a diaphragm or not. But just based on the number of beams that have been assumed braced this way over the years, my idea is that deck running parallel to a beam works well enough as a brace in (most} cases.

RE: Steel deck as lateral bracing

What is your beam span and bending moment. I have some journal articles from AISC and George Winters who has performed many lab experiments regarding bracing of beams and columns. In some cases he used short pieces of "cardboard" and found that the cardboard was strong enough to function as a brace.

RE: Steel deck as lateral bracing

For typical beam top flange bracing under gravity loads, I use the deck that is adequately fastened.

For brace frame beam buckling restraint during axial compression, I dont rely on the deck and rather frame interior infill beams perpendicular to the brace.  I connect the two with a detail that also restrains the bottom flange.

RE: Steel deck as lateral bracing

Poul75

I would use an attached deck (in either direction) to brace the member acting as a beam, but would provide some other lateral support to the member when acting as part of a frame.  For metal deck bracing a beam see thread507-186125: Steel beam and metal deck unbraced length.

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