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Drag Bars at Slab on Metal Deck

Drag Bars at Slab on Metal Deck

Drag Bars at Slab on Metal Deck

(OP)
There's a project in my office that is in a low seismic area and is using conventional structural steel with composite slab on metal deck and ordinary braced frames. I was helping size base plates and bearing plates during some last minute detailing before the project went out the door and noticed all of the slabs on metal deck have #5 drag bars wherever there is a braced frame. Would someone be able to explain the reasoning for this? I am assuming the drag bars are being used to enhance the transfer of forces from the diaphragm (concrete on metal deck) to the braced frames but I have never learned about this method in school and would like to understand the reasoning behind it.

RE: Drag Bars at Slab on Metal Deck

If these bars are labeled as "drag bar", I would take the note out. BTW, what is the framing plan and reinforcing pattern in the composite slab? Also, what explanation was given by the designator?

RE: Drag Bars at Slab on Metal Deck

(OP)

Quote (r13)

BTW, what is the framing plan and reinforcing pattern in the composite slab? Also, what explanation was given by the designator?
The framing plan is conventional infill beams range from W12x40s to W16x50s. Girders are typically W21s or W24s. The slab on metal deck is reinforced with welded wire fabric. I am still confused as to the reason for the "drag bars" but since they appear on level 2-5 they must be serving a purpose... Just not sure what exactly

RE: Drag Bars at Slab on Metal Deck

I believe the designer think there is thrust along the braced frame, and that WWF alone is inadequate to address it. I don't agree with it, but not against it either, as it is a prudent measure. I would just call out the bar size and length, and spacing, if multiple.

RE: Drag Bars at Slab on Metal Deck

Quote (OP)

...I have never learned about this method in school...

Ditto. I've attempted to explain the fundamentals below. The whole thing tends to be non-obvious because there are multiple ways that designers can choose to tell the story of how diaphragm shears get moved around. It's kind of a choose your own adventure thing.

You may also find value in this free resource: NEHRP Steel Deck Diaphragm Guide. It focused on seismic but all the the fundamental aspects of load transfer will apply to non-seismic situations.

RE: Drag Bars at Slab on Metal Deck

(OP)
Thank you to everyone that responded! Kootk your explanation was very helpful. I’m going to check out the NEHRP manual you linked.

RE: Drag Bars at Slab on Metal Deck

Yep, what Koot said. Typically we do this with field welded shear tabs or other huge bolted monstrosities at the beam/column connections. Recently I used added bars in the deck instead of dumping axial through the beam/column connections, like OP is talking about, along the frame line because I had SidePlate frames orthogonal to my braced frame lines. Apparently a 700kip drag force through a SidePlate kills the SidePlate :)

R13 what do you mean thrust along the frame?

RE: Drag Bars at Slab on Metal Deck

Quote (dold)

Recently I used added bars in the deck instead of dumping axial through the beam/column connections,...

Quote (r13)

I believe the designer think there is thrust along the braced frame, and that WWF alone is inadequate to address it. I don't agree with it,...

Any difference of the above mentioned actions?

RE: Drag Bars at Slab on Metal Deck

@r13: your use of the word "thrust" threw me off the scent a bit. It's not wrong but it's also not commonly used in this situation in the space that is North American building structural design. Rightly or wrongly, in my mind thrust conjures up an inclined compression force requiring lateral restraint.

RE: Drag Bars at Slab on Metal Deck

Quote (dold)

Apparently a 700kip drag force through a SidePlate kills the SidePlate :)

This touches on a point that I raised in my sketch. In my mind, the slab rebar:

1) Does a fine job of providing a belt and suspenders path for the load being considered.

2) Does a less convincing job of steering the axial load away from the beam line when it's not wanted there. That, particularly because the rebar is typically designed so as not to yield rather that to limit strain.

In your heart of hearts, how confident are you that the combination of your slab rebar and beam-column connection detailing actually does keep axial out of those side plates for both tension and compression? I'm sure that an appreciable amount of either probably voids the research that underpins that system, at least from the perspective of Sideplate's legal department.

RE: Drag Bars at Slab on Metal Deck

Thrust is an axial force more concentrated in a narrow band.


I would agree with this arrangement (nothing to do with drag).

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