Composite Action?

[msquared48]

Got another project where I have to analyze some steel bar joists for additional load. There is metal deck over the joists and a 4" total thi kness slab over that. The 22 gage deck is composite with the concrete and the deck is welded to the joists. There are no shear studs.

In the analysis of another engineer a few years ago, he apparently assumed that the bar joist, deck and concrete were all working together in partial composite action, modeling the joist as a W section with the transformed section of a partially composite beam with nominal shear studs.

Personally, I have a real problem with assuming any composite action for the steel bar joist assembly, as one would have to rely on the weld between the 22 gage decking and double top chord angle of the joist. If the decking was 16 gage, I would have to think about it a little harder. Just a little. My feeling here may create a huge problem for the client.

How do others feel about this?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[structSU10]

That is hard to rely on. I guess you could use typical shear values for a puddle weld, but those will be so low, your % composite action will be very small, and might not be usable anyway.

 

[JoshPlumSE]

SJI and vulcraft have some newer catalog which go over using "composite" joists. And, there are a couple of manufacturers that specialize in this (Hambro, Vescom?).

It's been a couple of years since I looked at it. But, I seem to recall that the method of transferring shear between concrete and joist was not usually related to studs. I remember seeing one with a deformed top chord and another that relied on friction.

My point being only that using composite action for that system may not as unreasonable as it first sounds.

 

[shobroco]

I'm not sure what information you can get from Hambro on their designs, since they supply a proprietary "pre-engineered system" for your building layout, but you might try pursuing it. Their joists get their composite action by having the top of the web members buried in the slab, so it's much more positive connection than welded deck.

 

[Ron]

Without shear studs, best you can hope for is composite in one direction. Otherwise, no.

Puddle welds without weld washers and inspection are not reliable for design.

 

[hokie66]

You can't rely on composite action with this assembly. This is really clutching at straws. All composite beam designs depend on attachments projecting within the depth of the slab, while the puddle welds would be right at the bottom, not even into the deck flutes.

 

[msquared48]

Thanks for the confirmatin guys. That really helps. My other colleague at work feels the same way too.

Nothing like stirring up the pot for a little fun, eh?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com