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HOLLOW CORESLAB to STEEL BEAM Connection to resist Lateral Torsional Buckling 4

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Vrpps EIT

Vrpps EIT

Structural
Aug 21, 2018
58
CA
Hi,

Its a 8" Hollow coreslab with 2" Concrete topping connection to a W10x33 steel beam. The connection (attached the connection detail) doesnt show shear studs on the compression (top) flange of the steel beam to resist lateral torsional buckling. However, it has fin plates and 4' Long strands of 1/2" dia bars connected and grouted in the joint, does this connection act as a restraint for the top flange of the beam to resist the buckling of the beam. It would be great if someone could help me understand this.

 
 https://files.engineering.com/getfile.aspx?folder=478a9847-155e-4b0d-8d4d-bcc1b8d6fe83&file=Connection.JPG
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IMHO, the grouted fin plates will work nicely to prevent LTB. Bracing has to keep the entire compression flange from "moving sideways" and trying to rotate. The beam's unbraced length becomes < 4', short enough to keep LTB from being a problem for any acceptable loading. This image gives an idea of how an unbraced beam moves and deforms due to LTB:

main-qimg-3b3b55683f6c533f2d85996e75ac7478-c_qoq7ts.jpg


[idea]
[r2d2]
 
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Looks pretty good to me: the grout would press against the fin in compression (depending on which way it wanted to buckle), and that would press against the slab. The only concern I would have is: where does the slab go off the page? If it ties into the lateral force resisting system and has a degree of stiffness and strength (acting as a diaphragm)......you should be good to go.
 
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Thank you very much! Is this a common type of connection detail for commercial projects where hollow core slab has to be connected to W-beams
 
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@WARose , yes the hollow core slab goes and connects with the foundation wall (tied down to the foundation wall - lateral force resisting system & act as a diaphragm).
 
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Around here where we do an insane amount of hollow-core on steel, we have 4" nelson studs welded to the beam instead of the fin plates at each hollow-core joint. then if there is a situation where the plank joints don't line up on each side of the steel beam, there is something to hook the joint reinforcement around.
 
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Now that I think about it a little more: I'd be a little worried about the grout getting in such a small gap (3/8"). I'd give some QC guidance to the field.

As a alternative, you might could check a minimal dead load friction transfer. If it's got that, you may not even need the fin. It doesn't take much to brace this stuff. (see Appendix 6 in AISC.)
 
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We use the Nelson studs as well.

Usually I try to choose a beam size which doesn't need to be braced. The labor in stud attachment and handling difficulty of a studded beam probably pays for a 50% increase in beam weight.
 
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I'm a precast engineer some of the time. Where I practice, it's usually weld plates on the bottom of the planks. For what it's worth, I'm also not a big fan of the wast strand detail. In most modern buildings, and with precasters strategically setting out plank to avoid partial width units, it's pretty rare for the grout keys to align either side of the beam. And breaking the cores to grout the strand in is a fair bit of work. We break and grout plank left, right, and center and tell ourselves it's as good as if it were monolithically cast that way. I'm skeptical of that and prefer to mess with the "product" as little as possible.

c01_s9ox0m.jpg



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
 
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WARose said:
As a alternative, you might could check a minimal dead load friction transfer.
Click to expand...

Friction wouldn't fly for me personally. I very much prefer a positive connection. The last thing that I'd want is an earthquake generating some vertical acceleration and a girder rolling over and dumping a bunch of hollow-core into occupied space.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
 
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KootK said:
Where I practice, it's usually weld plates on the bottom of the planks.
Click to expand...

So you're telling me they'd rather flip a plank over, grout in a weld plate, flip it back over, and then erect it? Seems like way more steps than just coordinating with the steel guy to have enough nelson studs on the beam.
 
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jayrod12 said:
So you're telling me they'd rather flip a plank over, grout in a weld plate, flip it back over, and then erect it? Seems like way more steps than just coordinating with the steel guy to have enough nelson studs on the beam.
Click to expand...

In a wet cast system they place the embed while the concrete is still plastic in the casting bed. No additional handling. I was initially shocked that a bottom side embed was even possible in the casting bed yet alone commonplace. It is commonplace though, I assure you.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
 
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I was always under the impression bottom side casting was a no go... nice to know that was a lie
 
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jayrod: It depends on the precaster. Top-side casting is generally a no-go. Top side is generally the open side in precast slab formwork and thus any embeds need to be carefully suspended in air via rebar or via some wood spanning over the top of the form which gets in the way of the forming machine or troweling. In actuality the shop guys probably just wet set anything on the top-side when I'm not looking...

We will often cast panels and things upside down and flip them after stripping if there are a lot of top-side embeds.

KootK made an important distinction that is worth noting. He mentioned wet-cast hollow-core and which is slip formed or otherwise non-extruded; for these you could place embeds in the bottom and the slip form machine can ride over them assuming appropriate clearances. By my understanding, this does not work with dry-cast (extruded) precast machines; but I could be wrong as I have no practical experience with dry-cast concrete.

Really it just boils down to the capabilities of the local precaster(s).

Ian Riley, PE, SE
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
 
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That makes sense. I'm fairly certain both of the big pre-casters here use extruding for their hollow-core.
 
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One thing to also consider if you design the beam braced is it will likely not be braced during the erection of the pre-cast. I got lucky once and caught this during shop drawings and had to upsize a couple beams.

I see the embedded soffit plate in my area, usually on alternating planks and alternating ends so they aren't positively attached at both ends allowing for some shrinkage and expansion of the planks.

Open Source Structural Applications:
 
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Celt83 said:
usually on alternating planks and alternating ends so they aren't positively attached at both ends allowing for some shrinkage and expansion of the planks.
Click to expand...

I've always considered this to be a not so clever sham. By the time that one grouts the keyways, and possibly adds a topping, you'll have restraint to all planks regardless of any skip placement of the embeds. And really, what else would we expect? Somehow the same mechanism that does give us diaphragm capacity somehow does allow relative slip between planks? KootK Revise & Resubmit.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
 
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"...it will likely not be braced during the erection of the pre-cast...had to upsize a couple beams."

Loading must have been mostly selfweight, right?
 
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KootK: hmm honestly hadn't given it that much thought was years ago and haven't had another plank job since, good point to consider on the next one I deal with. Thanks.

HotRod10: yeah small residential project self weight of plank was almost double the superimposed DL and Live loading.

Open Source Structural Applications:
 
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