Most efficient way to create bearing surface for decking?

[StructureMan44]

An existing 6in slab is poured directly on beams at 5ft spacing without decking (the original contractor must have used removal/reusable formwork). The top of the beam flanges are embedded .75in inches into the bottom of the slab without shear studs resulting in a 5.25in deep slab over the beams. We need to remove the slab in some areas and replace it with 1.5in deep grating (I assume you can saw cut concrete directly over the flange of a beam?). This leaves a gap of 3.75in between the top of the beam flange(s) and the bottom of the welded metal decking. 1.5in deep decking is used to match the decking on the rest of the project.

What is the most efficient way to fill this 3.75in gap while also providing support for the metal decking? I am planning on having the contractor weld an HSS3.5x3.5x1/8in running parallel to the beam directly to the top of the beam flange and then weld a 1/4in plate on top of the HSS section. Is there a more efficient way to do this?

Also, is there a way to calculate how much load an HSS section can take compressing two opposite faces and is there a way to calculate if 1.5in is enough bearing distance for the metal decking?

 

[KootK]

You mentioned both grating and decking. I'll assume that metal decking is the name of the game here. I'd weld a 4"W x 3.75"H WT to the existing steel beams. Whether you use WT or HSS, don't sweat the compression capacity of the filler. It'll be much higher than the crippling capacity of the deck.

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.

 

[KootK]

Actually, I think that I prefer your way. Just put the plate under the HSS and weld those two items together in the shop.

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.

 

[Brad805]

Does the slab have capacity or are the holes small enough to allow you to build an angle frame that hangs from the top of the slab?

 

[wannabeSE]

Decking manufactures usually provide minimum bearing length. You don't mention anything about the deck (or grate) except it is 1.5" deep. For the filler piece, I would consider using a 1/4" bent plate shaped like a channel with the webs down.

 

[StructureMan44]

Thanks for the feedback, the concrete slab does not have decking supporting it from below between beams. Sorry if I wasn't clear. The grating needs to match the existing which is galvanized 1-1/2”x3/16” bearing bars at 1-3/16” on center with welded cross bars 4” on center. The gratings panels are 36” wide and cut to a length not to exceed 6’-5” so as not to surpass 1/4" deflection. Here is a link:

http://www.mcnichols.com/product/64...ries2&navCode=avc:AV-7905&navCode=avc:AV-8379

I asked the manufacturer for the required bearing length for the metal grating and they said it's up to the engineer, is there a way to calculate this? The floor loading is 75psf.


I'm weary of connecting to the existing concrete to support the new metal grating as it has plenty of cracks. Is there anyway to know which is the cheapest option: the WT (does this need stiffeners?), the HSS & plate, or the 1/4" plate bent into a channel?

 

[dcarr82775]

I have used the same HSS with a plate shop welded to make up the difference in a similar situation. No complaints from the contractor. I have also seen the WT as Koot mentioned, but I prefer the HSS

 

[wannabeSE]

NAAMM has recommendations on bearing area on page 15 in their Metal Bar Grating Manual. See the technical literature (free download) at

http://www.naamm.org/mbg/MBG_Technical_Literature.aspx

 

[KootK]

The WT doesn't require stiffeners. It will be a sensible alternative if you wish to clamp your grating down rather than weld it down as you would have edges to clamp to. Maybe that's what you had in mind for your top side 1/4" plate.

For grating, deck crippling isn't a possibility and steel on steel bearing capacity is extremely high. If there's a check to be done I suppose it would be crippling/yielding/buckling of the 1/8" HSS walls. To be honest, I would never have thought to check that if you hadn't brought it up. 1.5 in of bearing will be plenty.

None of the alternatives is cost prohibitive. Some thoughts:

1) You'd have to stabilize the WT while welding it.
2) HSS involves flare bevel welding which can have QC issues.
3) Bent plate requires some shop effort but can be welded down cleanly.

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.