Holding down concrete to a metal roof deck for uplift?

[abusementpark]

I am designing a hurricane shelter and the owner wants concrete on the roof. We are doing 4" normalweight concrete on a vulcraft 1.0C non-composite 22 gage form deck over open web steel joists at 2'-0" on center. I need to find some way to positively secure the concrete to the metal deck for the high uplift.

Any suggestions?

 

[mmarlow]

I dont design in a huricanne region. But, maybe use a composite deck? The bond between the concrete and the ribs should be sufficient? Or a screw for fastening light gage steel to concrete? You could fasten from below. Or, a heavier slab on a bigger deck

-MMARLOW EIT

 

[kingnero]

Can you use welded studs on the web joists (after drilling through the deck plates?

 

[BridgeSmith]

For a secure connection, I would do as kingnero suggests and use automatically end-welded studs on the top flange of the joists. The 22 gauge steel deck may not be thick enough for a good connection to the concrete or the joists.

 

[abusementpark]

I thought about the studs, but have never done that before with open web joists. Can you weld studs to regular K-series joists? My joist size right now is 22K5.

 

[msquared48]

Talk to the joist manufacturer about the studs.

You could also use u-bolts at the panel points, unter the top chord, and protruded through the deck with washers and nuts as needed.

Mike McCann, PE, SE (WA)

 

[structSU10]

yes:

http://www.vulcraft.com/catalogs/composite-and-noncomposite-floor-joists

 

[Ron247]

I do not work with composite deck much but doesn't the composite ribs work for shear transfer laterally to get the composite action? I am not sure the ribs are designed to "bond" for vertical forces. IT would be best to study the ribs. They still occur on an inclined plane.

Stud transfer does sound more positive but you are resisting forces in a different direction than the stud is usually designed for.

What is your uplift per connection for components and cladding? 4" of concrete is only 50 psf while hurricane connection loads will be much higher. I think will you will wind up with a lot of connections.

 

[jayrod12]

Nelson studs are constantly used for developing tension forces. That would be my go to. Or more concrete, as that may result in a cost savings over the stud installation. How much uplift are we talking about?

 

[structSU10]

don't know if this makes sense, or if you can get it around you, but a steel edge creteplank welded to the joists may work for you too:

http://martinfireproofing.com/assets/secp_brochure.pdf

 

[abusementpark]

For an effective wind area of 10 sf, the ultimate components and cladding wind pressures are 131, 103, and 92 psf for the respective corner, edge, and interior zones.

 

[Ron247]

You may need more thickness as jayrod suggested. 4" concrete on 1" deck will have 3" in some areas and 4" in other areas. Time you leave clear cover clearance between stud head and top of concrete, plus the head thickness may not leave you much pullout concrete if you are in a 3" area. If the tension cone pops, connection is no good. Since you are 2' on center with joists, I would consider a lesser deck height if I can so I get more cone thickness. 5/8" versus 1" is still another 3/8".

 

[Ron]

The concrete will bond to the deck. You'd be surprised how difficult it would be to remove. That; however, depends on luck. I would go the Nelson stud route or use studrails.

 

[KootK]

- Rationally, I have a hard time imagining that the use of a composite deck profile wouldn't sufficiently bond the concrete to the deck. If you run some numbers on the the amount of horizontal shear transfer required of the ribs in order to effect composite flexural behavior, you'll find that number very high in comparison to the uplift demand at each rib form wind. Granted, the rib capacity is surely not the same vertically as it is longitudinally but, then, you probably only need a very small percentage of that value.

- Even if you get the concrete properly tied down to the steel deck, keeping the steel deck adequately tied down to the structure below may favor the use of nelson studs. If you're trying to tie your concrete down to the deck, I'm guessing that a tie down connection between the deck and the supporting structure is also required. It's not as though the steek deck itself will be adding much weight.

- If relying on deck ribs is likely to get you into a urinating match the the local AHJ over your ability to demonstrate capacity, I'd save yourself some grief and go straight to the stud solution. Are there studs used elsewhere on the project? For something relatively small like this, much of the cost of the studs may well be the mobilization and demobilization costs associated with bringing out the stud welding equipment. This might favor the use of screws or u-bolts as mentioned above.

 

[BridgeSmith]

The goal is resistance to wind uplift, presumably from air pressure on the underside of the deck, correct? That being the case, the uplift is on the steel decking, not the concrete. So, as KootK pointed out, bond between the concrete and decking is not the major issue; it's the connection between the decking and the joists. Providing a satisfactory connection to 22 gauge sheet metal seems questionable to me. It seems to me it would require either alot of self drilling screws or some fairly tricky welding. Granted, it's on a much larger scale and with different loading, but on every bridge deck I've inspected with corrugated steel decking, most of the welds were cracked.

 

[abusementpark]

KootK, you raise some good points.

I should note that this a very small building, approximately 2000 square feet. On a normal project of this size, I would never consider mobilizating a stud welding machine. I am not sure how bad that cost will be.

Also, my initial reading on composite joists indicates that the manufacturer is the one who designs the headed studs, which would add some cost on a small job, and also could be tricky since I’d have to get them to design primarily for uplift, not composite action.

And there will be a peer review done.

 

[KootK]

HotRod raises an interesting point as well. Unless/until air gets between the deck and the slab, there will be no force tending to separate them. There is no suction on the top, only a net push from the bottom. Unfortunately, it always seems like a bit of a dicey thing to rely on air not getting between things. If an upwards thrust on the deck bends the deck and causes deck/concrete separation at the deck bearing locations, it's all over.

Here's an idea, although likely not a great one. It ruins your deck continuity, makes deck fastening difficult, and may well be more expensive than mobilizing the stud welder. But hey, I already scanned it...

 

[abusementpark]

To follow-up: I spoke with someone at New Millenium and they said my project is too small to bother with an actual composite joist design (which he admitted was very rare even for large projects). However, he still recommended welding 1/2” diameter headed studs to the top chords of the joists to meet my needs for anchorage and designing the joists as non-composite, also specifying a minimum thickness of 1/4” for the top chords. He mentioned that he has seen this approach taken on several hurricane shelters in Florida.

I also spoke with a local steel fabricator who indicated I should assume the studs will be hand welded (too few studs to get the stud gun guy out there) and will cost about $5 per stud. I am estimating that it only adds about $3000 to the project, so I think I will just go with the studs, since that is what FEMA 361 depicts in one of their diagrams.

Thanks for all of the responses.

 

[chicopee]

Joist manufacturer advice would be my choice for recommendation.
Reading one of your comments"... Can you weld studs to regular K-series joists?" I am not sure that would be a good idea and instead what is done on bridge construction, the studs are welded to the metal deck which is thicker than what you propose for your project.

 

[BridgeSmith]

Not sure what bridge construction projects you've seen that done, chicopee, but on our bridges, the studs are always welded to the girder flanges, not the decking.

If I understand the project under consideration, attaching the concrete to the decking is the least of the worries. The uplift is primarily on the decking, so attaching the steel deck and concrete to the joists is the need. Studs are a good way to accomplish that goal. If the studs are to be hand welded, you might consider other options - channels, etc., since the main advantage of studs is in the ease of use provided by the automatic end-welding. Other types of connections may be easier to hand weld. I suggest discussing options with the fabricator or contractor who will be doing the welding, and see if they have a preferred option that will meet your requirements.