Uplift on Composite Concrete Slab

[cmrazik]

I am desiging the roof of a solvent storage room. Because I need to design for a 100psf force in the event of an explosion, there is a net uplift on the roof. I am not concerned about the deck-to-beam connection, I'll just design the welds for the uplift force. My concern is the bond between the concrete and the composite deck. Are there tables that show the uplift capacity of composite decks anywhere?

Thanks so much for any help you can provide.

 

[jike]

Why don't you use headed studs welded to the beams?

 

[cmrazik]

My supervisor doesn't think it is necessary. But that was my first choice.

 

[JAE]

Is there any suction on the concrete pulling away from the deck? If the explosion is below the deck, wouldn't it all be a surface pressure on the underside of the metal deck and thus, no separation? Or is this an impact effect with quick displacement that would try to pop the concrete off?

 

[cmrazik]

I am thinking along the lines that it will be an impact-type load. The explosion is from inside the building, so yes it is below the deck.

 

[dik]

Do you just need a light WWM layer to hold the concrete down to keep it from flying free?

Dik

 

[sundale]

My knowledge of blast loads is pedestrian at best, but 100 psf sounds small: these are psi type loads. Moreover, I think the design philosophy is that you WANT the roof to be blasted off, so as to direct the explosion force upwards in lieu of sideways to reduce shrapnel effect (if it is a detached building). Not an expert here...

The delamination of the concrete from the composite deck is your concern here and I do not have a good answer for that! Talk to Vulcraft about it, but this is a strange load case.

I agree with jike that HAS on the beam flange is a cheap and positive connection. An A706 DAS welded flat to the beam flange and lapped to rebars in the slab would also work good. These could maybe simply satisfy catenary tensile forces in lieu of a traditional flexural design (i.e. every few feet on center, WWF catenary transverse between rebar lines). The WWF might work by itself as catenary reinforcement.

You will need to assume some "sag"

 

[cmrazik]

All of you have some very good points. Thank you for your input.

I do have WWF in the slab so the slab should "stay together." My walls are CMU, so I am thinking of using #4 L-Hook bars at 2'-0" o.c. to attach the concrete deck to the sturcture.

Does this seem like a reasonable solution?

 

[dik]

Anchorage to masonry should be cautioned... There was a water treatment plant in Lindsay, Ontario that had a roof blow off and it lifted the first course or two...

Dik

 

[miecz]

cmrazik,

Under uplift pressure, the metal deck doesn't act as reinforcing steel. I'm surprised that WWF is adequate to take the tensile forces at the top of the slab. Are you counting on the non-composite metal deck to handle the net uplift pressure?

 

[AggieYank]

cmrazik, please don't be offended at all if I'm off here. I get the impression your boss took on a project involving blast loads, which is probably an area he doesn't have expertise in.

100 psf is a large live load, and is a good blanket design if you're not sure on the live loading on a typical floor. For a blast load though, which is more likely? 100 psf or 100 psi? Does your boss know? Do you?

I went to a SEAOT seminar in Dallas, one of the topics was designing for blast loads. Basically, what I got out of it is that the loading depends on lots of factors, none of which I'm experienced enough with to decide on my own. I'd hire an engineer who can provide you with loading from blast loads. The seminar I went to involved a talk about designing part of the airport in Dallas (DFW) for blast loads. The blast engineers gave the EOR forces which their walls / roof had to resist. I'm sure your project is a much smaller scale, but I'd still be hesitant to do this.

And again, if you or your boss has the experience / knowledge to do this, please don't be offended, and just ignore my uninformed post. But if it is the case that this is out of your area of expertise, don't be afraid to tell your boss you guys need help. I've read several posts on eng-tips.com involving junior engineers asked to do things out of their expertise. One was a boss left a post-tensioned slab on horrible soil for a junior engineer to design while he was gone on vacation. The consensus on here was that the junior engineer should not do the project, and wait until the boss gets back.

 

[cmrazik]

AggieYank,

No offense at all. I appeciate frankness.

The 100 psf live load comes straight out of the design code for rooms housing hazardous materials.

The room will be housing hazardous materials, but the hazard classification is low. We aren't looking at a bomb which would be a very concentrated force. If we were designing for bombs then I think we'd have to find another engineer.

I have decided to go with the composite slab desing after all, and ignore my supervisor. It seems the most logical way to control the building.

The other emphasis I must make is that I don't care if the building fails. The whole goal is to make the impact on other buildings as minimum as possible.

Thanks for the input.

 

[BSE05]

cmrazik

You are using the correct design loading. NPFA 68? Deflagration Venting has a section on this type of blast. Basically it comes down to 100 psf to contain the blast and an area to release at 20 psf. They give sample calculations for finding the area for relief based on a propane explosion. I have used this solution for propane, butane and rooms that dispense any type of flammable solvents. Propane has a pretty low LEL and using this model will cover solvent fumes. Blowing the roof off is a way to releive the blast, but they do make hatches for this purpose.

 

[stlje]

The 100 psf is 0.694 psi, a very small load for most blast effects. Blast effects are typically from bombs or from vapor cloud explosions and the like - I'm going to assume that the 100 psf is a nominal load for the hazardous material storage. In that case, why not design the slab to span between beams with top steel as the moment reinforcing? The WWF may not work, and probably won't be in the top, so use #3 or #4 bars at the apropriate spacing with bolster supports off the metal deck. Tie the deck to the steel beams with metal studs, and to the perimeter walls with bent reinforcing bars. Design the beams for the net uplift as braced non-composite beams. The rotational stiffness of the slab and stud connection should provide a torsional brace for the beam, so the bottom flange can take the required compression (you will need to check the stiffness of the beam web - see the LRFD AISC manual. (I'm assuming that you have steel beams.)