Forming concrete slab by hanging soffit formwork from steel beams 1

[ajk1]

Is it possible to hang slab soffit formwork from steel beams? It seems to me that 40 years ago this was routinely done with inverted U hangers over the top of the steel beams, but today contractors don't seem to want to do it. We don't want to use steel deck as a form because we are putting a urethane waterproofing membrane on the top side of the new concrete slab, and the soffit therefore must be left free to breathe (as per CSA Standard S413). The new slab is 5 storeys up, so forming it from the ground level below would not seem economical.

 

[gwynn]

This is how most bridge decks for steel girder bridges are formed. Concrete girder bridges are a little bit different, in that the girders are concrete.

Dayton Superior and National Concrete Accessories (NCA) supply the hangers you are thinking of. Just make sure that the beams can handle the slab load non-composite, if they are designed to be composite with the slab. That to me would be the difference between a building and bridge here. Bridges are designed for the deck as a non-composite load. If you are not the one designing the beams, the designer may not have considered this case.

 

[ajk1]

To gwynn - what you describe is exactly what I had in mind. Thanks. We are the structural designers for the project, so the composite action is accounted for. Thanks again for the clear response. Much appreciated.

 

[ajk1]

gwynn: Do you happen to know the name of the hanger? I was looking for it on National Concrete Accessories but have not yet found an inverted "U" type hanger.

 

[EZBuilding]

Have you researched to see if providing vented steel decks allows for the waterproofing membrane to be applied? I would reach out to Vulcraft or similar manufacturer.

 

[steele6707]

Try Dayton Superior C1, C2, C3, & C13. These are in a 2012 catalog. The problem is you still have to strip this from the underside and usually cut off the wires, so you are not too far from needing a work deck. Tying off while installing may also be a problem.

 

[KootK]

The handful of times that this has come up for me, the contractor has chosen precast permanent formwork from the alternatives presented. Sometimes hollowcore, sometimes solid plank depending on the durability concerns etc. I imagine that the economics is pretty sensitive to how much area you're dealing with. As I understand it, using precast in this way is relatively common in Australia. There's a term for the construction method but I can't bring it to mind just now.

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.

 

[Sawbux]

In doing industrial construction, it was common to support wood forms from bottom flange of the beams. Just run the joists & block up from bottom flange. May be too expensive for commercial work?

 

[ajk1]

Thanks for all the comments. Much appreciated.

To EZBuilding (Structural): yes we did, but our engineer tells me that the CANAM acoustic deck available in the Toronto area has perforations only in the bottom of the flute and they are only 0.5% of the area of the bottom of the flute. Perhaps there are acoustic decks with larger percent area of perforations. Some waterproofers are skeptical that even acoustic deck with greater perforation area will be ok.

To Steele6707 - yes, ok thanks.

To Kootk- The idea of precast is interesting but I don't like hollow core in garages, so it would be I suppose solid plank as you note.

To Sawbux - interesting

I had not thought about the fact the forms have to be stripped, so this might be difficult to do at 5 storeys up without building scaffolding below. Thanks for this reminder.

 

[KootK]

Perhaps there are acoustic decks with larger percent area of perforations.

Some manufacturers, in some markets, do produce a vented deck product which is actually distinct from the acoustical deck product (obviously both have holes). Whether or not not that product would suit your purposes is still debatable I suppose.

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.

 

[BUGGAR]

"There's a term for the construction method but I can't bring it to mind just now." We just called them precast soffits (Caltrans). If we could, we designed them as part of the structural system above and effectively got paid for our concrete forms.