best detail for concrete roof on metal decking supported on masonry

[ukengineer58]

Hi how would you detail a concrete deck on decking supported on masonry walls. Im looking here at the support detail at deck/walls. The deck forms a roof

 

[JedClampett]

Your question creates more questions. For instance, is the deck a shear diaphragm? How thick is the concrete? Does it need to be weather resistant? You state it is a concrete deck (?) on decking (assume that is metal deck). What is the span? What load do you have? Is there heavy snow, hurricane winds? Is this a habitable area? Does it need to be insulated? And there's more issues that need to be addressed.
And also, whenever you ask for the best of anything, you're asking a subjective question. It's like asking for the best structural analysis program or religion. We all have preferred details, but I wouldn't be as bold as to claim mine is the best.

 

[Ron]

My first question is "why"? If you are going to use reinforced concrete as your roof deck, why not just form and place the concrete instead of putting a steel decking in there as a form? If you take up too much space with the deck bearing on the masonry, it will decrease your connectivity with the walls, unless you dowel through the decking in order to tie the concrete in.

I think the amount of concrete you might save will be offset by more labor with the decking.

 

[hokie66]

Whether steel form deck or plywood, I don't see much difference in the detail at masonry bearing walls. You don't continue the steel deck over the wall like you would with steel beam supports. The deck is stopped each side, and has to be supported with shoring or by bearing on the wall if enough end bearing is available. The wall reinforcement is extended and cogged into the slab in either case.

 

[ukengineer58]

Ill try and adress the points. It is not habitable. It is contractor preferred option rather than formwork. The decking at the moment is spanning onto beams supported off the internal leaf. The deck continues over the outer leaf. The deck is used to provide head restraint to wall so needs to be fixed.

 

[hokie66]

So, as I understand it, the deck spans parallel to the wall. Why does it have to go over the wall? Why not stop it and cast the concrete onto the wall?

 

[ukengineer58]

Does it then restrain the head of the wall? Part of reason to extend deck is to guard against curling of slab raising brickwork by proving barrier.

 

[ErZaZee]

Decking will provide a better finishing of the slab soffit if required by the owner/contractor. Less steel is required and less form supports and scaffoldings. Just place it on top with shear studs on top of a reinforced cap beam. Or you can try the concept of a hollow core slab placed on top of a beam. Just be careful with the deflections. On top don't forget to consider your water proofing insulation and screeding. Make sure to check its structural capacity and stability.

 

[ErZaZee]

Try consulting a structural engineer before pursuing this project.

 

[ukengineer58]

We are structural engineers it is just slightly odd structure in terms of what it needs to achieve. Just gauging opinions

 

[RUS777]

Perimeter lagged steel angle as a support for deck.

 

[MiketheEngineer]

Without getting into all the details - just position a pour "stop block" at the edge of the wall/ roof and pour it. I might want to have some sort of studs in the wall to transfer any diaphragm loads from the roof - if needed. I would assume the steel is already tied to the wall??

 

[hokie66]

If this is unreinforced masonry, the slab to brick friction is the only thing restraining the top of the wall. Introducing steel in the bearing surface will decrease that restraint. I don't know why you would be concerned with "curling" of the slab. That is normally a term used in slabs on ground.

 

[ukengineer58]

Concerned as we have seen it happen.

 

[hokie66]

Restrained shrinkage of concrete slabs often causes cracking at the top of brick walls. Not curling, I think.