depression in composite slab

[structSU10]

An architect wants a 1/2" recess in the slab on metal deck to accommodate carpeting. Current slab is 5-1/2" total (3-1/2" over 2" deck) I don't have a fire rating requirement for the floor so the question is if they can reasonably cast the depressed area and we can have a thinner structural slab in those areas.

The areas don't relate well to the floor framing so any direct drop/step in the total slab would be awkward.

I also struggle to see how they would try and construct a small depressed zone within the pour without construction joints at that interface. Is there a way for this construction to be placed in one operation?

 

[KootK]

I also struggle to see how they would try and construct a small depressed zone within the pour without construction joints at that interface. Is there a way for this construction to be placed in one operation?

I am often amazed at what skilled builders can manage but, at first blush, I see it as you do: not particularly constructable without shuttering parts of the slab off and coming back to fill in the low bits as a second operation.

 

[jdcollins]

I've seen similar things done fairly regularly with cast-in-place concrete decks. In past projects we've had a continuous concrete slab pour that extends out over a balcony. In the area of the balcony, the finishers will provide a 1/2" step and then slope from the step to the concrete edge. If the 1/2" depressed area is very small, I've also seen styrofoam blocks or even plywood boards supported on isolated chairs.

Even wet concrete is fairly viscous stuff, and skilled finishers can do quite a bit. As long as you don't need the thicker slab, I don't see this as a problem.

 

[jittles]

Agree with jdcollins.

Recently specified a similar small step in the surface of a COMD into a bathroom so that the architect could flush out finishes and provide a small slope to drain. No special framing, didn't get any questions about it, they did it successfully, and I actually don't know specifically how they did it.

 

[MotorCity]

Depending on the size of the depression, they could either grind it down 1/2" or drop 1/2" plywood into the top of the pour and coat it with form oil. Aside from the constructability, does your slab still work with the 1/2" reduction in thickness? Can you still maintain minimum cover?

 

[Brad805]

A foam block will float unless anchored to the form. Depending on access, they can tool this after the concrete has set some.

Do you not have a contractor buddy to call? While we are all out in the field, this is a bit like asking an accountant for legal advice.

 

[structSU10]

Fair points - it is primarily a constructability issue.

I can manage losing 1/2" for my composite beam without a big issue. I can run 4" long studs everywhere and hit the AISC requirements everywhere.

 

[KootK]

Yeah, I've also dealt with plenty of balcony and shower depressions in CIP slabs in my day. There are things that make me wonder if it might be tougher on your metal deck setup however. Showers and balconies are usually small. It sounds as though your depressed area is large. Does one accomplish that with inlaid sheathing or blocking everywhere such that the underlying concrete could not be conveniently vibrated? Or does one just block out the perimeter and omit the blocking in between? If it's just a perimeter thing, does that mess with the methods typically used to screed steel deck slabs?

This probably can be done. I would seek confirmation of that from someone who has actually done it on a metal deck slab however. It might be a good question for the fine folks at SDI.