Maximum thickness of structural concrete slabs?

[met33]

Can a suspended structural concrete slab be too thick?

I am doing a preliminary "design" of structural cast-in-place concrete porch slabs on my new home, before I find a local structural engineer to do the real design.

The two-way slabs might be 12" thick, and the one-way slabs might be 8" to 12" thick. However, the total spans are relatively short, on the order of 8 feet to 12 feet.

Besides the expense, are there any structural reasons to limit the thickness of structural slabs relative to their span? I'm asking because it seems like the rebar might not be able to do its job if the concrete slab can't deflect enough to engage it. Is that a concern, and could it lead to brittle behavior?

See diagrams below for dimensions.

 

[Tomfh]

There are alternative design approaches eg strut and tie when concrete is so thick as to be considered “non flexural” but your 12” slabs spanning 8 feet are still flexural members.

Thicker concrete has larger minimum reinforcement requirements, and large shrinkage reinforcement requirements.

Why is you slab so thick?

 

[met33]

The slab only needs to be 12" where it meets the house. The rest could be thinner, especially if the porch stem walls are taller than the basement walls (which they currently are -- the house is under construction).

The two-way slabs are my main structural concern, because there will be rooms underneath (root cellars, and they'll also double as storm shelters -- even so, a 6" to 8" slab should be sufficient).

I thought the two-way slabs might be easier to design if the bottom is flat. But this would make them approximately 10-12" thick. They don't need to be that thick.

Another consideration is that I'd like to add sufficient reinforcement to the slabs to avoid control joints. It sounds like I should consider thinner slabs then, since thicker slabs have larger reinforcement requirements.

 

[HDStructural]

Besides the expense, are there any structural reasons to limit the thickness of structural slabs relative to their span? I'm asking because it seems like the rebar might not be able to do its job if the concrete slab can't deflect enough to engage it. Is that a concern, and could it lead to brittle behavior?

Are you calculating your thickness requirements assuming that the rebar won't be engaged (uncracked section)? Reinforced concrete is assumed to crack even at service loads. Even if the concrete doesn't crack under service loads, the strength of the concrete section would still consider the rebar in the concrete.

If you do the two way slabs as shown above, you will need a support spanning in the 11'-2" direction to support those slabs. Not sure if you have considered that.

I would expect the one-way slab you are designing to be 6"-7" thick or so with rebar for strength and Temp & Shrinkage.

 

[BridgeSmith]

For lightly loaded (AKA oversized) flexural members, AASHTO (and I assume ACI) requires the reinforcement for flexure to be sufficient for 4/3 of the design moment (or the cracking moment, if you care to calculate it).

 

[lexpatrie]

Why does that detail look so odd to me? I get the feeling that this is going to trap water against the wood.

For the concrete if the spans are modest, one could consider some reinforcing and metal "form deck" that's left in place after the concrete is poured. The metal deck is ignored in strength calculations so it can potentially rust away without damaging the strength of the slab. Usually it's got some galvanizing to it. Or you can get it painted if you really wanted.

 

[Eng16080]

A few thoughts:
[ol 1]
[li]I agree with lexpatrie that the detail doesn't look great. I would try to find an alternative detail that doesn't allow the potential for water/moisture to get trapped between the concrete and wood framing.[/li]
[li]Also agree that a metal deck form might be beneficial here. The only downside would be if you have an issue with seeing that from below. As a side note, how do you plan to form the slab with the curve that you drew above? Seems difficult.[/li]
[li]Off the top of my head, I don't think a thickness over 8" should be necessary here (maybe you already said that).[/li]
[li]I would probably just design this as a one-way slab if it was me. That keeps the detail the same throughout and simplifies construction.[/li]
[li]At least in my geographic location (getting snow), it's usually beneficial to have a minimum step down of 4" to the outside. Obviously, if there are accessibility concerns, then that doesn't apply.[/li]
[/ol]

 

[met33]

Thank you all for the replies.

The backstory is that the house is under construction (foundation has been poured, framing starts soon), but I'm making some on-the-fly changes because the 24" deep floor trusses that were delivered last week were mismanufactured. To stay on schedule, we're switching to I-joists (maybe 16" -- still plenty stiff for my spans) and cutting off the porch stem walls to be flush with the top of the basement wall (they are currently taller).

I previously had an engineered design for the slabs that goes with the taller porch stem walls and the floor trusses. To make a long story short, that design never made sense to me, it was sparse on details, and I couldn't get satisfactory answers to my questions about it. So I'm starting over given the new situation.

To respond to some of the comments:

1. I haven't made any calculations. The slab thickness that I'm considering ranges from L/20 (minimum for a simply supported one-way slab, about 6" to 7") up to 12" (because that's the height I need at the house). Also, yes, there will probably be a 6" step down from the house to the porch.

2. The smaller rooms that could be two-way slabs have intermediate supports (full-height 10" thick concrete walls; see foundation plan below). But it seems like these could be one-way slabs in the 11'2" direction, which might make things easier for the builder.

3. The bottom of the slab above the porch rooms will be framed with pressure treated wood covered in insulation and a waterproof membrane. The wood (plus temporary shoring) is mainly to support the slab while it's being poured. It doesn't need to be there after it cures, and I might take it out.

4. The curved profile under the slab could be made with the fill rock in the 40-ft long section of the porch. Due to an oversight, this 40-ft section of wall was designed as a standard basement wall (pinned connection at the top) so I'm looking into putting lightweight backfill in the upper 2/3 of the wall, maybe expanded shale aggregate, to reduce the lateral pressure on it.

5. The detail drawing where the slab meets the house was incomplete. See updated drawing below. There is a waterproof peel & stick brick flashing membrane between the house and concrete slab (H&B Textroflash). There's also brick veneer & brick flashing above it, to direct any water from behind the brick onto the slab to drain away. These porch slabs are covered with roofs, so the brick walls should see minimal water, except maybe at the edges of the porch (we don't get much drifting snow here).

This porch slab detail is typical construction in my region, for better or worse. I considered alternate approaches but ultimately decided to go with this, because they know how to do it and it seems to work in our climate, and because the other approaches have their own drawbacks (even more so if they're built incorrectly due to lack of experience with the techniques).

The main drawback of this porch detail in my view is that if water does get into the rim board area, it can only dry to the inside of the house. That's why there is just enough XPS inside the rim board to prevent condensation of indoor air (R-5 works for my climate if it's exterior insulation; but maybe it will be R-10 here, not sure yet) and the rest is Rockwool. This leaves some of the sill plate vapor open so it can dry inward.