Closing existing opening in 2-way slab

[Hoboneer]

How would we reduce up an existing opening in a 10" thick flat 2 way slab? I'm going form about 8x12 down to a 4x3.
My ideas are:
1. Epoxy anchor angles into 2 sides of the opening then drop in form deck with rebar and concrete, epoxy doweling the rebar into the edges of existing.
2. Dowel rebar into edges and do traditional shoring and forming.

Method 1 is preferred for speed of completion because this is multiple openings on top of each other, think a defunct stair opening over multiple floors.

 

[milkshakelake]

Method 1. That's how I've done it in the past. Use a 2" or 3" deck to span across the opening. Also, do a full slab analysis around the area to check top rebar, bottom rebar, long-term deflection, and punching shear. Even if you're closing up a staircase opening, that load used to be on one side, so the loading is a little different now.

 

[Hoboneer]

Yes, of course I need to analyze the existing slab.
I was also thinking of doing composite deck and not doweling the rebar (basically only the angles are doweled into the edges and not the rebar from the new slab) but then I have to worry about fireproofing, correct?

 

[milkshakelake]

Doweling the rebar serves the structural purpose of transferring the diaphragm shear, not fireproofing. If it's a small opening, it's not a huge deal to think about the shear transfer too much. If you neglect the rebar dowels, one can assume that the shear gets transferred through the connection between the form deck, the angles, and the epoxy anchors.

I'm not an expert at fireproofing so I'd leave that up to the architect. I'd think that the drop ceiling would serve that purpose. This doesn't seem like a case where you'd need to seal the joint for fire penetration, though again I'm not an expert.

One argument for adding rebar dowels is additional vertical shear capacity, if you need it. You probably don't. I just add it so I can sleep better at night, but I don't think it's strictly necessary.

 

[dik]

Look into UL D916 assembly. Used to be F 915 (I think), but couldn't find it.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[KootK]

The requirement to maintain the smaller opening makes it an unusually interesting problem. Will you trim that out with localized rebar in the deck slab which will kind of concentrate the loading on the slab edge of the existing slab? How is the new opening to be positioned within the existing"?

 

[milkshakelake]

@KootK That's an interesting point. I missed that part. As the master of field-assembled joists, I should've read more carefully. (By the way, I still have the certificate front and center on my wall.)

In that case, that changes everything. I'd just use regular formwork (option 2) and design it as two-way 8" or 10" slab. I thought about using the form deck with downturned angles to make something like an 8" (about 7" effective) two-way slab and just cut out the opening or block it out, but that's a lot of work with the angles, and might need extra shoring. I'm not too worried about the concentrated loading, but moreso the work.

 

[KootK]

I still feel as though the deck slab is going to be the "right" answer even if it is not the most structurally advantageous answer. There's space here for the deck slab to be 6"+ thick and thus have workable, flexural rebar reinforcing around the sides. If the deck had to be shored around the opening that might not even neuter the economy of the deck slab solution. Seeing as there will be bearing angles in the mix anyhow, perhaps it would also not be a big deal to add in a couple of angle "stringers" bounding the new opening and supported by those bearing angles. This is kind of why I want to know the relative position of the new opening within the existing.

 

[milkshakelake]

That could work, though I'd use something like a C6 channel or W6 instead of an angle to span the 8'. We'd be dealing with a 2" slab (2" one-way deck + 2" concrete). In that case, no shoring is needed. It will be tough to shore, since the existing opening is stacked and it'll require some beams spanning over the opening to support the shoring.

For something like a 4' wide opening, I wouldn't worry too much about the concentrated load. If that becomes a concern, use another channel at the end to spread the load out.

If the C6 channel or W6 doesn't work in flexure, it is possible to use a deeper channel or W member and support the bottom of the deck on the flange. Then we're looking at something like an 8" to 10" slab. I'm not a fan of that idea because this will definitely require shoring under the deck, not to mention more gravity loads at the concentrated ends. Better to use a small and light slab up top.

 

[KootK]

Starter detail to get the juices flowing.

As I wrapped it up, it occurred tome to make the following improvements:

1) Flip the bearing angle.

2) Go with an 8" slab.

3) Replace the top anchors with drilled in rebar that becomes slab top steel at the perimeter for a better bearing anchorage condition.

4) Add some nominal anchors lower into the slab just to hold it all together.

 

[KootK]

...though I'd use something like a C6 channel or W6 instead of an angle to span the 8'.

I feel that's overkill for what, in my scheme, would just be temp reinforcement for the deck prior to the concrete hardening.

In that case, no shoring is needed.

I meant shoring solely for the purpose of avoiding underside steel deck reinforcement at the opening.

It will be tough to shore, since the existing opening is stacked and it'll require some beams spanning over the opening to support the shoring.

Pish posh... git 'er done. The engineer can't be the only party solving problems here. The "shoring" could even just be steel spanning the opening.

 

[milkshakelake]

My idea:

Advantages:
-Less dead load
-No rebar

Unfortunately I'm not at my fancy artist tablet so that's the best I could do, and couldn't annotate much. I had an idea for how to connect the channel, but it was excruciating to draw that much, so let's just say that it's a ~1' long channel with a few epoxy anchors.

 

[dik]

If a 4'x3' opening, I'd likely use a C6 to span across and distribute the loading a little more uniform and use 1-1/2" deck with a 2-1/2" topping to minimise weight... and connect accordingly.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[milkshakelake]

I feel that's overkill for what, in my scheme, would just be temp reinforcement for the deck prior to the concrete hardening.

In my case, I'm not relying on the concrete at all. Only the deck and the steel supports.

The engineer can't be the only party solving problems here. The "shoring" could even just be steel spanning the opening.

Agreed with the second half, but definitely disagree with the first half. The engineer should solve as many problems as possible so people come back to them and they make tons of money. Then they use that money to rule the world.

 

[milkshakelake]

@KootK I just came to the same realization that the angle should be upturned.