Detailing of a new opening in an existing two way concrete slab 1

[PROFR]

I have an exiting two way concrete slab in which it is required to introduce new openings. The openings are square in shape and range in size from 2 ft to 5 ft. The existing rebar spacing is @ 16 in. I did a finite element model and the remaining reinforcement seems enough after the slab is cut. My only concern is that due to the large spacing of the rebar, there will be areas around the opening in which there will no rebar from the edge of the opening to the next existing rebar. Is this a concern? What is the best detailing concept to deal with this? should I still use anchored steel plates around the opening although I do not really need extra rebar area? The other issue is that I normally use U-rebar at the edge of the opening. I am not sure how to mimic the same concept in a new opening in an existing slab.

 

[r13]

In your model, check deflection around the opening, also the stress at the corners, from which the problem usually occurs. You can use rebar locator, or advanced device, to locate the rebars, if the slab haven't been cut yet.

 

[JedClampett]

Another thing to worry about is reinforcing development. Just because the bars are there, doesn't mean they're developed. But the good news is that concrete is pretty forgiving of openings, to an extent. Just don't go crazy.
Also, just cutting the opening can cause issues. There's going to be overcutting in the corners due to the saw diameter. A solution to that is to pre-core large holes in the corners and saw to those. I like doing that because the rounded corners are better for stress concentrations.
What I also like to do is put a curb around the new opening first, with the guise of having a good place to mount a hatch. Then make that into a concrete member that reinforces the edges.

 

[r13]

Two sound suggestions.

A solution to that is to pre-core large holes in the corners and saw to those.

put a curb around the new opening first

 

[dik]

Round corners are great and client should be aware that some cracking may occur.

Dik

 

[PROFR]

I am thinking of doing some strap plates at the underside of the slab and anchor them with concrete anchors designed to develop the plate capacity beyond the opening. The plate area should be equivalent to steel rebar cut by the opening and intersecting plates will be butt welded. What do you think?

 

[MegaStructures]

I would think for a slab in bending it would be best to develop the rebar at the edges of the openings with plate. Had a project similar to this, we cored the corners and made straight cuts to connect them like Jed said. Also chipped the concrete back around the rebar to give some room to weld to the new plate. If you don't connect the rebar to the plate I would imagine there would be no moment strength near the opening that could lead to some significant cracking.

 

[JLNJ]

I’m not a fan of the plate straps. What kind of FEM did you create? I usually just use the ACI two-way slab proportioning.

My feeling is that the straps won’t be effective until long after the rebar is over-stressed and the concrete has deflected unacceptably.

The location of the holes is important. The worst location is at intersecting column strips. Intersecting middle strips is probably the best location.

 

[PROFR]

Thanks JLNJ. So if the plate straps are not preferred. What is the best alternative to provide additional reinforcement at the opening edges?

 

[r13]

Profr,

What items are to be set on the opening?

 

[PROFR]

Retired13,
There will be a heavy HVAC unit just above the opening where large ducts will go through the opening. I will definitely need some edge reinforcement and will need also to develop the cut bars at the opening using a plate like Megastrutures suggested. The thing that I am not sure about is the best way to add edge reinforcement after the cut. Plate straps, FRP, ..etc. Any suggestions?

 

[JLNJ]

The best way is to figure out a way to make it work without needing to add reinforcement.

HVAC units are often in the 50 to 60 psf range, plus you may be able to justify 60 psf for maintenance access. If the existing slab was designed for 100 or 125 psf, you might be OK.

Then work with the mechanical engineers to size and locate the openings where they do the least damage.

Barring that, carefully chipping away the adjacent concrete, adding bars, and pouring it back is probably your best option, but don’t be afraid to apply some pressure to an architect or mechanical engineer.

 

[SlideRuleEra]

PROFR - Consider designing a suitable, oversized frame (shown in red) supporting the HVAC unit that distributes load away from the the opening. Reinforcement at the opening likely not needed.

http://www.SlideRuleEra.net

 

[r13]

IMO, the plates with through bolt is a viable solution. If more bending capacity is required that exceeds a single plate can provide, you may consider adding edge channel, fastened by threaded rod. The rod should stagger with the through bolts though. The contact face should be roughened, and grouted afterward.

 

[hokie66]

You made no mention of the location of the opening. If it is in the M-M area, you don't need to do much. If in a column strip, or worse, near a supporting column, big problem.