Strengthening an existing, suspended, simply supported concrete slab 8

[BFeng]

I recently looked at a project where an existing simply supported 24'x12' suspended concrete slab had a chase cut in the short direction by a mechanical ductwork installer.

The slab is 5" thick with #3 bars @ 12" o.c in each direction. The reinforcing is located in the bottom of the slab. There is also a 4" thick concrete topping on the slab.

Given the length to width ratio and the now lack of continuity in the long direction, the slab is grossly under-reinforced to support load as a one way slab in the short direction.

I would like to add an additional line of support at the mid-span of the slab in the short direction, but do have any negative moment reinforcing.

Any suggestion would be great appreciated and of course budget is an issue with the owner.

 

[msquared48]

Is the 4" thick topping slab structural at all? Any composite action between the two slabs?

If not, then you could sawcut through the 4" slab at least 1" into the 5" slab, install rebar in the slots, and epoxy backfill the slots. The slots need to extend 1/3 of the span to either side of the new support. If there is composite action between the two slabs, you could just do the same thing to the 4" slab.

I think Ron has some other ideas here too.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[asixth]

slab had a chase cut in the short direction

Do you mean that the chase was cut parallel to or perpendicular to the short direction span?

A simply supported slab with an aspect ratio of 2:1 with equal reinforcement each way will effectively behave as a one-way slab spanning the short span.

There is also a 4" thick concrete topping on the slab.

Is this topping bonded to the 5" slab beneath.

Personally, I don't believe the 5" slab has capacity to span 12' with No.3 bars at 12" crs with a 4" unbonded topping. It seems that the slab has been reinforced with the minimum 0.2% reinforcement with no considerations to the ultimate design loads or service considerations (I calculate the reinforcement to exceed the 60ksi yield under service conditions!!!).

 

[hokie66]

Cutting the span down to 6' with a central support should work fine. You would just have two 6' simple spans. A crack might form over the support, but I doubt that matters.

 

[csd72]

You can strengthen it using GRP bonded to the underside. This doesnt help deflection much though.

 

[paddingtongreen]

Agree with Hokie except I would sawcut a control line so the crack looks like you mean it.
Will you be jacking the slab up while you place the new support?

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[Ron]

Agree with others....cut the span down. If you are using a cast-in-place beam, don't forget to vertically dowel it. Agree with cutting a control joint centered on your new support.

Lots of other ways to do this...

Mike's method
pretensioned cables with shotcrete encasement
carbon fiber overlay with WWF bonded to bottom of slab (makes a nifty catch net when the whole thing fails!)

 

[asixth]

carbon fiber overlay with WWF bonded to bottom of slab

How are you going to be able to bond WWF to the underside of the slab. I would be a little reserved about using any type of FRP as reinforcement. The slab is highly overstressed and ACI440 requires that unstrengthened section has capacity to resist 1.1*DL+0.75*LL.

How is the slab performing currently, are the deflections excessive and noticeable?

 

[cancmm]

Bonded FRP could help with strength issues. Bolting 12' angles or strips of plate steel to the bottom of the slab could be a solution as well, depending on other design constraints.

 

[apsix]

The effectiveness of bonded FRP, carbon fibre etc. depends on the depth of the chase.

Can the chase be filled with a high strength non-shrink grout?

As others have said, I'm not quite understanding where the chase is, and how the original slab was ever adequate with the rebar quoted.

 

[BFeng]

Thank you for the very valuable advise. Probably going to add support at the midspan along with a sawcut control joint.

The owner inially thought the slab was grade supported until the mechanical installer saw cut the chase for the ductwork and realsed it was not grade supported. (Must have been quite a shocker). Absolutely no access to the under slab area until the chase. I established that the slab was grossly under reinforced and have told the owner that remediation is necessary.

Thank you again.