Coring through rc slab 2

[asixth]

Hi guys,

I have an architect who is doing some renovations on a building and requires to core through an existing rc slab with a nominal 75 and 150mm diameter hole (3" and 6" respectively) to pass through services etc.

It is my understanding that plumbers drill through slabs all the time and that engineers should be aware of this during design.

Is there anything that I should look out for?

The slab is flat plate and loadbearing walls and the holes are not located in peak moment regions. There are no pt tendons.

 

[PEinVA]

asixth,
In my short experience with this (~2 yrs), our office does a lot of renovation work, so we see this a lot. We usually follow the following procedure.

1. Review a design strip in both directions to make sure the slab has been adequately designed for the listed design load on the design documents (if the EOR listed it), if not use your best judgement as to what it would have been designed for based on year built, expected occupancy (you can check for architectural drawings for this), etc.

2. Review the location of the cores in relation to the effective area for shear at the column. Typically the architect would like to hide these penetrations right next to a column because since they will wrap it anyway, why not expand it a little include a few pipes, electrical lines, etc.

3. Review your design strip to see if you have extra steel in the strip and if they were to cut 1 or 2 bars (either top or bottom depending where you need the steel) if that would be acceptable for your design loads.

4. If you had to you could reinforce the slab with carbon fiber etc.

However in this case with such small cores, you could most likely avoid all steel and they aren't that large that it would affect your shear capacity unless he wants it literally right on the face of the column with 6" and 3" right next to each other, then it is a different story, but sounds like you should be fine.


That is how we typically review these things, not sure how others do it.

RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[ron9876]

Like RCraine said check existing design before coring. Check for reinf. placement at the proposed hole locations and adjust placement to miss reinf. if needed. Normally don't place cores too close together although there are conditions where placement is in a very minimal stress area.

 

[asixth]

Thanks RCraine,

Just to conclude...

The building was constructed in the 1970's and since then, the structural drawings have gone missing (not our design). The slab depth is 125mm (5") and despite my original post, the floor system is beam and slab construction, with the core penetrations situated near a column.

Without knowing what reinforcement is in the slab, the PE in charge of the job decided it would be best to x-ray the slab and core penetrations that miss the top reinforcement.

Thanks all for your time.

 

[JKW05]

With a beam and slab system, coring near the columns is less of a concern (I'm assuming the cores are not through the beams). Locating the rebar would be helpful, as a 3-inch or 6-inch hole could possibly be done without cutting any bars. Even so, with the slab and beam system, I would be surprised if cutting one bar in the slab would be much of a problem.

 

[youngstructural]

If you are at all concerned, you should avoid hitting any steel bars by ferrometric NDT works, and then line the hole with a length of steel boiler pipe to make the hole "dissapear" for compressive loading.

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[asixth]

I was out on site the other day and asked them how the coring went. They got in some expensive piece of machinery that x-rayed the slab to determine where all the reo is, and then they cored straight through the reo.

It looks like the slab may only have one layer of steel laid centrally, and I'm assuming they are about 12mm diameter bars or whatever was used back then (Nos. 4 bars maybe??). The core itself is 75mm diameter.

 

[hokie66]

asixth,

Just because only one bar was found by the core doesn't mean there is only one layer of reinforcing. The one in your picture is probably a "top" bar which ended up too low. That looks like more than a 5" slab.

 

[asixth]

hokie66,

It is 5", my original post listed 6" but I later amended it to 5".

 

[ali07]

Have they Intentionally cut the rebar just to knoe the bar cover, now a days these machine could tell about rebar spacing and depth, they don't need to cut the rebars. Mind you it's a flat plate not even flat slab. Your rebars are important for the stability, punching of slab and redistribution of moments. Try to locate cores in middle strip as much as possible.

 

[hokie66]

asixth,

When I said the slab looks more than 5" thick, I was just basing that on the width of my thumb and a #4 bar. But perhaps your thumb and mine don't match, and perhaps it is not a #4 bar.

 

[ryanac]

asixth - I am encountering the same issue with coring an existing slab to install drains that were never installed in original construction. What was the final verdict on your implementation of the mod's?

 

[Miner99er]

The skinny is,
Stay as far away from the col as possible (shear)
Don't cut rebar.
Have the contractor use GPR to locate reinforcing if possible.

 

[apsix]

And if it is a beam and slab system; next to the column is fine, keeping out of the beams of course.

 

[asixth]

ryanac,

Miner99er and apsix advice is good. There are lots of machines out there that can determine where in the concrete the reo is found.

The core location was away from the column, wasn't located in any beams and was in a negative moment region so we thought it was necessary to avoid cutting any top reinforcement, despite the fact they hit it.

Try to get out there while they are x-raying the slab because it's worth having a look at, and very expensive.

 

[ryanac]

Thanks to all for the advice. It has been much help.