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StructuralEng
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Category: Structural Engineering
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Experience:  Structural Engineer
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Hi I need to cut an opening in a concrete slab in a building

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Hi

I need to cut an opening in a concrete slab in a building in New York City. Slab is approx. 6-8 inches thick with rebar. Opening will be created to adjoin two apartments to create a duplex. Do you know what is required as far as supporting the slab after it is cut. Steel? I have other questions as well. I am a contractor in New York and I need a sub to cut the opening, and another sub to install the steel supports. I can be reached [email protected] I sincerely XXXXX XXXXX to thank you for your help.

StructuralEng : Hi
StructuralEng : I can help you with this
StructuralEng : Give me a few minutes to read the question completely
Customer:

hold on one sec

Customer:

Hi

StructuralEng : Is it a mild reinforced concrete slab or post tensioned slab?
StructuralEng : Two way or one way slab? Any beams or drop caps or is it flat plate?
Customer:

After the opening is cut I will need to install a staircase. Not sure if it will be straight or spiral. Also need to know the code and size requirtements for the stairs.

Customer:

Not sure.

Customer:

The building was built in 1968 in new York.

StructuralEng :

Ok.

StructuralEng :

Do you have access to any existing drawings?

Customer:

I might

Customer:

give me your email and i will send you the layout of the apartment it shows columns.

StructuralEng :

I'm not allowed to give out personal contact information - site rules, not mine. Can you upload them on mediafire and post the link here for me to view them?

Customer:

where is mediafire. Give me site address to upload to.

StructuralEng :

mediafire.com

StructuralEng :

You can also upload here with the paper clip icon, but it can give you problems sometimes if it's not in jpg format.

Customer:

ok hold on.

Customer:

hold on.

Customer:

here is link

StructuralEng :

Give me a few minutes to check it out.

Customer:

ok, when ready I will tell you exactly where I want to cut the slab to install the stairs.

StructuralEng :

That doesn't really help me

StructuralEng :

It has no structural information at all.

StructuralEng :

You'll need to do one of two things.

StructuralEng :

1) get your hands on the existing structural drawings

StructuralEng :

2) hire a local engineer to visit the site and do a LOT of analysis to determine what you can do.

StructuralEng :

If you've found my answer helpful, please "accept" it. I'll be happy to answer additional questions on the topic. If you could provide feedback, I would appreciate it.

StructuralEng :

Who is the prime contract on the job?

Customer:

Tell the difference between the two types of slabs.

StructuralEng :

A post tensioned slab has post tensioning cables and is typically thinner than a mild reinforce slab.

Customer:

I am the prime contact.

StructuralEng :

There is no architect on board?

StructuralEng :

I actually meant contract, not contact

StructuralEng :

a mild reinforced slab has only mild reinforcing steel. Do you know the difference between a one way beam and slab system and a two-way flat plate?

Customer:

my friend is architect. I just started looking at this. I bring him on on all my jobs for my clientds. He is not familiar with the slab in this building either.

Customer:

I am the general contractor.

Customer:

We do alot of high end work, but I never cut a slab before.

StructuralEng :

I can give you qualitative information, but you'll HAVE to have a structural engineer on board to do the actual analysis.

Customer:

Yes I know.

StructuralEng :

Have you been in the building?

Customer:

I will need a structural engineers report and stamped drawings in order to get approvals as well.

StructuralEng :

Are there steel beams with corrugated deck filled with concrete spanning between the beams?

StructuralEng :

What does the structure look like? Do you have any pictures of the underside of the slab system?

Customer:

Yes I have done about 60 apartments in this building, but never a duplex. Have joined two together on the same floor but not up and down.

StructuralEng :

So what does it look like? Is it concrete filled metal deck on steel beams? One way beam and slab? Two-way slab?

Customer:

It appears that it is a slab about 6-8 in. thick with rebar running through it. At perimeter wall over windows there appears to be a teel beam encased in concrete. The cielings are flat throughout the apartments.

StructuralEng :

Are the ceilings the under side of the structural slab?

Customer:

Around plumbing chases I have never seen any decking material.

Customer:

yes they are

StructuralEng :

Are the columns steel or concrete? Or steel encased in concrete?

Customer:

Seems like a simple flat slab to me, but again, I am no concrete expert.

Customer:

I believe they are steel encased in concrete.

StructuralEng :

And you can't see any steel beams below the slab and the slab is only 6" - 8" thick? What is the column spacing in each direction How tall is the building?

Customer:

For instance, if you look at the layout I sent you and you see the large column between the din. rm. and the hall, that column is concrete, but it may contain steel inside.

Customer:

Building is 48 stories. No visible steel anywhere. The only thing that I think I see that must be steel beams is on perimeter walls over windows. These I think are steel beams encased in concrete.

StructuralEng :

it would have to contain mild steel, but is it a W shape structural steel column?

Customer:

No idea.

Customer:

Do we need a copy of the as builts to determine?

StructuralEng :

Where do you need to cut the sla?

StructuralEng :

what is the size of the opening?

Customer:

just so you know, I am looking to cut the slab to the right of the bedroom column. From that col;umn to the closet wall of the closet that is located next to powder room. The doorway that leads into the vestibule area is going to be removed. As far as size I am not sure but lets say 5'x6'. Need to get staircase here.

StructuralEng :

Generally speaking, close to a column is the worst location to cut a hole in the slab. It will require the most remedial structural work.

StructuralEng :

If it's a two-way mild slab, this is the column strip and taking the majority of the load.

StructuralEng :

If it's a two PT slab, there will be a lot of tendons congregated there.

StructuralEng :

If it's a one-way beam and slab system, then you'll probably be cutting right through a beam.

StructuralEng :

Each of these can be overcome (except the PT slab), but you'll have to do some gymnastics to make it happen.

Customer:

Wow, I guess we really nust determine what it is. LOL. Also, it seems that there will be alot of work involved in shoring up regaurdless of what type of slab. Do you have any idea on cost to do something like this.

StructuralEng :

You'll likely have to shore more than one floor. An 8" slab weighs 100 psf, and the LL for residential is only 40 psf, so you probably need to shore the slab for three floors to distribute that 100 psf among 3 floors.

StructuralEng :

Additionally, being close to the column with a penetration drastically reduces the punching shear capacity of a two way slab.

StructuralEng :

Cost for shoring? You would probably know better than I would since you're aGC.

StructuralEng :

If you've found my answer helpful, please "accept" it. I'll be happy to answer additional questions on the topic. If you could provide feedback, I would appreciate it.

Customer:

Cant be done, this would mean we would have to shore in two others apartments that have nothing to do with this project. There is one apartment in the buiolding that is a duplex and the stairs are cut in the same spot that I would like to cut. I do not believe that there is any shoring in any other apartments below the one where the opening was craeted in the cieling.

StructuralEng :

There might be a way to hang some shoring off the column, but you definitely can't shore down to the slab and stop there.

StructuralEng :

No, not permanent shoring, just temporary until the structural work is complete.

Customer:

Even temp shoring while cutting is taking place is out. I cant do this in others apartments.

StructuralEng :

Then you'll have to get something designed to hang off the column.

Customer:

I knew this would be a nightmare!!! Ha Ha. So we would have to fabricate something to strap to the column that woud support the slab while we cut? There is no way to support the slab from the floor slab below? To Heavy, correct?

StructuralEng :

Right, because the dead weight of the slab above exceeds the LL capacity of the slab below

Customer:

This seems like it will be very costly.

StructuralEng :

It likely will

Customer:

I will find a good sub that specializes in this only and get a quote. First I will determine the exact type of slab. You mentioned that if it is a PT Slab that you can not do the cutting. Is this correct?

StructuralEng :

You might be able to, but it will require a LOT more remedial work and you will probably have to move it.

StructuralEng :

the opening, that is.

Customer:

So with this type of slab you can not cut close to the column. Is that correct?

StructuralEng :

Right, because you'll have banded tendons there.

Customer:

ok thank you so much for your help.

StructuralEng :

No problem. Best of luck with this.

StructuralEng :

If you've found my answer helpful, please "accept" it. I'll be happy to answer additional questions on the topic. If you could provide feedback, I would appreciate it.

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