Stair Opening in Exist. Slab

[Heldbaum]

Hi folks,
has any of you ever done a stair opening roughly 9'x7' in exist. RC building just next to the column? It's a 20-story residential building, two ways 8" flat slab - column structure. The client wants to create a duplex. It's at 10th floor. The opening would have to be located a few inches from the column so it's the worst location. I am quite familiar with ACI requirements when it comes to opening's location but there is nothing about openings strengthening to make it work. I cannot run any steel beams from column to column underneath the slab..I was thinking about steel flat bars connected to the columns and closing the edge of the opening from 3 sides with reinf. welded to the side plate. Does anyone have any experience with something like that? Any good reference? I see two obvious issues:
1. Punching shear
2. Temporary slab shoring - it's a residential building so LL 40 psf, and the concrete slab is min. 8" so DL is more than twice bigger.

I already spoke about that with my more experienced colleagues but I would like to get some insight from you guys. Thanks!

 

[dik]

Not a good location... do you have a partial floor plan of the area?

Dik

 

[KootK]

I cannot run any steel beams from column to column underneath the slab.

I'd start with punching shear because of this as it pretty much robs you of the ability to reduce punching shear demand.

1) See if it works without reinforcement (unlikely)

2) See if it's within the range of punching failure that could be remedied with the reinforcement options available to you. If not, game over.

3) If you have any workable reinforcement options, check with the client to see if they want the opening badly enough to pay for the reinforcing. If not, game over.

I'd be reluctant to get into the slab flexural reinforcing until I knew that punching shear could be dealt with somehow.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Heldbaum]

Dik,

I have partial architectural plans, columns locations close to the opening are marked in red.

 

[Heldbaum]

Some dimensions here

 

[Heldbaum]

Kootk,

Thanks for your input. That's what I am going to do. See attached sketch - that's what one of my older colleague came up with but I don't think he's got any calcs to support it. He wants to run plates as per sketch and attach it to the column next to the opening.

 

[racookpe1978]

Can you separate the two openings? Make two narrow slots for a single person-sized residential-type stair, rather than a more usual two-person up-and-down simultaneous stairway? (Could conflict with fire evacuation though.)

 

[Heldbaum]

Old construction drawing with columns location and dimensions

 

[BAretired]

Punching shear may not be too much of a problem. The column is rectangular and you are getting some benefit from all four sides. The east/west column strip could be an issue as you are removing almost half of it.

The opening is on the 10th floor. You could run a steel channel west to east from column to column tight against the wall under the 11th floor and use a hanger to pick up the southeast corner of the new opening. It means moving the stair opening north a small amount. Steel members supporting concrete structure would need to be fireproofed. The stair itself could presumably be of light steel construction and not fireproofed.

If the opening is L-shaped as shown on your first drawing, perhaps you could place a short post down to the 9th floor at the corner of the stair. The load would be fairly small.

BA

 

[NorthCivil]

See attached. A good read. I would personally try to discuss a change of plans with the architect. Make the floor plan revolve around the hole, not the hole around the floor plan.

That's a big hole you want to make, do able, but only in the right place. As has been said already, beware of punching shear if you are going to go anywhere near those columns

 

[dik]

I'm not sure about the stairs in a reflected ceiling plan. If you were to introduce a beam between the two columns along with fire rating, do you have enough headroom?

Dik

 

[Heldbaum]

Thanks for all your answers.

Dik, BA,
I can't introduce any steel beam underneath the slab, that's the issue..what do you think about the detail I attached in my previous post?
I am doing a research over the weekend and I will run some calcs on Monday..

NorthCivil
Thanks for sending the link.

 

[dik]

Is the plate, in elevation, with the four Hilti anchors attached to the face of the column. If so, then there could be issues with edge distance for the anchors and existing column reinforcing. In the event there are problems, it is nearly impossible to remedy the issue. I'd go for an HSS beam with intumescent coating for FRR.

Dik

 

[Heldbaum]

Dik,

Yes,correct. The column is pretty wide but I will check what the distance would be. Can you elaborate a little bit more an HSS beam? where would you introduce it ? As I said, as far as I am concerned beam underneath the slab is not an option.

 

[KootK]

..what do you think about the detail I attached in my previous post?

I doubt that the detail would be effective. If the intent is for the plates to serve as supplemental flexural reinforcing, I think that the horizontal shear transfer mechanism would have too much slop. If the intent is for the plates to serve as a beam, I don't think that the beam would be stiff enough relative to the slab to do much good. And it only makes it to support on one end.

Now that I see the layout, I agree with BA that punching shear may not be so critical after all.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Heldbaum]

Kootk,

any ideas how to attack the problem then ? I'd appreciate any suggestions/ideas. The intent was for the plates to work as supplemental flexural reinforcing

 

[dik]

The HSS beam would go under the slab with drypack or flowable grout, using ethafoam sheet or rod as a sealer. You might have to adjust your rise and run for headroom clearance. The HSS beam was selected because it is likely the most attractive solution. You can use a W section with furring, etc. and a drywall finish for FRR.

Sometimes with a renovation you have to live with what you can do and the lack of a beam may not be an option.

Dik

 

[BAretired]

What is FRR?

BA

 

[dik]

Fire Resistance Rating...

Dik

 

[BAretired]

I can't introduce any steel beam underneath the slab, that's the issue..what do you think about the detail I attached in my previous post?

Why not?

Your detail cannot be used on the south side of the opening. If column strip remaining after the cut is adequate, you're okay without anything. Otherwise, you need a beam.

BA