Stair Opening in Exist. Slab
Stair Opening in Exist. Slab
(OP)
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!
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!






RE: Stair Opening in Exist. Slab
Dik
RE: Stair Opening in Exist. 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.
RE: Stair Opening in Exist. Slab
I have partial architectural plans, columns locations close to the opening are marked in red.
RE: Stair Opening in Exist. Slab
RE: Stair Opening in Exist. Slab
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.
RE: Stair Opening in Exist. Slab
RE: Stair Opening in Exist. Slab
RE: Stair Opening in Exist. Slab
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
RE: Stair Opening in Exist. Slab
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
RE: Stair Opening in Exist. Slab
Dik
RE: Stair Opening in Exist. Slab
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.
RE: Stair Opening in Exist. Slab
Dik
RE: Stair Opening in Exist. Slab
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.
RE: Stair Opening in Exist. Slab
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.
RE: Stair Opening in Exist. Slab
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
RE: Stair Opening in Exist. Slab
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
RE: Stair Opening in Exist. Slab
BA
RE: Stair Opening in Exist. Slab
Dik
RE: Stair Opening in Exist. Slab
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
RE: Stair Opening in Exist. Slab
as far as I know that was the condition in order the project to happen. Architect was told that this opening can be done without any beam underneath.. otherwise there would be no project.
I know that if the opening was outside the column strip, there will be no issue..but it is not. I ran some quick calcs and it looks like Mx and My increase approx. 8 times when the slab is modeled with the opening.
Yes they need a beam there..
Thank you very much for all your input here, I appreciate it very much!
RE: Stair Opening in Exist. Slab
BA
RE: Stair Opening in Exist. Slab
Dik
RE: Stair Opening in Exist. Slab
I know beams are not allowed, but what about right around the column itself to make a shear cap? Cast a concrete shear cap, or add short section of vertical HSS bolted to the concrete column to increase critical shear perimeter.
Flexure is another story.