Opening in one way concrete slab

[nassau1]

Question:

An architect I work with is looking to place a stairway to join two apartments. The building is a 1960 vintage one way slab construction. Slab approx 6 inches thick.

There are concrete beams at approximately 20 foot centers.

the beams run east to west and the reinforcement in the slab run north south.

The hole is contemplated to be parallel to an interior beam. The hole is to be approx 10 feet long by 4 feet wide. so approximately 16 feet of slab will be "cantilevered" until picked up.

So 10 feet of reinforcement will be cut. I am concerned about cathing the top steel that is going over the interior beam as well as supprorting the slab.

Oh yea, the owner and the architect do not want to have beams supporting the existing slab since there is a head room issue.

I am thinking of placing plates or angles all around the opening and pre-heat and welding the bars from the interior over the interior beam, and maybe also catching the bars from the north on the positive moment. I still have to carry this lot and I am thinking of loading the slab below or ??? Thoughts? Has anyone cut a hole like this before. I know, "Do not cut holes in slabs" but can it be done. Thanks

 

[csd72]

I wouldnt be happy about this situation. With no beams, deflection will be an issue even if you can get the strength to work.

Can you get them to turn the hole around so you effectively have two 5' cantilevers - a bit more realistic.

Then just check the moment distribution of this against the moment in the existing.

 

[nassau1]

I did try to have the long opening go in the same direction as the main steel. But the layout was not acceptable.

The deflection is a question.
1- I think I can get them to accept plates along the bottom. I am thinking of attaching plate or small tubes to try to help the moment distribution. These would be attached along the direction of the main reinforcement (north to south). Thoughts.

2- When I am looking at moment distribution, how best to tackle this?

 

[civilperson]

Frame the perimeter of the hole with beams of the same depth as the e-w beams and continue them to a beam support location. Alternately, a series of vertical columns to a new footing can pick up the load of the slab where cut. Architects do not get what they want very often unless a sky hook is included in the budget!

 

[csd72]

I really dont think the plates will do much good, carbon fiber may be a better option but I would still be concerned about deflection and moment redistribution.

here are some references that may be some help:

http://www.cement.org/buildings/tech_note_floor_openings.pdf

http://www.concrete.org/members/ci2807mota.pdf

http://www.accessmylibrary.com/coms2/summary_0286-30252234_ITM

http://www.highbeam.com/doc/1G1-160640150.html

http://epubl.ltu.se/1402-1757/2005/87/LTU-LIC-0587-SE.pdf

If the client will not take no for an answer then often the best way to deal with them is to give them several options so that they can do a comparison.

In your case 3 options would be:

1. turn opening around (possibly no further work)
2. fibre reinforce bottom of slab
3. provide beams to underside of slab.

Often the architect is more willing to compromise when they realise their preferred option is going to blow the budget.

 

[nassau1]

Thanks for the info.

I read the Mota article and I ordered and received the Newman book from Mc graw Hill. Reference # 3 from the Mota article.

I know I cannot turn around the stairs. I think I may use some of the stair structure to carry.

 

[csd72]

Let them decide that - I would include it in the options anyway just so they know what they are paying for not rotating them. I have often found that they will reverse this decision when they find out how much it is costing.

Just do not allow anything that you are not comfortable with.

 

[jike]

There is no question, you will need to frame opening with beams or load bearing walls or posts. I assume there is a wall around the stairs, why cannot one type of these structural elements be used and be hidden in the wall?

If you cannot go underneath with a beam, how about above? I refer to these as upset beams. Maybe they can be incorporated into the stair construction.

By cutting a large hole in one span, you will be weakening the adjacent spans. If the original design was conservative, it may not be a problem. Also, the slab thickness should be a minimum of L/28 for a one way slab with 2 ends continuous but now by cutting a hole, you have one end continuous which would require a slab thickness of l/24 unless you compute deflections.

 

[csd72]

Jike,

That is why you leave a cantilever, to provide some continuity at the near end.

 

[jike]

csd72,

That works for one side, but the other side apparently is up against the beam (16 + 4 = 20). The slab on the other side would not have the benefit of the cantilever,unless it is an outside wall.

 

[csd72]

Jike,

I did realise that.

nassau,

You need to learn how to manage the architects expectations, just because they want it a certain way doesnt mean that you have to agree to it.

As I said above you can give them construction options - this is good politically as it gives them some flexibility and also shows that you have thought it through.

Another part of a response is to possibly give them zones where they can put the opening so they could possibly put it in another location that matches their layout.

Explain that as this is an existing building your options are much more limited than for a new build.

Do your background work, explore all of your options and then stand fast to your conclusions. As we say in climbing - the law of gravity does not compromise.

 

[pba]

Another option of course would be to remove the slab abd replace it with one desiged for your new loading and arranagement - That should be suitably expensive for the Architect to reconsider...

 

[nassau1]

Folks,

All true and all good. I did try to give them locations and they are set on this location.

The problem with posting below is this is the 11th floor so the floor below is also a slab.

I do need to check to see the affect of the l/28 vs l/24 on the interior slab.

And I will show them the solution with a transfomed/smaller opening.

Thanks again.

 

[hokie66]

The architect and owner are either being hardheaded in refusing to accept your advice, or they think your are a magician. I would just stand my ground and tell them they need to rethink this renovation option.