Flat Slab - Punching shear

[TaviB]

Hello guys,

I have a question. Some relatives want to buy a new flat.
- non-seismic zone (ag = 0,08 g)
- build with load bearing walls on contour and with a column in middle (30cmx30cm)------> structural system is a flat slab with perimeter beams
- slab is 15 cm thick, and the reinforcement plan of the slab is attached.
- Ved: punching shear force - 170 kN

There is a problem from my point of view, there is a pipe, which was put after the building was finished, and an opening was made just near the column. From my point of view, the punching shear capacity dropped with about 20% because of this.

There are some stirrups placed (see plan in right) over there, but when the opening was done, we could not see more then 1, so i do not think there are all of them (52). Maybe there are 25.

Do you think it should be considered a consolidation of the column? I ask this because i know that shear punch failure is quite a big problem. Or because of the small size of the opening no problems can occur?
Is the thickness of 15 cm a enough for a flat slab in this case?
Is the place of opening a good one in this case? I mean it is better to be there instead left of the column? Where the is a bigger span?

Thank you!

 

[KootK]

I agree with your 20% estimate. It's probably worse than that because your spans are unequal in the one direction which would generate demand for some moment transfer at the slab/column joint. I also question the value of stirrups in this situation. 6" is too thin for effective stirrups in my opinion. The further you put the opening from the column, the less impact the opening will have. That said, you'd have to move the opening quite a ways away to have a serious impact. The answer here is probably to get a structural engineer out to take a look at it if you're really concerned. In most cases, I doubt this would be enough of a problem to dissuade someone from purchasing a property. And there are definitely ways of reinforcing the existing condition.

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.

 

[TaviB]

Well, the opening is done and the reinforcement are already cut in the opening. Can you told me some ways of strengthening?
I would consider some steel L profiles (150mmx150mmx15mm) as a collar on at least 2 sides (right and left).
Do you think there is also a problem on bending moments? Should I consider some fiber carbon to strength the upper part of the slab?

 

[KootK]

I think that step one is to find a structural engineer to evaluate the situation and determine the nature of any overstress. That will inform any repair strategy. Angles won't do much of anything in this situation. I can't tell if there's a moment problem based only on the information available so far.

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.

 

[hetgen]

I think the slab may be acceptable as is without strengthening, but its hard to say without doing a proper review.

Flexural capacity of 15cm slab with ∅10 @ 15 top steel above the column will need proper review.

You should get a Structural engineer to check the slab capacity both in punching and flexure.

 

[dik]

Using the angle method to determine the 'punching shear perimeter' affected could result in a loss of 20%... might be something to look into. I 'sneak' a bit with flexural capacity... but, generally don't mess with compression (steel or concrete) or shear in concrete.

Dik

 

[hokie66]

Is this a multistorey building with multiple flats like the one you are concerned about? If so, strengthening just your floor would not give me much comfort.

Agree with KootK. The slab is only 150 thick, the designer tried to solve the punching shear problem with stirrups which do little good. Then a plumber made the problem worse.

I wouldn't buy it.

 

[KootK]

This is an appealing method for reinforcement. I'd still check in with Hilti on whether or not 150 mm is too thin for their system. I haven't dug into it.

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.

 

[TaviB]

Is this a multistorey building with multiple flats like the one you are concerned about? If so, strengthening just your floor would not give me much comfort

The building is only 2 level height, and the opening is only between first floor and ground floor. When I was on site 2 days ago I managed to catch the plumber and I insisted that the opening between floor 1 and 2 to be at least 30 cm far away from column.
I did a design after EC2 with the slab designed in SAP and the Ved was about 145 kN, which is a bit lower from my initial estimation.
The concrete without reinforcement, has about 470 kN/sqm punch shear capacity and the needed one is of 520 kN/sqm for slab without opening and 620 for the one with opening. For this stresses resulted a need of 20 stirrups 8mm diam. The designer included 52 stirrups around the column. So if it can be counted on the stirrups the designs checks, but if not no. If at least 50% can be counted on stirrups it would be still ok with the design.
If the opening is at 30 cm from the column the increase is only from 520 to 545, which is significantly better.
So the main question remains: how much can it be the stirrups taken into account? (and i think that the building being been in a non-seismic area is good and the system is affected mostly by gravitational loads).
And it would be a great help for me if you can point me out to some steps in order to install some steel collard around the column (a technology where is better described, how to bolt the steel profiles into slab and column, and what type of grout to use). I say this because i have a friend with a steel construction company and can help with steel profiles and what is needed.
If there is a increasing in column rigidity by steel collars, would the negative bending moments in the slab in the column area increase?

 

[rapt]

But EC2 does not allow the use of punching shear reinforcement for a depth less than 20cm!

Agree with Kootk and EC2 that your reinforcement is useless. 10cm deep legs with a spacing of 10cm does not help either.

Does you Ved include the effects of moment transfer?

Also a 15cm RC slab spanning over 5m is going to have deflection and vibration problems if it has not collapsed in the meantime!