Strengthening existing RC slab 2

[mar2805]

Hi guys!
I need some advice.
Please see picture atttached.

As you can see its an existing RC slab spaning in one direction.
Slab is supported by masonry bearing walls
The thickness is UNBELIVABLE for the given span. Only 5 inches!

The slab is said to me reinforced in bottom and top layer allthou I have my doubts in how the hell did they manage to put 2 layers of reinforcement in such a thin slab.
Ive done some calculations by hand and using FEM software and the reason for that large crack in the top layer of the support is clear.
There isnt enough reinforcement in the slab to satisfy the bending moment in the slab.
The problem now is that both, the bottom layer and the top layer were reinforced using the same area (said by the constructor).

Since thers a crack formed in top of the slab, the negative bending moment will tend to go to zero value and in return the positive bending moment in the bottom of the slab will increase (since this will become a simply supported slab system).
I calculated that the reinforcement in the bottom of the slab isnt capable of resisting "increased" positive bending moment, BUT the slabs hasnt started cracking yet on the bottom side.
These are good news.

I need to strenghten the slab.

My idea was to cast RC beam under the slab.
There would need to be 3 beams for each field spaning in the short direction.
This would be done by making formwork for beams under the slab and by pouring concrete thru holes that would be made in the RC slab.
Theres 3 beams would lower the bending moments in the slab to an value that is small enough for the reinforcement to resist it.
We have done some FEM modeling and analysis modeling the new RC beams that are not "monolitical tied" with the slab.

The numbers seem fine but there a lot of questions that are bothering me...

Has anyone done such a thing already?

Thank you

 

[BAretired]

You are proposing to add three new beams in the short direction spaced at 2.5m centers each side of the central bearing wall, parallel to the direction of the one way slab. Do you know how much reinforcement exists in the slab perpendicular to the proposed beams? If adequate, the slab will become a two way slab which should improve its strength substantially.

I wonder if it would be simpler to add steel beams under the existing slab, then fireproof them as required.


BA

 

[oldestguy]

Could the crack be caused by using a concrete mix of high water/cement ratio and it merely is the shrinkage that finally occurs as a result? Patch the crack and be done with it.

 

[hokie66]

A flexural crack does not necessarily mean failure. And the negative moment does not "tend to go to zero value". While the slab is a bit thin for that span, it may be quite safe, depending on the amount of reinforcement, the location of the reinforcement, and the loading. I would try to determine all these things before going further.

The crack in the top is likely due to flexure, perhaps because the reinforcement was not supported at the correct height. The lack of cracking in the bottom is encouraging.

 

[mar2805]

@BAretired

"You are proposing to add three new beams in the short direction spaced at 2.5m centers each side of the central bearing wall, parallel to the direction of the one way slab"

Yes, that is correct!

"Do you know how much reinforcement exists in the slab perpendicular to the proposed beams? If adequate, the slab will become a two way slab which should improve its strength substantially."

The contractors claims that he reinforced the slab with an reinforcement area of 335mm2 per meter width of the slab (that would be reinforcement area of around 0,52square inches for every 3,3feet)...
What is strange is that he claims that he used this reinforcement in BOTH DIRECTIONS!
We will check this!


@oldestguy and hokie66

Ive attaced some screenshots of our calculation.
If you look the picture attached, for continous slab, there is a negative bending moment above the support that has an value of 17,23kNm, wich would require an reinforcement area of 5,53cm2 per meter width of the slab (that would be 0,86 square inches for every 3,3 feet).
The slab is reinforced with ONLY 335mm2 per meter width of the slab (that would be reinforcement area of around 0,52square inches for every 3,3feet).

Not enough reinforcement area.

So it is very likely that this is a real flexural crack.

So since the slab is cracked at the top, weve made another system wich is basicly an simply supported slab with only an positive bending moment in the field and zero negative moment at the support
For this system we have an positive bending moment of 10,88knm.
To resist this moment slab need to have reinforcement are thats 5,17mm2 per meter width of the slab (thats around 0,80 square inches for every 3,3feet width of the slab)
As mentioned, slab has also only 335mm2 per meter width of the slab (that would be reinforcement area of around 0,52square inches for every 3,3feet).

Thats also not enough reinforcement area.

 

[mar2805]

Forgot the picture for the slab that behaves as an simply supported slab (zero negative moments at the support)

 

[hokie66]

But you are neglecting the reinforcement which does exist at the support. The moment is not zero just because the slab has cracked in flexure. The resisting moment provided by the actual reinforcement is still there. So your moment diagram will be displaced, but the end spans are not simple spans.

 

[mar2805]

So what you are saying is that there will be and re-distribution of moment diagram?
The negative moment will get smaller and positive will get bigger.

 

[KootK]

The slab is said to me reinforced in bottom and top layer allthou I have my doubts in how the hell did they manage to put 2 layers of reinforcement in such a thin slab.

The problem now is that both, the bottom layer and the top layer were reinforced using the same area (said by the constructor).

What is strange is that he claims that he used this reinforcement in BOTH DIRECTIONS!

Have you confirmed the presence of top steel in the field? My spidey senses are tingling with this. I suspect that you've got temperature and shrinkange level reinforcement in both directions at the bottom and nothing at the top. Perhaps, when your contractor mentioned top steel, he really meant the uppermost layer of the two bottom steel layers.

I'd assume that there's no top steel until you're presented with some evidence to the contrary.

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.

 

[mar2805]

Nope, he said he put an reinforcement layer in the bottom of the slab and another layer in the top part of the slab...he used reinforcement mesh....this will be determined soon.

 

[hokie66]

Good. Determine how much reinforcement you have, and then you can calculate the capacity. The capacity at the support will just depend on the amount of top reinforcement and the effective depth. Impose that moment, then you can calculate the moment at the midspans.

 

[BAretired]

Is this slab in a building under construction as opposed to an existing, occupied building? If so, did the contractor just size the slab and place the reinforcement according to his gut instinct or did he have drawings? It seems a bit unusual for the contractor to be offering information about an existing building.

BA

 

[mar2805]

On thursday, we are going to know the reinforcement layouts and areas.

YOu havent answered my question regarding your answer:

"So what you are saying is that there will be and re-distribution of moment diagram?
The negative moment will get smaller and positive will get bigger."

Is this correct or not?
If yes, how much will th enegative moemnt become smaller and how much will the positive moment become bigger?

Thank you

 

[bhiggins]

Once you figure out your flexural reinf. you can calculate the moment capacity of the negative reinforcement. Your total span moment is wl^2/8. The positive moment required is simply wl^2/8 - your negative steel moment capacity. The negative reinforcement will shift your moment diagram upwards by its capacity.

 

[KootK]

When you're out there doing your field investigation, keep in mind that it's quite common to drape a single layer of mesh such that it serves as both top and bottom reinforcement.


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.

 

[hokie66]

mar2805,
Yes, you can look at it as moment redistribution. The negative moment, as I said, is controlled by the amount of reinforcement and the effective depth. You can then compute the positive moment by knowing that value. At least, I hope you can.

 

[haynewp]

I don't know about the negative moment getting smaller part of your question if you are talking about moment redistribution. ACI 318 limits the redistribution amount if you start looking into that.

 

[haynewp]

In other words, check your Code before making the elastic negative moment smaller through redistribution.

 

[mar2805]

The investigation is finished.
Not good news....will report later

 

[mar2805]

Hi guys!
Look pictures attached

picture s1 and s2





Reinforcing in the lower zone is stoped right at the middle support.
We checked this 5 times (5 locations) since we didnt understand why the contractore didnt extend it to the next span.
Reinforcement in the upper layer extends around 1,5m into the span, both ways, wich is ok.

There isnt any reinforcement in the middle of the slab (top zone)

I tired calculating using moment redistribution.
My code allows for 30% of lowering negative moment but with certain neutral axis limitations.
Unfortunalty, the calculation doesnt pass the check.
Even if it did, Im skeptical because the reinforcement in the bottom zone is not continous thru the middle support, so Im having doubts if the section could be analyzed as doubly reinforced (calculating resistance moment with using top and bottom reinforcement).

To sum up.

There isnt enough reinforcement area in the top layer of the slab to resist bending moment.
There should be 50% more reinforcement area to pass the check. (335mm2 of reinforcement area is less then 553mm2 wich is how much it should be)

Same thing with bottom reinforcement area, only there should be 15% more reinforcement area (if the slab is analyzed as continous).

Im still thinking of casting two RC beams under the slab for each span but im not certain how to analyze and dimension them since this slab has already got deformations and cracks....its not as the beams and slab are cast at the same time where you would expect T section beahvior. This is big difference!

Your thoughts.