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Strengthening existing RC slab
2

Strengthening existing RC slab

Strengthening existing RC slab

(OP)
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

RE: Strengthening existing RC slab

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

RE: Strengthening existing RC slab

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.

RE: Strengthening existing RC slab

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.

RE: Strengthening existing RC slab

(OP)
@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.


RE: Strengthening existing RC slab

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.

RE: Strengthening existing RC slab

(OP)
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.

RE: Strengthening existing RC slab

Quote (mar2805)

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.

Quote (mar2805)

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

Quote (mar2805)

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.

RE: Strengthening existing RC slab

(OP)
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.

RE: Strengthening existing RC slab

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.

RE: Strengthening existing RC slab

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

RE: Strengthening existing RC slab

(OP)
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

RE: Strengthening existing RC slab

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.

RE: Strengthening existing RC slab

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.

RE: Strengthening existing RC slab

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.

RE: Strengthening existing RC slab

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.

RE: Strengthening existing RC slab

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

RE: Strengthening existing RC slab

(OP)
The investigation is finished.
Not good news....will report later

RE: Strengthening existing RC slab

(OP)
Hi guys!
Look pictures attached

picture s1 and s2

[url=https://postimg.org/image/g1ixch9mx/][/url]

[url=https://postimg.org/image/69js0ybgn/][/url]

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.





RE: Strengthening existing RC slab

(OP)
Guys, anyone? sad

RE: Strengthening existing RC slab

I haven't read the whole thread, but if your main intent is to flexurally strengthen the hogging zone of an existing slab, how about installing some near surface mounted carbon fibre strips/rods?

Link

RE: Strengthening existing RC slab

(OP)
Seem also like na option but pricey....

Anyone has any experience using steel profiles under the slab...

RE: Strengthening existing RC slab

The steel solution is common. Go for 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.

RE: Strengthening existing RC slab

Do you have noticeable vibrations on your slab? If you don't, maybe you can use fiber reinforced polymers to increase the hogging and sagging moment capacities. It is not very expensive for the quantities you will need. Check with your local providers.

RE: Strengthening existing RC slab

This may be a silly question, but have you considered replacing the entire slab with a CIP or precast one?

RE: Strengthening existing RC slab

If steel profiles are used, steel wedges will be needed between the top of steel and underside of slab to fill any gaps which will undoubtedly exist. Then the space may be filled with dry pack grout to ensure uniform bearing.

BA

RE: Strengthening existing RC slab

(OP)
@KootK
Im looking for an calculation egsamples for this.
Can find anything. Can you direct me to a book or something.
On thing more, there seem to be two different aproches for strengtening RC slab with steel profiles.
One is the use of steel plates that are bolted to the bottom of the slab and glued with epoxy. Together they increase the resisting moment of the RC slab
The other is to provide steel profiles to change or shorten the bearing direction of the slab.

@BeFEA
Same solutio as using the carbon fibers....

@Isan8440
Yes, but demoliting the whole slab is the last thing that I want to do.

@BAretired
This is the thing that IM most afarid of....since the slab already has some deflections there will be a lost of contact between the steel beam and the slab due to its curvature. Your idea of providing steel wedges is very clever! Althou I dont understnad where do you put the dry pack concrete.

RE: Strengthening existing RC slab

mar2805:
I was thinking of spacing steel wedges at about one meter on center or closer if need be, then filling the gap between wedge stacks with dry pack grout in order to provide continuous bearing between beam and slab.

If the slab has a lot of curvature, it may be necessary to use plates in addition to wedges in some locations. Tack weld steel packing to the beam to prevent movement due to future vibration.

BA

RE: Strengthening existing RC slab

Quote (OP)

Im looking for an calculation egsamples for this.

I'm afraid that I don't know of any. Really, though, it ought not be any more difficult than the design of any other steel beam. Figure the load and deflection requirements and you're off to the races. If you have space for the beams, I would think that solution would be preferable to the steel strapping. The strapping is likely to be more expensive than the beams and deflect more as well.

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: Strengthening existing RC slab

Don't be shy with your beam stiffness. Keep in mind, concrete is not a friend to deflection, so if your steel is able to deflect too much, it may not provide the additional strength you're expecting.

RE: Strengthening existing RC slab

(OP)
@BAretired

Do you mqaybe have anydrawings or sketches of what you are saying.
WOuld help a lot.

@KootK
So you would make calculation and design the steel beams for all the RC slab selfweight + all additional dead+live loads?
Basicly you would completly neglect the RC slab loading capacity

RE: Strengthening existing RC slab

Quote (OP)

o you would make calculation and design the steel beams for all the RC slab selfweight + all additional dead+live loads? Basicly you would completly neglect the RC slab loading capacity

Yes, for the steel beam detail. If you go this route and would like a starter detail, let me know. I've got something but it will take me 15 min or so to dig it up.

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: Strengthening existing RC slab

(OP)
@KootK
I would be very grateful you could dig something up.

Ill post an idea that I have also in my mind but would like you guys to comment on it.

RE: Strengthening existing RC slab

(OP)
Heres an idea.
See picture attached
https://s9.postimg.org/vvd3l30wv/steel-page-001.jp...
Slab is 5 inch thick RC slab
I would use 2 steel beams and position them under the slab.
At support (walls) I would fix the beams using steel plates wich would be anchored at concrete blocks that will be made.
Beams would be positioned as tangent to the concave shape of the slab, so there would be a lost of contact between the beams and slab at the support (max 1 inch gap)
I would use bolts to aditionaly fix the steel beams to RC slab wich would go all the way thru the slab and then I would pour non-shrinkage mortar/grout to fill the whole space between the slab and beams just to have an even contact surface between the two members. Small gaps that would be near supports will be closed before pouring of mortar/grout.

Your opinion guys.

RE: Strengthening existing RC slab

The slab is now two way spanning and will hog over the newly installed beams, perpendicular to the beams, under live load . There is no top reinforcement in the midspan, how does the unreinforced slab carry this moment?

RE: Strengthening existing RC slab

If the negative moment cannot be carried by tension in the concrete, the slab will crack and the negative moment will be zero.

BA

RE: Strengthening existing RC slab

Quote (OP)

I would be very grateful you could dig something up.

I had something quite different in mind. I'd imagined that you'd use a wide flange beam for the reinforcement. My detail won't work with your concept I'm afraid.

Given the depths of the slab and the reinforcing beams here, I can't see this working without making the steel composite with the existing slab. Non-composite, I doubt that the beams will be stiff enough relative to the slab for the scheme to be sensible. Are you working against a headroom limitation on the underside? How much depth can you add?

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: Strengthening existing RC slab

BAretired - So would you pre-empt the crack by saw cutting the slab, fill with a flexible sealant.

RE: Strengthening existing RC slab

Like KootK, I don't think those channels bent the wrong way will do much good.

RE: Strengthening existing RC slab

I too was thinking of a wide flange beam. I'm not a fan of the proposal by mar2805 (OP).

Yes, a shallow saw cut filled with flexible sealant would be a neat way to hide an unsightly crack. Whether or not it is worth the trouble depends on the finish to be applied.

BA

RE: Strengthening existing RC slab

(OP)
Is this what you have in mind?
See picture attached.

Few problems.
How would you solve the lost of contact between the beam and the slab due to slabs curvature?
There will be around 1/2 to 3/4 inch gap between them.
I doubt using steel wedges will be sucesfull.
Would you only fix the beam at the supports with no direct connection with the slab?

@KookT
I have around 7 inches free space under the slab.

RE: Strengthening existing RC slab

You could jack the slab upwards, install the beams and pack, release the jacks, slab will bear on the beams.

Alternatively fit some flat jacks between the beam and the underside of the slab. Both options will have the benefit of attracting both the self weight and the imposed live load from the slab.

Another option is to use an expanding cement to fill the gap between the beam and the slab, poured through a hole in the slab.

RE: Strengthening existing RC slab

That is what I had in mind. Dealing with the gap is easy, My detail resolves that with some adjustable bearing plates. The more pressing question, I think, is whether or not a 6", non-composite beam can successfully be designed for strength and deflection. If not, an underside strapping solution might be required. Or a composite beam.

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: Strengthening existing RC slab

See below for a potential strengthening strategy.

- beams added at regular intervals as reinforcing rather than isolated girder support.

- you probably get pretty good composite behavior but, for starters, maybe see if you can make it work by the numbers non-composite.

- start the fastening from the middle and and use the steel beams to straighten out the concrete to some extent. Might be cheaper than shoring and jacking.

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: Strengthening existing RC slab

(OP)
@KootK

So your idea is to use closely spaced steel profiles that would act as "reinforcing".
Basicly they would increase the moment resistance of the slab in one-way spannig direction.
Slab would still behave as one way spaning.

This is contatry of using 2 large steel beams, each positioned at 1/3 of the longer direction of the slab.
Seem my first post and posted sketch - 33 feet divided into 3 bays, each 11 feet wide to 13 feet long, wich would try to make slab span in two direction IF the steel beams are stiff enough.

RE: Strengthening existing RC slab

If the steel beams can bear on the masonry walls, that is the way to go.

BA

RE: Strengthening existing RC slab

(OP)
@KootK
You havent answered my previous question

"So your idea is to use closely spaced steel profiles that would act as "reinforcing""

RE: Strengthening existing RC slab

Sorry mar2805, I hadn't realized that there was still a question left outstanding. Yes, your interpretation of my suggested solution is accurate.

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: Strengthening existing RC slab

(OP)
Kootk thanks for the response, but I cannot agree with you on you last statement of this being an "reinforcing option" for the slab in one way direction.
I think that only epoxy glued steel plates and carbon fibre strips can be defined as reinforcing solutions that would increase slabs carring moment in the bearing direction.
These are all objects that "dont have" any Area moment of Inertia as they are not solid members resisting bending.
On the other hand, if you put an steel beam under the slab that has significant stifness and its area moment of Inertia, the slab will try "resting" on the beams and will probably in return have an effect of span shortening or reversing slabs direction.

RE: Strengthening existing RC slab

Hello guys,
I hope i'm not too late... it's an interesting subject.
So, from what i read here @mar2805 you are trying to prevent the failure of that poor reinforced slab. I think that serviceability and crack limitation are out of topic.
The failure can occur by:
- reaching the rotation capacity of the top section (rebar break)
- yielding the bottom reinforcement and creating a second yield hinge that will tranform the slab into a mechanism with 1 degree of freedom - large deformation and failure by reaching rotation capacity
(i can make sketchs if needit)

So the straightforward solution is to increase the bending moment capacity on the top or on the bottom. Increasing top capacity is hard on this stage so i will discuss the second solution : increasing the bottom bending capacity at the middle of the 2 spans.
It may appear as a simple topic but the calculations needit to be done for an exact solution can be quit hard and time demanding.
First you have to evaluate the bending yield capacity of the top section where the cracks are produced. I would suggest to go with the values of steel and concrete strength specified in the rehabilitation codes (consolidation codes) because they use the average (a lot more realistic) values not the probabilistic ones used in new designs, it's a big difference, about 20-30%.
After that you need to evaluate the rotation capacity of that section - same from consolidation codes - and check how much you have remain until is very bad; how do you do that? Measure the bottom deflection in the reality (in site) and calculate the theta (rational angle on the top) that correspond to that deformation (use any structural analysis software). Now you can have some ideea of how big is the danger of failure.
Every new added load on the slab will : 1. increase the rotation angle at the top and 2. increase de bending moment at the bottom (this is redistributed from the top because the yield hinge that is already at full capacity).
In this point with the maximum probable loadings and moment redistribution from hinge the bottom section will remain in elastic domain or will produce another yield hinge? If yes than you have a failure mode by losing the only remained restrained degree of freedom you have.
To avoid this you have to increase bottom slab capacity so that with the maximum loads will not yield (or yield with very small rotation - very little).
Increasing the bending capacity can be made in various ways but i think that KootK's solution with some modification is the optimum cost efficiency.
Creating a composite section is the best but is not so simple to check in design stage. The key in the making the steel profile and concrete slab to work togheter as a composite element is the shear connectors. If they dont hold than you dont have the extra bending capacity from the new steel. So first you have to choose a target capacity of the bottom composite section, as i said, you can choose the maximum bending moment from redistribution with the max loads but you have to check too if the deflection is not above the limit that produce steel failure from rotation capacity on the top section !
I would go with steel channels with mid holes for the shear connectors from steel reinforment - minimum 16mm diameter- fixed with chemical anchors to the slab. The span between the shear connectors and the number of them need it result form composite bending capacity calculation - just like composite deck on steel structures. The span between channels result from steel required to produce that bending moment - you can set the span to be 1 m and choose the profile dimensions from thet bending necessary.
So you can see that selecting a optimum solution can be quit tricky...






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