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

 

[mar2805]

Guys, anyone? :-(

 

[Trenno]

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

 

[mar2805]

Seem also like na option but pricey....

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

 

[KootK]

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.

 

[BeFEA]

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.

 

[lsan8440]

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

 

[BAretired]

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

 

[mar2805]

@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.

 

[BAretired]

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

 

[KootK]

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.

 

[jayrod12]

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.

 

[mar2805]

@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

 

[KootK]

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.

 

[mar2805]

@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.

 

[mar2805]

Heres an idea.
See picture attached

https://s9.postimg.org/vvd3l30wv/steel-page-001.jpg

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.

 

[pappyirl]

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?

 

[BAretired]

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

BA

 

[KootK]

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.

 

[pappyirl]

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

 

[hokie66]

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