rebars as anchors - connecting existing and new RC elements 3

[gmd255]

I dont have much experience with reconstructions so I hope my questions wont be silly. Id really like to know what do you guys usually do/suggest in situations like this.

I have to connect a new RC slab (140 mm thick) to existing RC slab (same thickness). There will also be some new openings (span 3 and 5 m) in existing masonry walls. Because of this I will design RC BEAMS where existign walls used to be.

I will connect RC beam/slab to an existing slab like this: drilling holes in existing slab, then epoxy + rebars.

What Im wondering is how to make sure that new stirrups has sufficient anchoring since existing slab is only 140 mm thick. Do you guys design stirrups by considering a shear flow between existing slab and new beam?

About connecting slabs together - I was thinking about doing it in the middle of a slab since slabs are 140 mm thick, but I d feel better if I could connect it top/bottom.
Is it ok that all shear gets transferred by relying on the reinforcing bars only? I would normally chip a bit into the existing slab so that a part of the shear gets transferred with bearing on it which means it isnt 100% relying on the rebars, but since im also adding a RC beam below a slab it can be problematic...

I will appreciate any suggestion, advice, photo or sketch.

Tnx in advance.

 

[SLTA]

I personally am a belt and suspenders kind of engineer. Any deterioration on those rebar and you have no vertical load capacity at that edge - a floating slab. Have you considered adding an angle of some sort off the new beam to assist in supporting the new slab?

Please remember: we're not all guys!

 

[SlideRuleEra]

gmd255 - Design the epoxy/rebar dowel connection's size, location, and spacing based on manufacturer's (e.g. Hilti) recommendations. In a 140 mm slab, the edge distance ( <70 mm, at best) will probably be the controlling factor. Don't try to crowd too many dowels into the corner. This is self-defeating since drilling will make "swiss cheese" out of the existing concrete. Interference when drilled holes hit rebar in the existing slab will likely force field changes in dowel locations. A compromise on a reduced number of dowels may be necessary.

Take SLTA's advice for additional support at every edge where slab-to-slab epoxy dowels are used.

I have marked up your sketch to show what may be improved conceptual locations for slab-to-slab epoxy dowels:

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[KootK]

Do you guys design stirrups by considering a shear flow between existing slab and new beam?

Yup.

What Im wondering is how to make sure that new stirrups has sufficient anchoring since existing slab is only 140 mm thick.

Yeah, that's tough. You obviously can't provide standard hooks at the top of the stirrups. Nor will you likely be able to develop the bars for Fy/shear friction with the drill and epoxy install. A little off-reservation judgment might be required. Some options:

1) Try to design the beam as non-composite so that the stirrups don't have to be developed into the slab.

2) Use the bearing wall dead load as an effective clamping force for shear friction.

3) Choose not to worry about developing the bars for Fy into the slab.

I'd probably go with #3. I've seen some research that you can develop a fair bit of shear flow resistance even without any dowels.

but I d feel better if I could connect it top/bottom.

I wouldn't sweat this. Just get your shear connection and accept that the joint may open up a bit. You could tool the joint so that it does so cleanly.

I would normally chip a bit into the existing slab so that a part of the shear gets transferred with bearing on it which means it isnt 100% relying on the rebars, but since im also adding a RC beam below a slab it can be problematic...

1) SLTA's approach solves this.

2) Were it aesthetically ok, you could widen the beam to provide a ledge.

3) A ledge wouldn't need to be but a couple of inches. Perhaps you could increase the cover on your beam stirrups so that a 2" ledge could be chipped into the existing slab.

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.

 

[dik]

At the re-entrant corner, I would likely treat the -ve moment as a fixed end condition of a propped cantilever and use a heavier bar at the centreline of the beams... maybe a single 25M (#8) and reinforce the balance accordingly.

Dik

 

[hokie66]

I would want to chip out the existing slab to half the width of the wall. That will serve the dual purpose of providing for bearing for the new slab and giving you access to cast the beam. As to the stirrups, I would provide them with different leg heights, and 180 degree hooks on both.

 

[gmd255]

tnx for replies! SlideRuleEra - I agree, good advice.

I would want to chip out the existing slab to half the width of the wall. That will serve the dual purpose of providing for bearing for the new slab and giving you access to cast the beam. As to the stirrups, I would provide them with different leg heights, and 180 degree hooks on both.


hookie66 - im having some troubles understanding what you suggested (English language is not my 1st language so I have so issues with expressions). Can you please pst a sketch? I d really appreciat it.

Kootk - thanks, helped a lot.

I think you guys were suggesting something like this:

 

[KootK]

You interpreted my stuff correctly. You might need a little more stirrup cover on the one detail in order to do the drill and epoxy business correctly.

Hokie's proposal would work well if you can keep that bearing wall above properly supported during the intermediate stages. You probably need some shoring no matter what path you take. I think that he's proposing having one of the stirrup legs project up into the existing slab concrete that will be chipped out at replaced. That would be a slick way to get a 135 hook on that stirrup leg. And perhaps you could space your ties to satisfy your shear requirement with just that one leg.

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.