top reinf for a cantilever beam and existing slab to be enlarged 1

[geo321]

Hello,

I have 2 independent questions

1- For a cantilever beam where the top reinforcement can not be anchored in the column since the ldh (ACI318-08) is bigger than the columns dimensions, is it ok if i extend the top reinforcement into the adjacent slab and make a class A splice ? (refer to the attached section). (PS i dont want to decrease the bars diameter and increase their numbers)

2- An existing reinforced concrete solid slab needs to be enlarged. Dowels will be planted in the slab. A hole of 15 x bar diameter will be drilled and bar along a bonding agent will take place. The connection between the existing and the new slab is a pinned connection. Correct?
If i need to have a continuity and a moment connection, i should demolish a part of a slab and ensure that the bars have the adequate development length. Correct?

Thank you all

 

[geo321]

Oups , forgot the attachement

 

[BAretired]

1. I think it is okay to anchor the top bars into the slab
What is the purpose of the bottom bars hooked each end?
How will you engage the column steel into the cantilever?

BA

 

[BAretired]

2. Please explain your second sentence. It make no sense to me.

If you need continuity, you could do as you suggest or use a welded splice for each bar.

BA

 

[KootK]

1) Your goal here is a moment connection between the beam and column, right? If so, I don't think that the proposed detail is acceptable as it would be possible to open up a diagonal crack across the joint (bottom right to top left) that would not cross reinforcement. I believe that your left side column rebar should extend to the top of the beam and then hook to the right and out to the end of the beam. Let me know if that's not clear and I'll provide a sketch. Another alternative is to lap the beam top steel with the left side column rebar as it rounds the corner. Either way, however, there's little to be gained from running the beam reinforcing out past the column.

2) The connection will possess some degree of fixity but you're right, I would assume it to be pinned unless you've actually lapped the new and existing rebar for the required rebar tension. That said, you can often create a post installed rebar connection that laps new and existing bars without chipping away the existing concrete. See this doc for some ideas: Link

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.

 

[geo321]

To BAretired,

I have attached a sketch illustrating the above (sketch 1 is for question 2 and sketch 2 is for question 1 - not very logic - excuse me).

To Kootk,

This is how i am planning to make my column reinforcement.

Thank you for your answers

 

[KootK]

Works for me so long as your right facing column bars are lapped with the beam top steel. Depending on the height of your beam and width of your column, you may want to continue your column ties up through the 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.

 

[bookowski]

Are these related to the same issue, i.e. you are trying to add on a balcony to an existing structure? If so that may change the answers you are getting.

 

[geo321]

No bookowski. They are 2 independent questions. I have already specified it in my initial thread

 

[rapt]

For question 1, you would need to check how far back into the slab you need to extend the reinforcement for the moment in the slab from the cantilever. Firstly, the slab depth is significantly less than the cantilever depth, so it may actually need more reinforcement than the cantilever side needs. Depending on the length of the slab span compared to the cantilever length the moment could drop off very slowly and the slab reinforcement may not be sufficient for the moment induced.

For question 2, do the hooks at the ends actually overlap fully as shown? With dowels, I think you need to make sure that there is sufficient vertical reinforcement at the edge of the existing slab to catty the shear.

 

[geo321]

Hi rapt, dowels will be checked versus shear friction. As for shear reinforcement, it will be verified as u mentioned. Regarding the overlapping, there will be a full overlap for the new slab and 15 bar diameter for the dowels which will be accommodated in the existing. (Don't know if I made myself clear).
Thank you all for your replies

 

[KootK]

One glitch with the shear friction proposal is tat, per code, you have to develop the bars for fy either side of the joint. And that would mean Ld here.

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.

 

[geo321]

Yes kook but since u are using a bonding agent the ld will decrease for the dowels running in the existing slab or wall. For the new slab, ld is a must.

 

[KootK]

I agree. So long as you're developed for fy by some mechanism, it doesn't necessarily have to be Ld. I'd recommend going with one layer of dowels at mid-depth of the slab. It can be tough to install the dowels close to the top and bottom of the slab without spalling the concrete. This would affect your Vc shear capacity locally but that's rarely critical in slabs.

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.