New RC slab + existing RC beam 2

[greznik91]

Hello, i got a problem.

i dont know what to do about this:
existing slab is 200 mm thick, existing beam is 250/500 mm
new slab is 200 mm thick

i have to connect new RC slab to existing slab - which is pretty standard (view A-A: rebars anchoring in the middle of a slab and control reabars for shear forces). But in this case I also have an opening in the existing slab (for staircase), so I have to connect my new slab to existing beam (view B-B). i dont know how to do that since beam is only 250 mm wide. I havent done that before. What do you suggest/do in case like this? I have to connect new slab to existing - i m not allowed to do new support (wall, column) next to existing structure.

 

[Once20036]

I`m not sure that I see the problem with going into a beam rather than a slab.
Your bars will only have 50mm of cover at the top of the slab, the bottom of the slab, and the top of the beam. With such a small edge distance, I think that increasing the embedment of the bar would hurt more than it would help.

Personally, I`m not a huge fan of this type of detail due to the small edge distances mentioned above. I prefer to bolt an continuous steel angle into the side of the beam to provide a bearing connection rather than a shear connection. Perhaps you'd be comfortable with a similar detail at the beam.

 

[greznik91]

Tnx for answer but my cover would be approx. 10 mm not 5 mm since slab is 200 mm thick and i ll anchor it in the middle...

 

[KootK]

So long as the connection is transferring shear and not moment, the beam condition should be fine at a dowel embedment of 150 - 200 mm. Shear dowel connections don't benefit all that much from deep embedment anyhow.

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.

 

[greznik91]

KootK, agree. What is your opinion on bolting a continuous steel angle into the side of the beam as Once20036 suggested.

 

[Celt83]

for this application we would typically do a steel angle bolted to the face of the existing concrete beam and either a composite or form steel deck for the new slab, ideally a steel deck that works for the 3 m span unshored. This gets you bearing for the slab, higher edge distances for the anchors into the beam, and potential for some cost savings on the construction end with no form work or shoring of form work needed.

 

[KootK]

KootK, agree. What is your opinion on bolting a continuous steel angle into the side of the beam as Once20036 suggested.

It's a fine solution that has some advantages but it probably wouldn't be my choice here for the following reasons:

1) You may need to fireproof the angle.

2) If it's exposed sofit, someone may have to look at the fireproofing on the angle.

3) If you've got no other structural steel on the job, I'd hate to add just this one piece.

4) If you've got form work elsewhere, it's not a big deal to use form work here.

5) The dowels are pretty easy and 100 mm edge distance is plenty.

Obviously, I'm making some assumptions about the nature of your project and your construction environment.

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.

 

[Celt83]

KootK makes some excellent points regarding the steel angle option.

My usual hesitation with direct dowel from the slab is any top steel in the beam cross section may be getting close to that 100 mm mark depending on how its detailed, especially at that interior column.