How to cast an inverted beam over a slab that has already been constructed? 2

[Ryuichi]

Q1: How to cast an inverted beam over a slab that has already been constructed?

One part of the slab has no beam under due to vertical clearance restriction under it, thus making the beam-less part vulnerable.
This is called a two-way 3-support slab. Where only 3 beams support the slab. One side, the longer span, 5.5m length has no beam.

I am thinking of adding an inverted beam to an existing slab.
Q2: Will an inverted beam strengthen it like a normal beam would do or if not will it help at least?


My other option is adding CHB wall under the beam-less slab. (2m-3m length where the remaining non-walled is the entrance).
Q3: Can the CHB wall fulfill the purpose of the supposed beam or if not will it help at least?

Q4: Should I connect the rebar of the existing slab to the inverted beam?
or
Should I not connect and let them be, let the CHB wall under it act as the load bearing.

Actual 3D Model: (The inverted beam along with the stairs are not yet installed)

Related Reference:

 

[hardbutmild]

Firstly, I think you shouldn't do this just based on our suggestions and you should get an engineer to look at it.
I'll try to answer your questions, but bear in mind that I might say something wrong.

Q1: Based on your drawing I guess you plan to add the beam next to the existing slab and not on top of it. I drew a picture. So you want to do the one on the left as I understand it.

Now, I don't actually know why you want to make this beam. If you need it because an engineer said that he needs a support for the slab at that edge then you'd need to connect them. To do that I guess you'd have to drill a horizontal hole in a slab and put reinforcement that looks like a letter L. Now the actual size (diameters and lengths) of the bars and their distance I'd suggest you get from an engineer (not us, someone who'll sign on it). You'd then fill that hole with concrete.

Now If the slab doesn't need the beam (it looks like that to me) and you need it just to make a step before the stairs then I suppose you don't need to connect them (if someone knows any reason for it, please tell).
That kind of answers the Q4 I guess.

Q2: It could help but it could potentially make it worse (depends on the way reinforcement is positioned inside a slab).
Q3: It should be similar to a beam, but you might have a problem with a slab under this one because now you have a wall on it. Beam seems like a better solution to me.

 

[BAretired]

Q1: How to cast an inverted beam over a slab that has already been constructed?

Surely the contractor had something in mind when he poured the slab. If it looks anything like the 3D model, the upturned beam, together with the upper slab can be poured together (monolithically) making an "L" beam. The beam-less edge of slab must have sufficient reinforcement projecting out so that its reaction can be carried by the new beam. This detail will need a bit of thought on the part of your structural engineer.

One part of the slab has no beam under due to vertical clearance restriction under it, thus making the beam-less part vulnerable.
This is called a two-way 3-support slab. Where only 3 beams support the slab. One side, the longer span, 5.5m length has no beam.

Call it what you like. It is not acceptable without edge support on the fourth side.

Q2: Will an inverted beam strengthen it like a normal beam would do or if not will it help at least?

Yes, if properly analyzed and detailed, it will be adequate.

My other option is adding CHB wall under the beam-less slab. (2m-3m length where the remaining non-walled is the entrance).
Q3: Can the CHB wall fulfill the purpose of the supposed beam or if not will it help at least?

That is an option if the wall has an adequate foundation below it.

Q4: Should I connect the rebar of the existing slab to the inverted beam?
or
Should I not connect and let them be, let the CHB wall under it act as the load bearing.

Either option is acceptable provided the details are carried out properly.

BA

 

[Ryuichi]

I feel some very small vibrations above the part of beam-less slab and would like to remove if possible.

I think connecting the inverted beam to the slab (by breaking/chipping the edge of the slab) is a lot of work.
than
Letting the beam-less edge be supported under with CHB wall.

The projected CHB wall has a beam under that is roughly 2m-2.5m which will at least support the center of the
beam-less edge but would not extend to support the whole 5.5m length.

Another purpose of the inverted beam is to carry the hallway slab above it then the stairs.
The beam-less edge will have a wooden wall and a door that separates it from the hallway and stairs.

 

[BAretired]

I think connecting the inverted beam to the slab (by breaking/chipping the edge of the slab) is a lot of work.

Can't argue with that.

than
Letting the beam-less edge be supported under with CHB wall.

But it is supported for a length of only 2.0 to 2.5m, leaving 3.0 to 3.5m unsupported. Is the slab reinforced adequately for that condition or will it develop unsightly cracking when future load is applied?

The problem should be assessed by the structural engineer for the project.

BA

 

[msquared48]

There is a project structural engineer, correct?

Mike McCann, PE, SE (WA)