is this possible - cantilever RC slab 8

[mats12]

Hello

Residental building, the interior RC slab in not at the same level as the cantilever slab.
The cantilever part is non walking surface - only self and permanent load + snow load + wind load.
Can this be reinforced properly so it could work even tho slabs are not at the same level?
Dont ask why this cant be at the same level - it has to be like this acording to architect...

Thank you for help.

 

[WARose]

Can this be reinforced properly so it could work even tho slabs are not at the same level?

Assuming you have adequate space for the girder/slab turndown? (That part isn't dimensioned.) Most definitely. I've handled worse (although in industrial settings).

Just eyeballing it....you are (also) going to need some healthy RC columns for the girder to frame into as well.

 

[MIStructE_IRE]

Sure, no problem. Its at a lower level for weathering. Normally you’d expect a 150mm step or so.

It does however leave you with a thermal bridging issue - but fear not, Schoeck Isokorb have the solution to that..although they don’t come cheap.

This image shows level slabs but if you review the Schoeck brochure or contact them they’ll give you a design for the step detail. I’ve done it before without any issue.

 

[KootK]

Yeah, it's a good detail for keeping water out of the interior of the building and, certainly, better than the reverse. Do mind your anchorages and development lengths of course.

 

[engineering_patrol]

make sure that cantilever is at same level to the slab in order to ensure continuity of the reinf. bars.

 

[rowingengineer]

Eng patrol, that isn't required as per the above post you can have the at different levels as long as you detail it correctly. See KK post.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[BAretired]

The way KootK is detailing it makes much more sense than cantilevering the slab. Cantilever two deep beams, one each side. Frame the end beam as a simple span between the two cantilevered beams and span the slab as a simple span, restrained slightly at each end.

BA

 

[r13]

I like BA's proposal. Also, watch out for potential of ponding

 

[hokie66]

I doubt that BA's two beams at the end suit the plan of this structure.

But I also have problem with KootK's approach. I would want some continuation of the exterior soffit level into the interior. The top bars just hooking in the beam doesn't give me enough confidence.

 

[KootK]

I take exception to "just hooking". I specifically mentioned correct consideration of development and anchorage. Approached skillfully, the "anchorage" part will result in the requisite soffit extension, even if that extension exists within the beam width which is what I would recommend for formwork simplicity if that is feasible.

 

[hokie66]

Exception noted. My comment stands.

 

[Tomfh]

Is there any reason (Eg ceiling) why the exterior soffit can’t continue inside of the beam?

Also, Could the beam simply be wider?

 

[mats12]

Thank you for feedback.

The beam at the end of cantilever is not the part of a bearing structure - its span is large (10 m) and it cant be supported because of geometry and architect demands.
I should make this more clear in my OP.

@Kootk - to me this detail is more like (1) which is a no go in this case than (2) which is what I had in mind.

Dimensions of the beam:

 

[rowingengineer]

The detail KK has proposed is a fixed ended slab detail aka 2, the only part we are not agreeing on is the detail of the tension bar. Hoikie seems to think something like this would be better

 

[Tomfh]

I think kootk’s will work, but agree that the hairpin plus diagonal is more decisive.

 

[HTURKAK]

I suspect that the long term deflection will be an issue..the cantilever slab thickness 150 mm , I think, will not satisfy the deflection criteria .If the roof drain gully near the bldg, ponding will also be a probable risk. Moreover, due to compatibility torsion with the torsional resistance of the beam 300X600 mm ( if the span is 10m ), the tip deflection of the cantilever slab will be amplified..

The typical span to depth ratio for cantilever slab is L/10 .. increasing the thickness to 200 mm could be an option but the own weight will be too heavy . If i were in your shoes, i would prefer to construct the cantilever portion with steel (light steel frame post installed to concrete structure).

 

[hetgen]

Is the slab a roof or an accessible balcony? Looking at the 3D sketch it looks likes just a roof, not a balcony.

If that is the case, I think, the slab could be designed as a one-way spanning thin section if you add two upstand cantilever beams supported on the walls below to support the edge beams. (see the markup below)


You may need to provide drainage openings through the web of the beams to connect the three-compartment formed due to the upstand.

 

[jayrod12]

I'd be doing something like hetgen shows. And also likely using those cantilevered beams to support the fascia beams you said can't be used for anything. Provide a similar cantilevered beam at the far end of the flat roof and your slab becomes simply supported between beams all around. Seems the most confident.

 

[KootK]

@Kootk - to me this detail is more like (1) which is a no go in this case than (2) which is what I had in mind.

No, as rowingengineer alluded, I was absolutely detailing to your [case 2], true cantilever behavior. In my experience, this setup is both common and free of reported problems. it's nothing more than a light duty slab fold in my opinion.

 

[dik]

put scuppers in the upstand... then it drains...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik