A Cantilever slab with no backspan?
A Cantilever slab with no backspan?
(OP)
Hi,
I am in the process of reauthenticating the design of a villa already in progress made by a former colleague who left our company. The villa consists of a G+1 floors, with provision for a future floor.
Currently, Superstructure works up to the 1st floor columns are completed, and the contractor now is in the process of fixing the formwork for the 1st floor beams.
We have a 250mm thick in-situ concrete slab with 3-sided supports, and 1-sided cantilever. However, I am concerned that the slab does not have a backspan, as the rest of the villa is constructed using hollow-core slabs. I would appreciate your advise on how we could possibly develop the rebars sufficiently to develop the necessary rigidity in the beam/slab connection with minimal architectural changes.
Please refer to plan extract attached. Note that the beams are 500mm deep.
Thank you for your support.
I am in the process of reauthenticating the design of a villa already in progress made by a former colleague who left our company. The villa consists of a G+1 floors, with provision for a future floor.
Currently, Superstructure works up to the 1st floor columns are completed, and the contractor now is in the process of fixing the formwork for the 1st floor beams.
We have a 250mm thick in-situ concrete slab with 3-sided supports, and 1-sided cantilever. However, I am concerned that the slab does not have a backspan, as the rest of the villa is constructed using hollow-core slabs. I would appreciate your advise on how we could possibly develop the rebars sufficiently to develop the necessary rigidity in the beam/slab connection with minimal architectural changes.
Please refer to plan extract attached. Note that the beams are 500mm deep.
Thank you for your support.
RE: A Cantilever slab with no backspan?
2) If possible, I'd try to make the slab work with just the three sided, simple supports, relying on no back span action.
3) if the hollow core plank has a topping and the elevations work out favourably, you might run some balcony top steel over the first couple of planks and into the topping as a redundant, belt and suspenders measure.
RE: A Cantilever slab with no backspan?
Looking at it again, I'd check to see if 1/2 the width of the 10" slab and reinforcing could span the 5+m (col to col) supporting the total load... and then treat the transverse slab as a cantilever about the line joining the columns... just as a first 'kick at the cat' if this is more than ample, I'd be happy with it... if more than 70%, I'd go a lot deeper into it.
Do you know how it was initially analysed?
Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?
-Dik
RE: A Cantilever slab with no backspan?
Unless they contributed to weight, I would not likely consider them at all... just cloud the problem...
Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?
-Dik
RE: A Cantilever slab with no backspan?
RE: A Cantilever slab with no backspan?
RE: A Cantilever slab with no backspan?
Also tell us how you modelled the slab and what load (ultimate or serviceability). I assume uncracked, short-term properties but tell us the relevant details. Pretend we know nothing about your structure and analysis except what you tell us.
RE: A Cantilever slab with no backspan?
RE: A Cantilever slab with no backspan?
Hopefully my crappy sketch is self explanatory.
Ps - the dashed line isn’t a beam. Just a delineation between two slab span directions. It would work with an FE analysis in my view.
RE: A Cantilever slab with no backspan?
@KootK: I thought about that but as you said, its not fitting with the architecture.
@MIStructE_IRE: You mean design the yellow slab as 1-way (North to South) and the purple would be cantilevered off that slab?
RE: A Cantilever slab with no backspan?
SDL = 3 kN/m2 so the total DL = 0.25*25+3=9.25 kN/m2 is this true ?
My suggestion will be use drop beam and reduce the thickness of cantilever to 150 mm. Proposed beam shown with red below..
RE: A Cantilever slab with no backspan?
RE: A Cantilever slab with no backspan?
How does it deflect if you delete the cantilever area?
RE: A Cantilever slab with no backspan?
RE: A Cantilever slab with no backspan?
As noted above, the 1/2" high strength bars @ 6"o/c top and bottom could help...
Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?
-Dik
RE: A Cantilever slab with no backspan?
But that doesn't account for bearing of the precast slabs. Maybe the beams need to be widened to accommodate both the precast slabs and the balcony.
RE: A Cantilever slab with no backspan?
I'll revise to 300mm thick or place the beam in the middle as others suggested.
Thanks for your input everyone.
RE: A Cantilever slab with no backspan?
realistic deflections? also long term deflections?... if you can add a beam, that's the easiest and you may be able to use a 150 slab...
Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?
-Dik
RE: A Cantilever slab with no backspan?
When using 300mm thick slab, using same applied loading for Fcu = 35N/mm2, then:
* Deflection from linear analysis, with inertia modified to effective moment of inertia (using ACI-318 method)= 2.8mm (SLS case, assuming 100% of live load conservatively)
* Calculated long term deflection modifier = 1.733
* Long term deflection = 1.733 x 2.8 = 4.85mm
* Allowable deflection = L/480 = 2800/480 = 5.8mm
Consequently, the slab design is ok provided bottom rebar is increased to 16mm @ 150mm to suite ULS requirements.
RE: A Cantilever slab with no backspan?
Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?
-Dik