Bubbledeck / Cobiax voided slabs modelling

[Anonymous]

Hi,

I will start a new model for a 14 storey building in Adelaide (Aus) and the slab system is going to be Bubbledeck (slab with internal sferic void formers).

My question is how would be the best way to model these kind of slabs in Robot? I have done this in 'Scia Engineer' but never in Robot and I don't really know which floor system to use.

Have anyone done this before? Thanks for your time!

Cheers

Ramon

[Artur.Kosakowski]

I'm not familiar with this system and I'm not sure what is the final goal you want to achieve but if your objective is to reduce weight I'd assign it a new material with density being 2/3 of typical concrete. 

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[Anonymous]

How about Hollow blocks slabs Mr Kosakowski?
I need to model a ribbed slab
When I do ,Robot conseiders it as a homogeneous slab and I cant get the moments or RFT of the ribs
is there a way to do it ?

[Artur.Kosakowski]

Please check:

http://forums.autodesk.com/t5/Robot-Structural-Analysis/T-slab/m-p/3185418

http://forums.autodesk.com/t5/Robot-Structural-Analysis/Beam-offsets/m-p/3332921

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[StefanoPasquini6790 · Accepted solution]

Hi,

I used cobiax technology sometimes, also with good results in comparison with the calculation report of cobiax engineers.

I've done this way:

1) I've defined a new concrete material, with self weigth reduced. This reduction depend by the dimensions and density of the bubbles;

2) I've applied to the floor panel (slab) the inertia reduction coefficient. Usually in cobiax technology the reduction coefficient is about 0,88-0,92. For the definition of the right reduction inertia coeff. I have an excel spreadsheet, unfortunately is in Italian. Cobiax engineers give me this spreadsheet, if you don't have it, try to request to your local cobiax engineers. Let me know if they don't have...

3) take care to model correctly the unbubled zones for punching with a slab that have the full inertia and the full selfweight.

This way you can model the slab correctly and calculate the required and provided reinforcement.

At the end take care to share your results (reinforcements, deflections an so on) with cobiax engineers for comparison.

Hope I help you, cheers.

[Anonymous]

Thanks StefanoPasquini6790!!

I reckon this is what I'm going to do. I was already thinking on this option but I wanted to make sure there was no 'direct' option as there is in Scia Engineer.

The Bubbledeck engineers here in Australia provided the self weight of the 'bubbled' areas so I can work this out. For the inertia reduction I will use Scia Engineer which allows you to vary size and spacing of bubbles and slab thickness and then gives you the coeficient (and also the self weight reduction which I will compare with the one provided).

Thanks for all the quick responses guys!!

Cheers 

Ramon

[StefanoPasquini6790 · Accepted solution]

hi,

one more trick. You can also model the slab as a bidirectional holed one, using this function

you can calculate the equivalent box hole that is equal to the bubble volume. The only thing that you have to know is that with this kind of slab you can't calculate the provided reinforcement, and you can't have the drawings...

It's your choice, try to make two models, one with the bidirectional slab and the other with equivalent slab and make the comparison.

Let us know your results, cheers

[Anonymous]

Hi,

Thanks for the tip. I thought about this one but initially dismissed due to the rectanular nature of the grid...but you are right I might just find the equivalent weight and stiffness. 

I will post the comparison once done (it will take a while as the architects are still adjusting grid locations), I will start modelling in a few weeks and getting results in a few more...I'ts a 14 storey building with 2 transfer slabs at 2nd and 3rd floor with first 3 levels using concrete framing (car[ark) and then concrete walls and bubbledeck 2 way slab (apartment)s.

Now that I have you i would like to ask you If you have modelled a composite slab (concrete over steel sheet) supported by steel beams and columns (beams not used as composite). I reckon that this is not an uncommon situation at all and surely Robot must have a way to do it?

I'm trying to do it but having problems with moments on slab and beams...please check this out:

http://forums.autodesk.com/t5/robot-structural-analysis/composite-slab-over-steel-beams-getting-wron...

Any help (reply in that post) will be much appreciated.

Cheers,

Ramon

[Anonymous]

Hiii which software you used to get results on analysis of this voided slab

[miguel_serraTTQEY]

I would just modify the stiffness and weight of the slab, design normally for bending. When considering these kind of slabs, with top and bottom layer continuity, im pretty sure you can model them with modified weight, bending stiffness and axial stifness. Torsional stiffnes is always tricky, but im pretty sure it won't matter much if using Wood and Armer method for design bending moments as the extra bending is adressed. In terms of deflections, you never really really know, no mater you are if using normal or hollowed. It's always +- 50% in reallity (humidity conditions, time of propping, water content, cement and aggregate type, your grandmother birthday, dog ate my calcs, and so on). If really worried about deflection and dont mind being a tad honerous, reduce torsion stiffness to 15% or so. Deflections really get large by doing this. If it works with this stiffness in your model, it will always be good on site. If you were using simple ribbed/waffle slabs, i would always recomend this, for hollowed, not so much.

The one thin you should check by hand is the shear in the ribs. There are some particular fenomena in ribbed/hollowed slabs shear that must be adressed, like the edge ribs combined shear (checked at 45º).