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Meshed slab supported by beams

9 REPLIES 9
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Message 1 of 10
Casparov
10175 Views, 9 Replies

Meshed slab supported by beams

Hi

 

I'd like to support a concrete slab by a concrete or steel beam.

 

I'm interested in the moment distribution for the beam, and in the mean time would like to know the moment distribution for the slab (e.g. Bottom reinforcement...). How can a model be set up which combines those two results? Because once the slab has been meshed, also the supporting beams are divided into elements and the expected (global) moment distribution for the beam isn't shown.

 

See attachment 1: meshed slab.

See attachment 2: slab without finite elements and 'normal' moment distribution in the beams.

 

Thanks in advance

9 REPLIES 9
Message 2 of 10
Rafal.Gaweda
in reply to: Casparov

Hi

 

I am not sure whether I am answering for your question or not but in case of RC beams you can turn on option of considering slabs and then see results for such beam in Forces table.

 



Rafal Gaweda
Message 3 of 10
Casparov
in reply to: Rafal.Gaweda

Hi Rafal

 

I can't see the image above.

 

What I would like to build is a model that combines both slabs and beams (be it steel or concrete beams) in such a way beams and slabs don't work together (i.e. the rigidity of the slabs should not influence the results created for the beams). So, the beams only act as a line support for the slab, and the slab only distributes its load onto the beam (e.g. Moment in the beam = PL²/8), as we'd do in an easy and quick hand calculation.

 

As you can see in the model in attachment, slab and steel beams are both influencing each other (working together)?

 

The load onto the slab equals 100 kN/m², and the slab is 10m by 10m. So the load onto the beams equals 500 kN/m as the slab is only distributing load in towards the beams, resulting in:

  1. Moments M = PL²/8 = 6250 kNm (i.o. 5265.91 kNm)
  2. Shear force V = PL/2 = 2500 kN (i.o. 2233 kN)

So actually, I'd like to know if there's a possibility of both seeing a moment and shear force distribution in the slab, and the moment and shear force distribution for the supporting beams as we'd expect.

 

To make my question a bit more understandable, we're designing a structure which uses prefabricated (also prestressed) hollow floor slabs and beams. To determine which slabs we should use, the contractor provided us with an easy to use guide containing all resistance moments and shear forces for any of the slabs and beams we'd like to use. For this, we assume the connections between slab and beams are not rigid (moment connections...), but the beams are just supporting the slabs.

 

Thanks for your appreciated advice.

 

Regards,

Casper

 

 

Edit: the slab in the above example is 300 mm high, C12/15 concrete quality; Deck slab (one-way): Shell FE, One-way load distribution and no diaphragm. The supporting beams (simple bars) are HEA1000, S235 steel quality.

Message 4 of 10
Rafal.Gaweda
in reply to: Casparov

If you want to see such results you have to use cladding with unidirectional distribution of loads instead of slab panel



Rafal Gaweda
Message 5 of 10
tony.ridley
in reply to: Rafal.Gaweda

Casparov,

 

You don't need to use cladding with unidirectional load distribution.

 

Instead you can use an orthotropic slab with no transverse stiffness.

Note that Robot won't let you put in zero (0) for "n2" in the definition box.  Enter it as 0.00001, then run your calculation.  When you re-open the definition dialogue box it will be blank.  

 DEFINE ORTHOTROPY.png

 

This will then get you pretty close to the results you are after

 

BENDING MOMENT BEAM.pngSHEAR BEAM.png

 

Message 6 of 10
tony.ridley
in reply to: tony.ridley

note my slab is 250 thick solid concrete, shell element, no diaphragm.  I have not used linear releases on my slab at the beam interface like you did.

Message 7 of 10
Casparov
in reply to: tony.ridley

Tony

 

Thank you very much for your help. Your solution resembles the closest to the results I expected.

 

Thanks again.

Casper

Message 8 of 10
OllieT
in reply to: tony.ridley

Tony,

 

I'm amfraid this is a beginner's question,

I have attempted to replicate your model of the meshed slab problem. I set up two beams placed a floor slab on them. As you described previously, I set the slab as orthotropic with no transverse stiffness.I loaded it with 100kn/m2, calculated and obtained the moments and reactions in the beams that I would expect, ie the load seems to be transferred to the beams ok.

However, try as I might, I cannot obtain a moment in the slab that i would expect. In fact it's not showing any moment. When I run the calculation I get a warning telling me there are separate structures.

 

Any ideas?

 

 Regards,

 

 

Oliver

Spoiler
 

 

 

 

 

Regards,

 

Ollie

Message 9 of 10
Rafacascudo
in reply to: OllieT

Your panel is not meshed.

You have to change your panel calculation model marking " finite element type" and " analytical(finite elements) in the picture below.

Then on " mesh options" ,set a meshing size, and meshing method.

model calculation.jpg



Rafael Medeiros
Robot Structural Analysis

Message 10 of 10

Hello Everyone,

 

just to make it clear in my mind: If you want to model a concrete slab properly you either have to model it as a shell using finite elements shell or you can model it as "No Finite elements". If you do model it as a shell you will have stresses and required reinforcement in the slab but you will not have loads distributed to the supporting beams. Therefore your beams required forces and moments will be highly underestimated and in the same time your columns forces would probably be overestimated.

 

On the other hand if you do model your slabs as "No finite Elements" then the forces in your beams will be as expected, the columns moments and forces will be less and you will have no information on how to design the slab. 

 

Is that correct? If it is, could you advice what should be the proper way to design each one of your elements (columns, slabs, beams). Because the two different scenarios drive you to significantly different results.

 

 

 

 

 

 

 

 


@Rafacascudo wrote:

Your panel is not meshed.

You have to change your panel calculation model marking " finite element type" and " analytical(finite elements) in the picture below.

Then on " mesh options" ,set a meshing size, and meshing method.

model calculation.jpg



@Rafacascudo wrote:

Your panel is not meshed.

You have to change your panel calculation model marking " finite element type" and " analytical(finite elements) in the picture below.

Then on " mesh options" ,set a meshing size, and meshing method.

model calculation.jpg


 

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