How to design reinforcement for RC T slab (T beam) in Robot?

[Anonymous]

Hi

I'd like to compose a concrete slab with beams underneath: the beams are spanning between columns, and should work together with the slab.

What's the best way to implement this in Robot?

At the moment, I've defined a slab (let's say 250mm high), and beams (also 250mm high) with an offset so they "touch" the bottom of the slab with their upper part. The total build up will be 500mm. Is this correct, or is there a different method to define a slab with beams at predefined locations?

Thanks!

Artur Kosakowski edited the topic for better findability and marked posts as the solution. If it did not work as an answer, please post your reply in this thread so that I can unmark the solution. Artur Kosakowski

[Romanich]

Hi,

I was doing some comparisons, but in Russian. Take a look

[Artur.Kosakowski · Accepted solution]

I would propose yet another approach that you may find useful especially in cases when you actually want to design a T shape beam in the RC Beam Design module after importing it from the model. In such case use of offsets that correctly 'model' stiffness of a slab cause existence of axial force that we usually don't want to 'see' while calculating reinforcement inside the beam (offset cause the bending to be replaces by set of the compression force in a 'flange' /panel/ and the tension force in 'web' /bar element/ or vice versa /span or over supports/). My approach is to model a rectangular beam ('web') with no offset but with artificially increased moment of inertia (see the attached picture - IY of rectangular beam multiplied by ratio of IY of  T shape beam / IY of rectangular beam). This approach causes 'increase' of bending in the beam (T shape beam effect) while no axial forcse in the beam being generated.

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[LuisCRodrigues]

Artur,

In your example, you have increased the moment of inertia by a factor of 1,7.

When working in a model with a lot of different sections with a lot of different spans, what is your criteria?

Do you use the same factor for all the beams?

Do you use define a factor for each beam/span?

[Artur.Kosakowski]

As I wrote I usually increase  IY of a rectangular beam multiplying it by the ratio of IY of  T shape beam / IY of rectangular beam. 1.7 is just the sample value I typed; could be 2.3 or 1.5.

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[Anonymous]

OK if I undestand it well,

                                        by doing that this way the user can design this transformored fictiv section with the T-Beam module design in ARSA ?

[Rafal.Gaweda]

Exactly

[Anonymous]

Dear Artur

Pls can you refer me to the litrature backing up your solution to your post in 10-10-2011 12:23 

I am solving similar problem with continous reinforced beam with T and L-beam interaction with slab

also considering that when there is a negative moment at the support I dont want the T-L-beam advantage according to Eurocode

[Artur.Kosakowski]

This is the approach I found as best matching the situation however I don't know if it is described somewhere. For your case you can divide the beam into 3 parts and increase Iy for the middle one only leaving the parts close to supports 'as they are'.

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[Tuctas]

  The dispositions of Eurocode that you are referring to, concern the design of T beams not the analysis. In analysis the effective area of slab is along the member. In design, the effective area concerns indeed the case that the compression due to bending acts on that.    

[Anonymous]

Yes you are correct (Tuctas), it is the design I am refering to. 

so how do I handle the design in RSA to account for it?

[Tuctas]

What about defining T shape beam (not rectangular) directly, without the need to increase manually the moment of inertia and then use Artur's method? The effective width is given by EC provisions and should be assigned in the window of T shape beam definition (although I think it should be given automatically after defining slabs or claddings).

[Artur.Kosakowski]

---

@Anonymous wrote:

Yes you are correct (Tuctas), it is the design I am refering to. 

so how do I handle the design in RSA to account for it?

---

The RC Beam module internally limits 'too' wide flange to the the maximal effective width allowed by the code.

[Tuctas]

 That helps the situation.. Do you mean that when the allowed effective width is exceeded then it turns automatically equal to the allowed value or just that a warning message is appearing after that?

 Besides, i hope that the RC design module takes in to account the effective width only when it is under compression isn't it?

[Artur.Kosakowski]

 Do you mean that when the allowed effective width is exceeded then it turns automatically equal to the allowed value

Correct.

 Besides, i hope that the RC design module takes in to account the effective width only when it is under compression isn't it?

I'm not sure what you  mean. The tension is to be transferred by rebars (concrete is not intended to work against compression only) so the width of the flange seems to be irrelevant for this scenario.

[Tuctas]

O.k, so if the flange is under tension, Robot makes the design in the same way as it would do for rectangular section. I just asked for confirmation.

[Anonymous]

It looks like your reply in 04-14-2013 01:17  will work because it is a similar technic used in SAP and ETAB. I will check it. however it is difficult to know the inner workings of RSA because there are no much published items on it. 

Also I would want to believe with this whole BIM thing, interoperability and better way to interprete design intent is achieved. 

[antoine.carimalo]

In general case of slabs with beams (supported slab or T slab for exemple), I think the inertia modifier must be choosen to obtain the same deflection in both :

- provided slab reinforcement cartography

- beam modulus

If it is not true I am affraid about bending moment distribution betwen slabs/beams

Errors may be bigger if we apply deflection correction in the Slabs/Beams elements finaly design separatly, isn't it?

[t.sautierr]

If you do that, defining directly a T-Beam then you will have the same problem in other location if you keep the exact properties of the slab :

Avoid that part of the slab pannel acts with the beam and then create a king of ("double" flange).

To me, we have got to main solutions :

1) pannels with exact properties of the slab (thickness, material, ..) + Arthur's methods based on the I tbeam / I rect and rectangular beams in the problem.

2) pannels that doesn't interact with the beam + bars defined as T beams. In this method, the question is how to be sure that the pannel does't act with the beam -> it doesn't take any tension/compression forces so :

a) reduced thickness of the pannels to something insignificant (then the design of the slbs is made separately)

b) Modulus of Young reduced to something insignificant (then the design of the slbs is made separately)

c) play with the stifness matrix of the panel (more tricky).

finally to be able to design both slab and beams instracting together, to me, method 1) or method 2 c) with parameters set to giver flexural stiffness is main direction but no flexural stiffness in the main direction of the beams (usually slabs span perpendicular to the beams ... but not always.) 

[Μηχανικος]

Hi,

After going through the whole thread its still not clear to me what is the best way to define a monolirhic beam + slab and how to carry out the design.

However i understand that i should never use the offset command to move the beam below the slab.  Basically this means that the offset command is uselless, so why do they include it in the software in the first place.

Next i also understand that i should never define a T-beam in my model which means that this feature is also useless,

I am new to RSA and i am trying to learn the software and its very dissapointing to see that there is no simple way to define a monolithic design between beam and slab. 

I know there is a way around this by increasing the moment of inertia or by increasing the Youngs modulus so as to increase the stiffness but how accurate is this and how am i going to present the reinfoecment drawings to the client if i cant use the T-beam feature?