## Robot Structural Analysis

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# T slab

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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!

# Re: T slab

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# Re: T slab

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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.*

**Artur Kosakowski**

# Re: T slab

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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?

# Re: T slab

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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.*

**Artur Kosakowski**

# Re : T slab

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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 ?

# Re : T slab

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Exactly

**Rafal Gaweda**

# Re: T slab

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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

# Re: T slab

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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.*

**Artur Kosakowski**

# Re: T slab

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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.