beam/slab deflection

[Anonymous]

Hi,

I have modeled a slab that is supported by a combination of wall, columns and an RC beam at one side.

I have reduced the stiffness Iy and Iz of the RC beam to account for long term deflection and avoid any torsion from the slab.

However, what I see is that I get a quite big slab deflection (17mm) when checking with RC design - required reinforcement. However, when I check the deflection of the beam I get only 7mm. Is there a lack of connection between the beam and the slab so I get these values?

[Artur.Kosakowski]

As the elastic method is based on scaling the values from the static analysis try to run the deflection verification with the stiffness update method instead.

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[Anonymous]

Thank you for your reply.

The slab deflection is obtained with the stiffness update method (FEM).

The beam deflection is obtained after I run the calculations at the geometry tab.

It seems that the slab and the beam are not working together?

[Artur.Kosakowski]

They are however you can't see the results of beam's deflection for a model with 'cracked/reduced' stiffness of the slab. The only way would be to  reduce Ig value in the panel thickness definition by the factor that corresponds to the reduced slab stiffness and then calculate such model. 

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]

What is the best way to obtain long term deflection for the beam?

I have reduced its stiffness by 0.3 x Iy. Wouldn't that be accurate when running static analysis?

Also, the slab shows 17mm deflection at the location of the beam. If they work together that should be how much the beam deflects as the beam is supposed to be supporting the slab.

[Artur.Kosakowski]

What is the best way to obtain long term deflection for the beam?

I have reduced its stiffness by 0.3 x Iy. Wouldn't that be accurate when running static analysis?

You may consider designing a beam reinforcement first and look at the obtained deflection. Then you can 'update' the model with the corresponding reduction of the beam's IY

Also, the slab shows 17mm deflection at the location of the beam. If they work together that should be how much the beam deflects as the beam is supposed to be supporting the slab.

Correct however when you run static analysis for a model with reduced Iy of the beam the stiffness of the slab is not reduced (so slabs starts to 'carry' the beam) so the obtained value of the beam's deflection is less than this (e.g.7 mm) instead unless you also reduce slab stiffness (Ig) (this is done when you run the verification with the stiffness update).

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.

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