Reduction of (Ig) for orthotropic slabs

[Mohammed_Ata]

Dear, All 

Hope all of you are fine here in our preferable forum

While modelling an orthotropic slab (Ribbed slab), there is no an option to make reduction of moment of inertia for the slab section ! 

How can we make such reduction or is there something missed ? 

This option is available in homogeneous slab and not available in orthotropic slabs 

[mustafahesenow · Accepted solution]

1

[Mohammed_Ata]

Tgank you so much, @mustafahesenow, i really do this as you described but hope an option of reduction to be developed in the future versions ! 

Great thanks for your reply and interest  

[Mohammed_Ata]

Dea, @mustafahesenow 

ACI provisions state that the reduction factor 0.35Ig is for flat plates and flat slabs ! 

Does this means that orthotropic slab shouldn't be reduced in inertia, so robot doesn't add it ?! 

Your replies are appreciable 

[mustafahesenow]

1

[Anonymous]

Hi @mustafahesenow

Could you please explain why reactions due to selfweight change after reduction of I(g) for the orthropic slabs. My expectations was that this should remain constant just as the reduction of other structural elements.

Thank you.

[Artur.Kosakowski]

Hi @Anonymous

As the load distribution is based on the stiffness of elements (structure deformation) the more rigid the element is the more load it 'takes'.  The would e no change only when you calculate the load 'assigned' to e.g. supporting walls by the tributary are of the slab with disregarding the deformation of these columns as well.

I hope that @mustafahesenow agrees 

If I managed to answer your question(s) press the Accept as Solution button please. This will help other users to find solution(s) much faster. Thank you.

[Anonymous]

Hi @Artur.Kosakowski

Thanks for the response @Artur.Kosakowski. so does it mean that reduction in rigidity results in reduction in self weight?

[saclovitzky · Accepted solution]

The total self-weight of the structure(DL) will be the same after reducing moment of inertia of structural elements(columns,rc beams ,slabs etc.).

As its  reduces stiffness,  so does the moment attracting ability. Some of the moment which was present at this section (for example at beams ) goes to other areas which are not yet cracked ( for example columns ).
This reshuffling of the stiffness in the whole structure leads to redistribution of moments. Which means some of the loads were transferred but not lost.

Generally, the purpose of reducing stiffness of concrete sections is to model for cracked behavior of concrete.
They are "only" applied to concrete members because it cracks under loading.

Those regions which were "un-cracked" and received extra moments from cracked regions will also crack once the concrete in that region reaches its tensile capacity limit.

So this cycle of moment redistribution continues until all the members have been cracked. Steel reinforcement which sits idle before this stage now starts taking these redistributed moments.

Hope this helps.

[Anonymous]

Thank you so much @saclovitzky. it makes perfect sense.