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Slab deflection and crack

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Message 1 of 3
toha_mh
1305 Views, 2 Replies

Slab deflection and crack

Hi,

I am facing some problems regarding slab.

 

1. At first, from Results > Maps the deflections values comes acceptable. But when I go to Required Reinforcement calculation, message comes that the deflection value has exceeded the limit. However, after I click on verification with FEM, it again reduces deflection value.

 

So what is the difference between these deflections? Which one is the nearest to reality? What does the "verification" actually do? When should I click the verification and when I should'n?

 

The whole thing is not clear to me. Can you please explain in your own words?  Because I tried to read the help files and also some previous posts. But didn't understand it clearly.

 

2. Sometimes the cracks also exceeds the limit. Why does this happen and how to reduce it?

 

Please don't mind if my questions are so simple. I am still a student. So there are a lot of things I still don't know. Maybe that's the reason I don't understand these simple things.

2 REPLIES 2
Message 2 of 3
Artur.Kosakowski
in reply to: toha_mh

1. During linear static analysis value of deflection is calculated for elastic (uncracked) material (concrete). During the RC Slab req. reinf. calculations Robot estimates the average 'cracked' stiffness of  the slab and based on this value increases the deflection. This is estimated method. The stiffness update one calculates real 'cracked' stiffness of each of the element of the mesh and runs static calculations for such modified model. This is more accurate approach.

2. Your SLS load is too large for the slab with the properties (e.g. thickness) as defined. You can increase reinforcement or/and increase thickness of the slab as well as change its grade.



Artur Kosakowski
Message 3 of 3
Anonymous
in reply to: toha_mh

It is very conservative method to check all elements with with creep and crack factor I think it can be improved in such complex software as Robot.

According to Eurocode 2 you should check the deflection for total load (based on quasi permanent combination) and additional deflection based on frequent combination. Actually the floor has to be checked for two values of the deflection (additional 1/500 L and total 1/250 L). Is it not possible to make this checks easier for constutors who are making the calculation with Eurocode2?

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