Beam cracking correction

[karolisKZK6A]

Good evening, 

I just found a weird behavior in ROBOT and I was wondering if someone could explain it to me.
For some reason when calculating beam longitudinal reinforcement with cracking correction, ROBOT increases required reinforcement area even if the cracking is 0mm. 

Here is an example. I just modeled two simple frames. Both frames are the same, loads are the same and the only difference is the cracking correction parameters for the beams (one is on and other is off).

As it can be seen in picture No. 2, required reinforcement is different between beams despite cracking not appearing in a beam. Can someone, please, explain what is going on?

[karolisKZK6A]

Just in case someone wants to see the file here is the download link:

https://www.transfernow.net/dl/20250725D0lCCjL8

[Simau]

Hi @karolisKZK6A 

To focus on the bottom reinforcement, it's best to work with this example.
Perform tests by playing with the crack limit.

[karolisKZK6A]

I cannot open the file you've provided, as my robot version is older. 

All I want to do is to calculate all beams in the project and cracking to be taken care of.
The least I want  is to go check every beam one by one, whether it is cracking and only then change settings
for robot to increase reinforcement. As otherwise I am over reinforcing the element.

I really don't understand why does software increase required area if cracking doesn't occur.

[Simau]

You don't need to do this in 2 steps

Robot increases reinforcement only if necessary.

I use your model, without columns to make comparaison easier, with and without cracking taken into account and all seems logical.

[karolisKZK6A]

The way you tested it makes sense. In both tests you performed - cracking occured and robot increased longitudinal reinforcement for it to be within the allowed limit. My issue is that when cracking correction option is checked and cracking doesn't occur, robot still increases required reinforcement area for some reason. 

I am now designing beams in a project and by default I just check the correction parameter on for software to make sure cracking is correct. But now I noticed, that in beams, where cracking didn't occur, Robot still notably increased required area (Just the way I showed in a frame example). 

[Simau]

In your example, cracking is not so critical, that why you don't see any difference btw with and without cracking.
That's why i told you to test other cracking allowable limits

[karolisKZK6A]

There is a misunderstandment going on. 

In my example i just showed that cracking is 0mm in both beams, but in the beam, where "reinforcement check (cracking)" is checked on, required reinforcement 
area is higher in one of them despite everything being the same. 

Of course in this case, area required doesn't differ all that much (even though it should be the same), but the project I work on,
the difference is quite a bit larger and I was wondering why is this going on.

[Simau · Accepted solution]

Yes indeed, it's weird.

But if you choose exposure X0 instead of XC3, there will be no difference in reinforcement.

[karolisKZK6A]

Wow, indeed choosing X0 makes it the same as XC3 for 0mm cracking. Maybe someone else has any ideas and could give any explanation.
Thank you for your contribution.

[CaioSaporito · Accepted solution]

Hi @karolisKZK6A.

Check another parameters in General Tab and you'll see this. 

The minimum code reinforcement became different.

Best regards,

Caio

[karolisKZK6A]

Hey, that explains everything. I wonder why does robot increase minimum reinforcement if everything else stays the same.

I assume that EC2 clause 7.3.2 formula (7.1) is the culprit for this behavior.