## Robot Structural Analysis

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# Lateral buckling - critical moment

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Hey there. I'm trying to figure out exactly how Robot calculates lateral buckling in a simple supported beam. It seems that no matter how i define the bending moment of the beam Robot insists on calculating the critical moment from a uniform moment. As you can see in the picture below it uses the british standard ENV311(F.1) which states that the calculation method can only be used if:

*"The elastic critical moment for lateral-torsional buckling of a beam of uniform symmetricalcross-section with equal flanges, under standard conditions of restraint at each end, loaded through itsshear centre and subject to uniform moment is given by"*

but as you can see in the picture below my moment isn't uniform? Is there any way i can tell Robot (apart from the Load parameters definition, because that doesn't seem to be working?) that I __do not__have a uniform moment and it should use another norm to calculate it?

- Regards Martin

Solved! Go to Solution.

# Re: Lateral buckling - critical moment

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*Is there any way i can tell Robot (apart from the Load parameters definition, because that doesn't seem to be working?) that I do nothave a uniform moment and it should use another norm to calculate it?*

This is exactly the location to make this selection. This selection will govern C1, C2 and C3 values as described in the Tables F1.1 and F1.2 from the link you provided. As you can see below the value of Mcr is different.

*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: Lateral buckling - critical moment

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Sure it gives 2 different values, but it uses the ENV311(F.1) for both scenarios... the norm clearly says it has to be uniform moment, but your blue marked load types are not uniform moment?

# Re: Lateral buckling - critical moment

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I'm sorry but I'm not sure if I understand your point correctly. Could you describe what in your opinion should be the way of handling the LTB design of the beam you described please? In Robot it is the user who decides what kind of load is applied to the beam and based on this decision the steel design module calculated Cs values necessary to find the value of Mcr. My understanding is that the F1.1 and F1.2 tables from the F1 point of the code describe what values of Cs should be used depending on the load type and boundary conditions of the beam. Mr is just the resultant value.

**Artur Kosakowski**

# Re: Lateral buckling - critical moment

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Since the bending moment isn't uniform i would believe it should use the general formula **F.1.2** to find Mcr, instead of the basis formula **F.1.1**?

# Re: Lateral buckling - critical moment

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bump

# Re: Lateral buckling - critical moment

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It actually does. The F.1 is the reference to the F.1 Chapter rather than the formula itself. If you look few rows above the Mcr one you should see values of C1, C2 and C3 with the reference to the F.1.2 sub chapter of the F.1 chapter (again not to the formulas). If I remember correctly these C values are not used in the formula F.1 at all.

*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: Lateral buckling - critical moment

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True the C factors are not used in formula F.1, but if it only refers to the F.1 chapter, how am i supposed to know if it has used formula (F.1), (F.2), (F.3), (F.4), (F.5) or (F.6)?

If i go through the load parameters and such i can figure it out ofcourse, but it would be nice to get some sort of confirmation from Robot just to make sure it has used the right formula?

# Re: Lateral buckling - critical moment

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Robot uses the F.2 formula which is the general one.

*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**