Mcrit Value calculate by Model Incorrect?

[Anonymous]

In analysing a steel gantry model, my primary beams fail due to lateral torsional buckling. Take member #151 as the example. The RSA calculated value of Mcrit is in the region of 250 KNm. I have made another model of the member in question #151 and applied the most onerous load case and come up with a similar Mcrit. When running the verification my member exceeds the capacity greatly due to ltb.

When using an alternative software (LTBeam) I get my value of Mcrit to be 1941 KNm. If I input this value into the member parameters manually my verification runs fine and the beam is well within capacity. The utilisation is dictated by the sectional strength rather than the ltbuckling capacity.

How does RSA calculate the value of Mcrit? is there a reason why the value is so low, what changes need to be made to the model in order for the automatic RSA calculation to be correct.

In addition to this I cannot switch on the DSC algorithm? the suggested path to toggle it on does not seem to exist.

If you could shed some light on this I would be very grateful as manually inputting Mcrit seems like a very long winded process for future projects.

Many thanks,

James

[Artur.Kosakowski · Accepted solution]

Hi @Anonymous

When using an alternative software (LTBeam) I get my value of Mcrit to be 1941 KNm. If I input this value into the member parameters manually my verification runs fine and the beam is well within capacity. The utilisation is dictated by the sectional strength rather than the ltbuckling capacity.

Is there a reason why the value is so low, what changes need to be made to the model in order for the automatic RSA calculation to be correct.

This is because you created a model and defined the design parameters in such not in the way Robot's steel design module expect them to be. If you create the model with beam "design' elements defined among their supports (columns) the value of Mcr will be much higher:

See also: https://forums.autodesk.com/t5/robot-structural-analysis-forum/critical-bending-moment/m-p/6438745

 How does RSA calculate the value of Mcrit?

See: https://forums.autodesk.com/t5/robot-structural-analysis-forum/lateral-buckling-critical-moment/m-p/3253094

In addition to this I cannot switch on the DSC algorithm? the suggested path to toggle it on does not seem to exist.

 

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