buckling in ThII

Hi,

I've drawn a truss and supported it in the top chord perpendecular on the plane (Ty and Rx, global buckling) but in some combinations of height and division of the truss, the second order theory component gives a warning about buckling in 1 or 2 modes. This also happens when I reduce the loads on the top chord, there are no loads on the bottom chord. For example:

> 92 kN/m no warning

83 kN/m buckling in 1 mode

50 kN/m buckling in 2 modes

<50 kN/m no warning

Since the truss is supported in the top chord against global buckling, this warning must be about the bottom chord? But the bottom chord doesn't have compressive forces, then what could this warning be about? And is there a way to check where it buckles (the modes in the warning)?

Thank you in advance!

Kind regards,

Cansu

Attachments:

Parametric N truss with different shapes.gh, 77 KB

Replies to This Discussion

Permalink Reply by Karamba3D on May 5, 2017 at 12:46pm

Hi Cansu,

you can use the 'Buckling Modes'-component to see how the system buckles (see attached file).

It seems as though the sideways and torsional stiffness of the lower girder is not sufficient to stabilize the vertical posts of the truss system.

My explanation for the strange behavior of your system: In case of the load of 92kN/m the tension in the lower girder is sufficient for stabilizing the system against sideways movement whereas at lower load levels the compression in the posts dominates the system response.

Best,

Clemens

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