Slenderness Ratio for Truss Design Using ETABS

[Simran7]

Designing a truss in ETABS and have a query regarding slenderness check done by the software. For the length part, I am considering full length with (K = 1.0). On running design in ETABS, it fails the chord due to excessively large slenderness ratio in its plane. I understand that it is considering the radius of gyration only for the bottom chord and not truss as a whole. Has anyone faced and resolved this issue before?

Thanks.

 

[Ron247]

I am confused by your drawing. What does 2L mean since the entire truss is L?

 

[KootK]

What does 2L mean since the entire truss is L?

My money's on double angle chords.

Has anyone faced and resolved this issue before?

In the plane of the truss, the effective length of the chord for flexural buckling is generally assumed to be the distance between truss panel points. Maybe a little less if you feel like getting fancy and trying to account for continuity etc.

 

[Simran7]

@Ron247 - As KootK said, it is double angle chords. Sorry, mentioning 2L is probably not relevant to my question.

@KootK - I agree with the effective length and that what ETABS is using for the check. But how do I force it to use the radius of gyration of the whole truss instead of just the bottom chord's in ETABS?

 

[KootK]

But how do I force it to use the radius of gyration of the whole truss instead of just the bottom chord's in ETABS?

You wouldn't as you want it to be the radius of gyration of the chord itself and not the truss. You are considering buckling of the chord itself rather than buckling of the whole truss. As such, the flexural stiffness of the chord alone, as indexed by its radius of gyration, is the relevant parameter.

 

[jittles]

Is it failing with KL/r > 200 for compression, or is it failing due to L/r > 300 for tension? We're talking about the bottom chord here, but perhaps you have a wind uplift case you're considering that is causing compression in the bottom chord.

In any case, I agree with KootK - you're just looking at the bottom chord member itself.

 

[Ron247]

I agree with the others, the physical length is the distance between points that brace the segment from buckling.