Buckling Analysis Results

[Settingsun]

Hi,
I'm new to Staad and am running my first buckling analysis. The structure is a walkway consisting of two parallel aluminium trusses at about 2m centres with a floor between them. The floor is at the level of the truss bottom chords. Overall span is 25m and the truss bays are 1.4m long.

I've applied the dead loads, live loads, and handrail loads with ultimate limit state factors using Repeat Loads to combine the primary load cases. The maximum compression force in the top chord is around 500kN (from P-Delta analysis) and the analysis reports that the buckling factor is 4.0 (buckling analysis).

To determine the effective length of the top chord, do I simply solve the equation below?
(4.0 * 500E3 N)= pi^2 * E * I / Le^2

This gives me a bit over 3m as the effective length for elastic buckling which is a bit shorter than I expected so I wanted to check whether Staad reports results the same way as I'm used to from other software. It seems to have its own way of doing things in some other areas...

 

[JoshPlumSE]

Truss bays are 1.4m long. The top chords are tied together in some way at the 1.4 m mark, right?

Going back to a simplified method and assuming fixity and sway, the k value would be 2.1. So, KL = 2.1 * 1.4 = 2.9 meters. So, 3 meters seems relatively reasonable to me.

Now, I don't really know how the tops chords are braced / connected to each other. So, this is really just a wild guess. But, as far as wild guesses go, it sounds like you're in about the right ballpark.

 

[Settingsun]

Thanks, Josh.

The top chords are braced by U-frame action. No direct connection between the two top chords as that is the walking path.

You're probably right about the order of magnitude of the effective length. I was thinking of the case of a single truss where the reduced eff length is due to the varying compression force in proportion to the 'bending moment'. In this case, I have that plus the U-frame action. So it's telling me the U'frames are about 45% of a standard brace working in tension (ie fully effective).