Torsion on truss Mutually opposed lacing system 2

[Italo01]

Hello,

Studying Eurocode 1993 1-1, i've found, in topic 6.4.2.2 (Constructional details), the recommendation about the orientation of the lacing planes on built-up columns shown below.

I've never used this mutually opposed lacing system but i see it in my region with a reasonable frequency and i think nobody consider the torsional effects. I'm planning to do some research on this topic and maybe write a paper about it if i verify that the reduction on member strength due to this torsional effects is too great and if there's no paper already written on the subject. Is anyone aware of such paper or has analyzed a structure of such type?

If Eurocode brings a recommendation about it and says that there are torsional effects, i think that someone has done research on the topic but i couldn't find it. Also, Would you suggest a frame analysis with the diagonal ends offset or a complete FEM analysis.

Thank you in advance.

 

[Italo01]

Hi Lomarandil,

I'm soryy for the late response, i did what you suggested, especially because the buckling load of the chord wasn't matching the analytical calculations and i found that the connection has a great influence on the buckling load. This is what i found.

1 - The first buckling mode for all models was Out-of- plane buckling and it was almost equal to all, as expected.

2 - The second buckling mode was In-plane buckling and, for the welded column, the value was close to the one obtained using the formula present on timoshenko`s "Theory of elastic stability". (k[sub]eq[/sub]=2,07, instead of k[sub]eq[/sub]=2)
For the bolted column, the buckling load was much smaller. (k[sub]eq[/sub]=2,45)
This load was not sensitive to the diagonal direction.

3 - The chord buckling was much greater for the welded connection.
Bolted connection/ Parallel diagonals:
k[sub]eq[/sub] = 0,96
Bolted connection/ Crossed diagonals:
k[sub]eq[/sub] = 0,85
Welded connection/ Parallel diagonals
k[sub]eq[/sub] = 0,61
Welded connection/ crossed diagonals
k[sub]eq[/sub] = 0,56

 

[BAretired]

Laced columns are a strange assembly and difficult to analyse. I would avoid them if possible.

 

[Italo01]

Hello,i did some research to try to find the buckling load of this buckling mode below.

The AISC design guide (Frame Design using Web-tapered Members) presents a method to calculate Constrained-axis Torsional buckling and i found the situations close. The equation presented by the design guide is shown below.

When i apply the equation for my particular case i found a good agreement. The distance between nodes(L[sub]binside[/sub]) is 120cm and i found a load equivalente to L[sub]binside[/sub] = 115cm, which could be explained since the bolts are closer than the nodes. The situations are not exactly equal since for the truss, these points on blue circles in the image below are not braced.

What do you guys think about this approach?

Maybe i'll change parameters to see if the load keep in good agreement with the equation.