BiaxColumn Design (Double channels joined by lacing at one face, and continuous plates at back face)

[Bridge_Man]

Hello Engineers,

I am assessing an old tower leg that is composed of double channels joined together by plates lacing at the front face and continuous plates the back face.

I am having issues trying to design this column for flexure, the easiest assumption is to consider this acting more or less as a box or an HSS but is that true? should I be checking it for LTB or not since it could be considered as a box. and if the section saw flexure in the direction of the lacing, can I consider that the lacing might see compression or tension. so many questions but am not sure about the answers. I haven't found much discussion about this topic.

Any thoughts or resources I can use. Thanks !

 

[dik]

None recent... came across a bunch of mat'l pre-computer and haven't retained it. It's a good thing to model using FEM, not necessarily as a 3D solid. It behaves similar to what you've described except the lack of solid material (only straps) on the faces leads to greater deformation and a bit of a shift and increase in stresses and a reduction in stability. Other than that caveat, I cannot add more. I wish I would get these things correct the first time.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[KootK]

...the easiest assumption is to consider this acting more or less as a box or an HSS but is that true? should I be checking it for LTB or not since it could be considered as a box.

I agree completely with your box assessment and would not personally bother with an LTB check. Yeah, the lacing is less shear stiff than a solid plate. There's no way that undoes your box-ish-ness in any meaningful way though. At worst, the thing is like a space truss on one of its three sides.

...and if the section saw flexure in the direction of the lacing, can I consider that the lacing might see compression or tension.

I get your concern but I wouldn't bother with that either. If the lacing attempted to act as the tension or compression "flange", it would tend to stretch or compress accordion style and just warp the channels out a bit laterally in doing so. Pish-posh.

 

[Bridge_Man]

Dik and KootK , thank you guys, I feel better now about my assumptions.

Regards!

 

[Lomarandil]

I'm on mobile, so can't provide the PDF, but if you need to sharpen the pencil further, Caltrans provided some good research on behavior of built up laced sections for flexure in both directions and torsion as part of the Bay Bridge project.

This one:

https://www.researchgate.net/public...d_Members_of_San_Francisco-Oakland_Bay_Bridge

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just call me Lo.