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Transmission Towers- Staggered bracings

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ab21

ab21

Structural
Aug 3, 2017
20

Hi,

I have a quick question about a transmission tower I am currently modelling in PLS Tower. I dont have the drawing with me at the moment but I need a quick answer. I have attached a sketch to show staggered bracing in tower legs. Can someone please help me to understand what the L/r ratio for the leg is? If I am right it would be 2.4 in x, y and z axis (according to AS3995)? I think because buckling length for member L is twice length in both directions and there is a 1.2 factor in the standard as well (or any standard for the matter I want to know the principle).

If that is true, then whats the point of having a staggered bracing because buckling length is big, wny not have normal symmetrical bracing?

Many Thanks

 
 https://files.engineering.com/getfile.aspx?folder=9798cba8-f4bd-4729-8d74-bf2aecec9da1&file=sketch.JPG
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In case someone doesn't normally look at this room, I'd also post in the Structural engineering and other subjects room. It's a basic structural question, not just for towers.
 
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For Tranmission Towers, and any 3D space frame, you have to look at the KL/r ratios in three dimensions, not just two.

Remember that.

Mike McCann, PE, SE (WA)


 
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I can only speculate, but they may have decided the connection simplicity of the staggered braces was valuable enough to deal with the mid-length bracing forces that would develop.

----
The name is a long story -- just call me Lo.
 
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Because when your structure was originally designed the 1.2 staggered bracing factor wasn't there, and because the controlling buckling direction for single angles is in the "z" (weak principal axis) direction. With staggered bracing vs. standard bracing the "z" unbraced length was halved while the "x" and "y" stayed the same. So an increase in strength (rx or ry controlled vs rz) for the same amount of bracing.

But full-scale testing proved that staggered bracing wasn't as strong as theorized, so the codes added the 1.2 factor to compensate.
 
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