Lattice Steel Tower Leg Braces supported on Tension-Only Members 3

[PackRust]

Hello,

On the analysis of a very old tower, where the lower legs (member 27) are braced with redundant members (31-35) that also connect to the diagonals (member 12), we consider the diagonals as tension-only members due to the high L/r slenderness they have (between 300-400). If the tension-only member buckles, are the redundant members (31-35) connected to them still fully performing their function of bracing the main leg (member 27)? Would appreciate very much your take on this problem and any advice you can provide, I'm running the analysis on PLS-TOWER software.

Thanks

 

[dauwerda]

Do you have a copy of ASCE 10?
This is the document used in the US for the design requirements of lattice towers.

 

[PackRust]

yes I have a copy ASCE 10, the 2015 revision. I haven't read the whole code but can't find anything related to this issue, still looking though.

 

[straub46]

Are there not additional braces in plan view? Something like the snippet below, that prevent the diagonals from buckling in the opposite direction, thus limiting their KL/r in compression and making this a moot point? I know a lot of tower manufacturers add those braces in, but I don't remember about the older ones.

Go Bucks!

 

[JoshPlumSE]

I would have thought this was a communications tower subject to the TIA code. But, I suppose this is based on use.

Since my experience is with the TIA code, I'll say the following:
a) The TIA-G code give limits on L/r based on "shall preferably not exceed". Note: gotta the the "Shall" in combination with "Preferably". LOL
b) That limit is 150 for leg members, 200 for main compression members (other than the leg), 250 for secondary members, and 300 for tension members (other than rods or cables).
c) What you do when the preferable limit is exceeded isn't said.

Personally, I'd say that you'd be required to reinforced that member 12 if you make any major changes to the tower. I suspect if you try to run this model in PLS-Tower or TNXTower (which I'm more familiar with) then you'd find that any analysis that includes geometric non-linearity would diverge at any load level where the member 12 buckles.

Members 33, 34, 35 would be required to resist a minimum axial force of something like 1.5 to 2.5% of the compression force in the leg.

 

[PackRust]

there aren't additional barces on plan/section view.
I've also seen other towers that have those braces to support the diagonals against buckling in 2 directions, but this one doesn't have those.
The analysis using PLS-TOWER does show the diagonal (member 12) receiving compression forces above it's buckling capacity, I would assume these would bow out of plane since the bracings hold them against buckling in plane. But I'm really not sure if the main leg remains braced if the diagonal is buckled and the braces are supported on a buckled member.