60° Angle Legged Self Support Tower

[smithw018]

I am working on running a SA on a existing 120' SST with 60° angle legs. The issue I am having is the tower is failing even prior to adding any loads. Was wondering if there is anything special that you need to take into consideration when working with this type of tower. (using TNXTower)

 

[atrizzy]

I dont know the program but it sounds like it could be a number of things... have you checked foundations? joint connectivity?

 

[EngineerEIT]

What is failing exactly? Legs in compression? Diagonals? Bolts? It's been a few years since I've used tnxTower but I don't recall anything "special" about 60deg legs besides using the built-in tool to create the leg shape.

 

[msquared48]

If no one answers your question here, either call Peter at TNX Tower, or perhaps Josh Plum in the forum here could help you. He used to work at RISA where TNX Tower originated.

I used the program five or six years ago, but you might want to start by checking your units. Then make sure your joint conditions are correctly identified.

Inputs can be tricky.

Do you have a users manual to follow? That helped me a lot, but I did have to study and interpret it on occasion, as well as talk to Peter.

Mike McCann, PE, SE (WA)

 

[msquared48]

Also, 60 degree tower legs are typically of one manufacturer, with the member properties included in the data within the program.

Who is your tower manufacturer?

Mike McCann, PE, SE (WA)

 

[smithw018]

Thanks for the insight all,

The legs are failing in compression. I have spoken to Peter briefly about this tower and I am wondering if it could possibly be due to steel grade..? I used A36 as default. I cant seem to find anything on that contradicts that.

 

[msquared48]

Do you have the manufactures drawings?

The steel grade used is listed there.

If you do not have the drawings, you need to contact the manufacturer and get them.

Mike McCann, PE, SE (WA)

 

[JoshPlumSE]

Okay, shifflerized angle legs that are failing in compression.

It's been awhile for me and Tnx/RISA Tower, but your situation doesn't sound all that unusual. I'm thinking this is probably an existing tower built under older codes, and they're trying to add new a new dish or antenna to it.

I didn't get too much into retrofits when doing tech support. But, here are my thoughts:
1) There are a number of preferences / settings that can affect code. Like whether or not to consider leg bending in the code check. You might walk through all those check boxes to see if any of them significantly affect the code check. If they do, then it may be reasonable to choose the other option.

2) Are all the leg braces / struts modeled? You might take a closer look at unbraced lengths and k values if they aren't. Heck even if they are you might be able to reduce some unbraced lengths slightly based on clear span rather than center to center distances.

3) If this compression failure is occurring pretty close to the base, then adding in some extra bracing or reinforcing the tower shouldn't be all that difficult.

4) As long as the tower analysis isn't diverging then I suspect there are still cost effective options to solve the problem.

 

[msquared48]

As I recall, I analyzed an older tower of the same type a few years ago and the lower two sections were failing under load.

Ended up reinforcing the legs in place with steel plate across the angle legs between the panel points to close the angle legs.

Had to use another program, Shapebuilder, to get the section properties and create a new section manually within the Tower program. Worked for me.



Mike McCann, PE, SE (WA)

 

[ENGINEER92]

Never done tower analysis before. Our engineer that use to do tower analysis for us, before he retired, would talk about for some towers that were originally designed per REV-F, that he would have issues getting them to converge when he ran them for REV-G. There was just that much of an increase in wind and ice load between REV-F and REV-G.

 

[WARose]

I don't know if the program takes into account the shifflerized legs.....but you may want to check out a old AISC Journal article and hand check some of them:

'Design Strength of Schifflerized Angle Struts', AISC Journal, 2nd Quarter 1991

 

[TheDW]

Is it just the bottom section of leg that is failing? How close is it?

Double check the output to make sure the capacity/demand make sense - if I remember correctly, we had to input the shifflerized leg properties in manually...could have fat fingered something...