Tube-Steel Tower Frame 2

[DCEngr1]

I need to analyze the capacity of an existing 60-foot, three-leg, tube-steel communications tower for moment and tension at the base. The 1-1/4-inch diameter legs are 16-1/2 inches on-center and of 50K steel tubing. What are the factors for allowable stress design with regard to tension & bending? This structure was built in 2009. Thanks . . .

 

[paddingtongreen]

I would not have expected this question from an experienced engineer. If I did receive this from an experienced engineer, I would expect much more of the information necessary to provide an answer. Think about it.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[DCEngr1]

I am not particularly familiar with steel design, the current factors or the factors for the 2009 time period for 50k steel allowable in steel design, with the standards for tower design, or with the code applicable to this type of structure. I have been involved with concrete design, and my experience with steel is limited. I am not trying to lay out the entire problem. I am asking for the specific information. If you can assist me, that would be helpful. That is the purpose of this site for those who want to be helpful. Thank you.

 

[fancypants]

Look in your Handy Dandy copy of AISC 360-05 chapters D, F, E, and H. I am adding the compression chapter in also since you should have some at your tower base.

 

[fancypants]

I wanted to also say if you do not have a copy, you can download the specifications from the AISC website.

 

[dhengr]

DCEngr1:
If you are “not particularly familiar with steel design,” maybe you shouldn’t be doing steel design like this. Remember..., practice within your experience level and area of expertise. Sites like this are ‘for help’ but not for a refresher course in steel and steel tower design, assuming you had a first course at some time. There are many basic concepts involved, but it is also kind of a specialized design area and problem, with some of its own codes and design considerations. And, fishing the internet is a darn poor substitute for real answers to these kinds of basic questions. What you really need is a mentor in your office or immediate area, with some experience in steel tower design. Someone who can help you through the various code sections, do some sketches and details with you, actually interacting and talking about the important points of that detail or condition, and watching over your shoulder until you gain some experience on this problem. Ultimately, you better be able to “lay out the entire problem” when you are being asked to “analyze the capacity of a 60-foot, three-leg...” Paddington was suggesting that you consider the problem, and your weak OP, a bit more deeply, so you stay out of trouble, for lack of the full picture.

 

[MiketheEngineer]

If this was built in 2009 - there should be all kinds of drawings and calcs lying around about this project??

BTW - good tower design takes some expertise. There is some kind of industry standard for them - I think...and not so cheap software....

 

[MikeDB]

If this tower is in the US, TIA-222-G is the standard for the design of communication towers. The standard is an LRFD design and somewhat follows AISC 360 and ASCE 10. The standard requires a check of wind from three directions, normal to the face, on the apex and along the face. As such, leg tension is rarely the controlling factor for the leg. Assuming there is diagonal bracing, the towers are normally designed as space trusses, so bending is not a controlling factor. Not sure if you are checking the design of the tower or the foundation, but with such a small face width, all three legs are probably on the same foundation and it won't have any uplift. Is this a guyed tower or self supporting? The small legs and face width sound like a tv or ham radio tower for home use, so it may not have been designed to anything.