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Anonymous Poster #1

Calculating Tower Base Moment

12/07/2011 5:40 PM

I am to provide the foundation design to a 60 foot communications tower. This is not a tower I have been involved in the design of and know little other than it is 60 feet tall, 3 2 1/2" dia pipe legs, and angle braces. Each side is 24" forming a triangle. Pretty standard stuff near as I can tell. Without redesigning the entire structure is there a way to obtain the base moment of the tower? All I am looking for is the Base Moment as the force to each leg could be obtained from the base moment. I do not have a manufacture or any design documents for the tower itself. Any Suggestions?

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#1

Re: Calculating tower base moment

12/07/2011 6:33 PM

Yes, find out who made it and ask them.

Posting anonymously and asking for engineering advice on how to perform these calculations is suspect, maybe just by me, but none the less............

Why would you be erecting a tower with no paperwork in the first place?

Sorry, can't help you.

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#2

Re: Calculating Tower Base Moment

12/07/2011 7:15 PM
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#4
In reply to #2

Re: Calculating Tower Base Moment

12/07/2011 10:35 PM

Lyn \ Wareagle,

I can't find a definition of "moment" in this context.

Anyone got a definition in context...I googled, but get a definition in context.

I call it a learning curve... Since I don't know.

Thanks,
Sapper

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#5
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Re: Calculating Tower Base Moment

12/08/2011 5:08 AM

First, I am The SolarEagle, a bird of peace and enlightenment....usually

Second, if you will follow my first link in the preceding post, you will have your explanation...It has to do with force exerted on mast...

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#3

Re: Calculating Tower Base Moment

12/07/2011 8:49 PM

You don't say where you are. Where ever that is, you should demand certified foundation loads.

Short of that, you could calculate the positive and negative capacities of the bottom sections and use those in the worst configuration.

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#6

Re: Calculating Tower Base Moment

12/08/2011 8:30 AM

Whenever I got involved with designing a tower foundation (there have been many over the years), the tower manufacturer's engineering department (staffed with Licensed/Registered Professional Engineers) always supplied the horizontal & vertical force reactions for each leg (if a lattice type tower) or the individual base (if a pole type tower).

You must contact the tower manufacturer and request the design reactive loads.......make sure that the information is sealed and signed by a Licensed Professional Engineer registered in your state certifying the data.

HTH...

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#7
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Re: Calculating Tower Base Moment

12/08/2011 11:45 AM

This is a used tower that was purchased by a wireless provider. They came to me as an engineer to obtain a foundation design. I have not been able to locate any manufacture or other information on the tower. I have in my contract that I did not design the tower, and they have signed a release of liability for the tower design. I agree it would be much better to have the reactions design data in hand. That said this tower will go up with or without a foundation design. I feel like if the foundation is safe then I have done my best to make a bad situation at least as safe as possible.

It is like being pregnant now that the baby is there you have to deal with it in some way. Do nothing is not an option. I will be calculating the point of failure of the 3 legs and using that to calculate a maximum base moment. From a maximum base moment with appropriate factors of safety design the anchorage connections and footing. I personally don't like it but is the public more or less safe with me involved? Am I putting the public at risk with this design? I feel like that without me involved there is greater risk than with me involved.

Thanks for the input. KM

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#8

Re: Calculating Tower Base Moment

12/08/2011 2:54 PM

Dear Lost One,

Since this is a used tower, is there a possibility that you can gleam the accurate dimensions and the individual structural steel element's properties via a field visit? I'm asking about this because you can then utilize the data and eventually determine the reactions for the legs after inputting all the pertinent structural data into a 3-D FEM structural engineering program....or better yet RISA 3-D or equivalent software package.

Of course all of this this is extra work, and you should charge the client accordingly because it is after all a charge in scope of services.

Am I correct in assuming that you're a Professional Engineer that has done structural engineering design and analysis?

I would recommend using the ASCE - 7 publication to ascertain the gravity loads, wind loads, and if applicable (I don't know where you're located), any ice loadings.

Good luck with this adventure!

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#9

Re: Calculating Tower Base Moment

12/08/2011 3:32 PM

Yes I am a structural engineer and yes it will have ice loads. The good part is that it will be located on a solid rock out crop. Bearing will not be a problem. Digging a hole might be. I am looking at rock bolting and other means of securing the tower. It looks like I could be spending the weekend getting my model to run. I have a new software that the company is wanting demoed that I may try. Nothing like a software learning curve to slow a project. I have done mostly buildings the last several years so it will be fun to get back to the mechanic of materials. I was hoping someone had a the silver bullet but lacking that I will simply model it and go.

Thanks for the feed back.

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#10

Re: Calculating Tower Base Moment

12/08/2011 10:51 PM

A self supporting tower like this has a drag coefficient and you add the wind drag at 200 miles per hour for each one foot vertical segment times the height into a total windage moment = the force that wants to blow it over = the moment your foundation must resist. The foundation will be a mass of concrete or a rock mass that will resist this by it's nature and thus be safe. If you want an extra safety factor, build for 300 MPH.

these links may help, page down as many as you need to find a reference you can use.

more tower search

Tower search

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#11

Re: Calculating Tower Base Moment

12/09/2011 12:40 AM

The tipping moment is dependent on the wind loading.The wind loading on the structure itself PLUS the wind loading on what ever is strapped to it including the vertical cabling.

Start with what is being hung and work from there.

Wind loading data is obtainable from the antenna and feeder manufacturers.

Have a look at TIA 222g.

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#12

Re: Calculating Tower Base Moment

12/09/2011 12:48 AM

Very Simple in your case.

Find out the steel cross section of each of 3 corner main pipes- say X mm sq

Yield point of most mild steels=30Kg/mm sq.

So you can see -- the corner posts will Yield/Bend/Destroy when lifted by 30X Kg force

And the Foundation must not Pull Up at That point( maybe it should?) .So your design should merely ensure that :

  • The Leg Foundation shall not lift at 32X KgF pull . And that design of Yours
  • Shall take the Dead Load of the( Tower mass divided by 3 + 30X) kGF

Rest is simple-- ensure that:

Soil resistance to Driven Pile; Bearing strength of Pile;Soil Bearing Str ength.

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#13
In reply to #12

Re: Calculating Tower Base Moment

12/09/2011 2:28 AM

You've not done any tower work it seems.

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#14
In reply to #13

Re: Calculating Tower Base Moment

12/09/2011 4:31 AM

It seems your Seemings are ill conceived

mm

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#15
In reply to #13

Re: Calculating Tower Base Moment

12/09/2011 6:49 AM

He does not seem to realize that the dead weight can be smaller than the turning moment in a high wind. I have seen three legged towers toppled when one leg was lifted up by the wind load. This tower will have a single base, so it's only risks are the dead weight load - which is easy to calculate based on the bearing ability of the earth, and the force that wants to blow it over- which needs to be resisted by a number of 45 degree angled piles that act to block this rotation about the base by adding a huge mass of earth via the piles. That said, you can also do it with a very large base mass - at greater cost.

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#16
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Re: Calculating Tower Base Moment

12/09/2011 7:43 AM

Aurizon you hit the bull's eye. Thanks

Rest is easy for OP

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#17

Re: Calculating Tower Base Moment

12/09/2011 1:59 PM

I figured the overturning would control and it did by a huge amount. Then keeping within the kern on the design caused for a pretty fair slab of concrete. I could try and beat in piles but this is basalt and that is expensive. I looked at a rock bolt to engage the bedrock and with mobilization and installation I ended up with just more concrete. Footing concrete in place is about 100 $/yd so you can buy a fair amount of concrete cheaper than most other options.

Thanks for the feed back. Nice to have a sounding board to bounce ideas off of.

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#24
In reply to #17

Re: Calculating Tower Base Moment

05/10/2020 6:18 PM

As you probably don't have information on the wind speed the tower was designed for, and you might decide or be instructed to design for a higher speed, I would check the legs for local buckling under compression loads (close to the base).

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#18

Re: Calculating Tower Base Moment

12/09/2011 2:23 PM

I'm not a tower designer so this might be a stupid question but at 60ft tall x 24in base, wouldn't that require guy wires?. If so, then don't the guy wires secure it against wind loads so the base moment loads are not significant.

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#19
In reply to #18

Re: Calculating Tower Base Moment

12/09/2011 3:11 PM

Most likely this is an existing steel triangular trussed-type tower, and at 60 feet tall it most likely it will not require guy wires. Think of the typical HAM radio tower in someone's backyard.

From what I've gathered (from OP's comments), the base of each leg maybe s a "FIXED" connection which is designed to resist Horiz. and Vert. force reactions and overturning moments.......all transmitted into the concrete foundation, and then into the underlaying Basalt bedrock.

Yes, any option with grouted rock bolts is going to be quite expensive and require additional concrete depth (mostly for resisting punching shear). One way to skin the "Kitty", and avoid rock bolt installation, is to provide several (preferably 3 minimum) reinforced concrete "key-ways" sunk down into the rock to help resist the horizontal reactions, thereby leaving the remainder of the foundation concrete to resist the vertical reaction forces and overturning moments.

===signed CaptMoosie, PhD, PE

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#20

Re: Calculating Tower Base Moment

12/12/2011 4:38 PM

Contact a reputable tower manufacturer (Rohn). Have available the number of antennas and the types of cable. Also let them know the heights of the various antennas. Does the area have a wind speed average? Also icing conditions should be provided for. Lastly soil conditions. Don't guess on this, if you have any doubts as to the soil, consult an engineering firm. I was putting together a quote for a 60 foot tower, fortunately one of the principals was a friend, he informed me the site had been a former sand quarry. We ended up using two 12 foot cesspools.

You might be dealing with a homemade tower.

If you can't get a manufacturer to help, visit some of the local towers. You may find a knowledgeble installer. Check your yellow pages for riggers. Under no circumstances do this on your own the first time.

Lastly, find a good insurance agent

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#21

Re: Calculating Tower Base Moment

08/26/2012 2:55 AM

Hello Anonymous

This question sounds precarious to me, why would you need to "provide the foundation design " of an existing tower?

Design of tower foundation is done for new towers, and is done by professional structural engineers who are experts in this field. Whosoever, has tasked you to do this job is unreasonable person. He/She shall, instead of tasking you this job, task a professional in the field of towers.

We all understand that forces in the legs can be obtained if you have the base moments, obviously. And in absense of the manufacturer's design data, a full tower survey has to be done, then a wind load analysis be done, then you need to run a structural analysis to get the base or support moments that you are looking for. There is no rule-of-thumb for this structural analysis job that you seek for.

Being a professional engineer in this field, I recommend you get in touch with a PE in your area and get this job done by them. I do not want to discourage you, but it can be a hazard if you, being not in this feild, carry our this type of jobs. Hope this helps you.

Fareed Siddique

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#22

Re: Calculating Tower Base Moment

12/11/2018 6:09 AM

I am to provide the foundation design to a 60 foot communications tower. This is not a tower I have been involved in the design of and know little other than it is 60 feet tall, 3 2 1/2" dia pipe legs, and angle braces. Each side is 24" forming a triangle. Pretty standard stuff near as I can tell. Without redesigning the entire structure is there a way to obtain the base moment of the tower? All I am looking for is the Base Moment as the force to each leg could be obtained from the base moment. I do not have a manufacture or any design documents for the tower itself. Any Suggestions?

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#23
In reply to #22

Re: Calculating Tower Base Moment

04/27/2020 11:11 AM

Yes. Follow the advice above given to Anonymous Poster #1.

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