I inherited a project where a 90 fo 1

[Lutfi]

I inherited a project where a 90 foot high guyed steel tower to be relocated from one site to another. I can not find any available data on the force in the guy cables. The tower has cables on all four corners. Each corner has an inner and outer set of cables anchored to helical type anchor in the ground. There are two cables on the inner anchor and three cables on the outer anchor.

In the absence of any data and he owner does not want to spend lots of money on nonlinear analysis, her is what I am about to do:

1. The cables are 0.25 inch steel with breaking strength of about 9800 pounds.
2. I plan on designing the foundation for the new anchor point to the break point of the cables (i.e. 2 cables*9800 pounds = 19600 pounds).
3. resolve he cable force to Fy and Fx
4. Design footing to resist uplift (Fy) and sliding force Fx.

Is my approach sound or is to conservative? If it is, what is a good percentage of the breaking load should I design the footings for and be in the safe zone?

Your input is greatly appreciated.

 

[flamby]

Lutfi,
I agree with your approach. While designing for uplift and sliding, try to keep the factor of safety as 1.000 only. The breaking loads of cable are normally 3 to 6 times the service loads. This factor of safety will automatically be inherited by you.
flame

 

[Stumpy]

Lutfi,
Your approach is sound, but may be overly conservative. As I recall, cables are normally sized with a factor of safety of at least 4. I'd back that out right away to get the design force. Seems like FS=1.5 against uplift and sliding would be appropriate.

 

[Ron]

If the tower was existing and will be moved, find out if it was designed under a prior version of the governing code. The design could be deficient.

A 90-ft. guyed tower is not very high so I would expect the stresses in the cables to be relatively low. At the least, for your own protection, do a beam analogy analysis of the tower to check the validity.

 

[Lutfi]

I want thank you all for your valuable input. Ron, I already looked at dates of tower installation. The tower is designed to meet the current code. Actually it is not that old. It is being relocated to a new and more suitable site for the users. NO one seems to have the load in the cables. I also did a quick beam analogy and came up with loads in the range of breaking load divided by FS of 4.

My conclusion is to divide the breaking load by 4 and use the allowable as the maximum in the cable. This will yield reasonable force. I will use 1.5 FS for uplift and sliding. I think this is in line with Stumpy's input.

Thanks for the input everyone.

Thanks,
Lutfi

 

[Busman]

Lutfi,

I have analyzed somewhere around 150 towers in the past few years. Ron is right, this is a relatively short guyed tower and the loads will be low, but the guys are also relatively small compared to the big boy towers. Where did you get the 9800 pound breaking strength for a 1/4" guy? My data shows (data varies for the smaller diameter cables) that a 1/4" EHS cable has a breaking strength of 6650 pounds. Regardless, the TIA/EIA codes allows a 2.0 factor of safety for the guy wire stresses for towers below 700'. I'd just be conservative and use 1/2 the breaking strength as the cable force and resolve to the Fx and Fy components.

 

[Lutfi]

Busman,

I am sorry, the cable is 5/16 not 1/4 as was stated earlier. I have TIA (1996 edition)and you absolutely right they call for SF of 2 for towers below 700'. I will use the SF of 2 in lieu of 4.

Thanks for the input.

 

[Busman]

Lutfi,

One more thing, if the tower is relocated to another county, the TIA winds speeds may be higher and an analysis may be neceassary to evaluate the tower members. Check your code for the wind speeds if this is the case.