High Voltage Tower "engineered" Fall Distance

[Jdubb16]

We are currently working on a building design that will be funded with HUD money. HUD guidelines preclude a building from being constructed within the "Engineered" Fall Distance of a high voltage tower. We have contacted the power compnay (Dominion Virginia Power) to get the required info and their response was that our towers are not going to fall and they sent a 4 page boilerplate letter out to HUD. Well HUD is tighening their lending requirements and this letter (which has worked in the past) does not work anymore. They want verification that the building is not within this fall distance.

I am a structural engineer that primarily does building design, so this is out of my area of expertise. However, I am trying to assist the owner and architect to keep a very big project moving.

How do I determine, calculate or figure out what this fall distance is. What info on the tower do I need or does this HAVE to come from the power company.

If it has to come from the power company, the owner may have to get thier attorney involved so they will release this info.

Thanks for any help you can give me.

Jdubb

 

[MikeDB]

Looks to me like the worst case scenario would be an arc equal to the height of the tower with the center of the arc at the structure leg which is closest to the building. The wires attached to the structure could extend farther than that due to wind blowing the wire away from the center line. Your surveyor should be able to measure the height of the structure and get the coordinates of the legs for you.

As a side note, how much right of way does the power company have and are there any restrictions in the easement against building on the right of way?

Mike

 

[transmissiontowers]

Transmission towers rarely fall over like a tree chopped down in the forest which is the safest assumption that MikeDB pointed out. When the legs buckle, they pretty much just fall on top of themselves, and I assume you are talking about a latticed tower and not a steel pole or wood H-Frame.

You can determine the tower height or the height of the tallest conductor using a level and a 45° triangle and a tape measure. Sighting along the hypotenuse of the level triangle, walk back until you can see the top of the tower or the top conductor. Use the tape measure to find the distance from your feet to the tower leg and add the height above ground to your eye.

Or hire a surveyor to tell you the tower height.

_____________________________________
I have been called "A storehouse of worthless information" many times.

 

[k2panman]

transmissiontowers - So how do you calculate the "engineered fall distance" from a structure? I am an electric utility engineer, and I just had a designer for a new HUD residential project in my office asking for this info. I have a 138kV line with three single pole steel structures, and a substation with two guyed single pole structures, all close to the property line. I know the structure heights. If I give him the "chopped tree" distance, that severely limits his project's buildable land area. Is there an acceptable method to calculate the "engineered fall distance"?

K2ofKeyLargo

 

[transmissiontowers]

Hi K2panman;
I am a structural engineer and design and analyze transmission towers and poles for a IOU in the South. Since engineers like to be safe and will usually give the worst case for any answers, I would assume the chopped tree distance for your structures. If they were latticed towers, you might assume 1/2 the tower height but to be really safe the entire height would probably be the answer. I have never personally seen a steel pole failure from a real world event but have seen pictures of pole tests with failures.

In your case, both the guyed and free standing poles might fail by the anchor bolts failing, pole shaft buckling, base plate weld failure, guy failure, etc. All of these would happen at the base and the pole would fall over like a tree.

For a more definitive answer, contact your pole supplier. They are in the business of designing many poles and might have come across this term before. They probably have done tests to verify designs and might be willing to tell you what they have observed in their testing.

Thomas & Betts; Valmont; Sabre; FWT are some of the pole suppliers that you could contact.

If your utility is going to send any structural engineers to Ft. Worth next month for the ASCE/SEI seminar, there will be 500 experts all in one place and they could ask questions.

http://content.asce.org/conferences/ets2009/general_information.html

With the HUD involved, I'm surprised someone is not scared to death about EMF, although you get more exposure from your toaster oven than standing on the edge of a transmission ROW.

_____________________________________
I have been called "A storehouse of worthless information" many times.

 

[Polecat]

I imagine that it's a bit late to provide any useful information on this project in view of the fact that the thread was started back in August.

Still, I would offer the following just for the record:
I have designed many pole structures as well as having analyzed some lattice towers. Quite often a municipality might require an engineer to provide a "fall zone letter" to certify that a pole structure will not collapse beyond a certain radius (especially with telecommunications poles).

This is easy to do if you are the one specifying the pole strength, since you can design a pole's slip joint connection that would just meet the code but, overloaded beyond that, would collapse outside the fall zone. Then you size all the parts below it to have a higher overload factor than does the joint.

A lattice tower is an entirely different story. You would have to either obtain a backup file of the pole model (hopefully done with PLS Tower), or would have to get the shop drawings and create the model from scratch (and trust me, you don't want to do this unless you have nothing else to do for many, many days). Then, you would run the model using the worst load case provided by the utility company and see where the weakest members are.

If you're lucky, you might be able to beef up members and joints in the lower part of the tower so that the upper part of the tower that would lie outside the zone would have to collapse first. In any case, it would be mammoth job, and depending on how the tower was made, you simply might not be able to accomplish the objective.

Barring that, you could replace the lattice tower with a slip joint monopole and design it as described above.

In short, there is no easy out. As TransmissonTowers has said, it is surprising that HUD was not more concerned about the EMF than having the tower collapse.

http://www.spiraleng.com

 

[transmissiontowers]

My utility was asked this same question just a couple of weeks ago by a home builder in order to qualify the house for a FHA loan. Our response was to give the structure height which most anyone could find using a surveyor with the comment that we design the structures to withstand NESC loading.

This implies the structure will fall over like a tree and is the worst case distance. I have heard that some municipalities require telecom towers to be 3 times the structure height away from any habitation or road which is very severe and must assume that the antennas would fly off when the tower fell like they were thrown by a trebuchet.

_____________________________________
I have been called "A storehouse of worthless information" many times.

 

[stevenal]

After reading Polecat's post, the answer is clear: There is no engineered fall distance, because it was not required for line construction. But since there is HUD funding for the project, here is our estimate for re-engineering the line to include a fall distance.