Do we absolutely need a concrete base for poles/towers? 5

[Gitakaren]

Hi, first of all, great site you guys have here.

Currently my boss has given an idea which I don't know is feasible or not. He wants to create a 50 ft pole foundation which does not use concrete. Instead, he wants to use a design that mimics tree roots, where the pole will be embedded into the ground and branches will spread out underneath to give leverage so that the pole will not move/overturn.

I have tried to find similar foundation types without much success. I am not very experienced in foundation analysis, and so have little knowledge as to why such foundations have never been used. Seeing as how concrete actually bonds to the foundation, as opposed to dirt, my boss is willing to accept that the branches will have to spread out and cover alot more area that a concrete slab. But I'm stuck thinking whether that will be enough.

Any comments/tips are very welcome. Thanks.

 

[Lion06]

Why in the world would you do that? You would have to excavate a great deal more dirt than providing a simple spread footing.
Also, I have no idea what kind of analysis you would do to check the soil pressure under a 'tree root' foundation.
What material would you be using if not concrete? How would you embed the 'roots' without excavating a ridiculous amount of soil?

 

[DMcGrath]

Quite an interesting idea, and given enough money and engineering effort, it could most likely be done. Concrete is simple, relatively inexpensive, and there are years and years of experience behind it - that is why it is so common. As far as moving goes, even the biggest, strongest towers sway a bit. I have been up many cell phone towers, and if the wind blows, they sway an inch or two.

I picture your "concreteless" foundation as a series of horizontal members buried in the soil in a radial pattern, but I would also anchor each of them with a helical pile or two. You will need to consider both uplift and compression for each radial member. Then there is the question of dealing with the considerable moment transfer from the vertical pole to the horizontal foundation. Sounds like you need a geotech and structural engineer on staff.

 

[graybeach]

Gitakaren, What you describe sounds like a pile foundation with a grillage in place of a concrete pile cap. Easy enough to determine the forces on each pile (branch), but you need a geotech to tell you the soil resistance.

 

[JAE]

With spread out "roots" you would potentially create problems if there are any other underground utilities that need to be placed close by...in the future there would be a higher potential for excavations to damage the roots and reduce the pole stability/safety.

 

[SteveWag]

I am involved with the re-erection of a welded steel coaster and was getting some estimates from one of the local crain guys. He told me that he was setting a 55 foot high by 20 foot wide sign on metal pin foundations. He advised me that the pins were being installed the next day, and maybe I could use them for some of my foundations. I watched the install and was not impressed. First, the locations of the pins was determined. The three holes about 20 feet deep were drilled about 10 feet apart in a straight line, with the center hole being at the pin location. Then three expansion anchors were installed, with the expansion being about 19 feet below grade. Then the Pin, with a 6-inch hole in the middle (6-inch pipe x 15-feet long)was centered over the middle hole, a 100 ton jack, with a hole thru, was set on top of the pin, a strong back (beam assy 22-foot long with three holes at 10 foot spacing) was set on center, on top of the jack. The strong back was tied down with three rods connected to the expansion anchors, passing thru the beam and secured with nuts. The pin was them pushed into three day old fill to a depth of 12-feet. The expansion anchors were removed, holes filled, the second pin installed as the first, the sign erected, done. I pass the sign every day and it's still OK after one year. I am not going to use them for the coaster.

This link is to the manufacturer:

http://www.metalfoundations.com/

I have photos if anyone is interested.

Steve wagner

 

[Gitakaren]

Thank you for all the replies. It is very much appreciated.

DMcGrath, you have pretty much nailed the basis of the design. I am currently analyzing the forces similarly to how you have it outlined, to see how deep and wide the branches need to go. I will take into consideration the soil resistances and underground utilities as well.

The reason my boss wants this type of design is to cut costs on the concrete foundation building required (we work closely with a steel factory), but I'm wondering if the amount of soil that needs to be excavated is going to be worth the trouble, as I have read from your posts.

 

[csd72]

The company that I was working for in the UK looked at this for mobile phone towers.

They were using conventional concrete piles and were therefore limited as to the uplift they could take in the pile. They ended up putting concrete weights on the platform to minimise uplift.

In your case the screw piles would have almost as much uplift capacity as compresion capacity and this should not be an issue.

I doubt that you will save anything on materials costs, but you will save on installation time (and therefore some site related costs).

You should get a structural engineer involved in this as there are many construction issues to consider.

 

[jike]

Concrete foundations have the inherent property of heavy weight which is used in the design to resist the overturning forces. With a light weight foundation, the uplift forces need to be resisted by some sort of tension piling into the ground.

Perhaps a helical auger might be able to provide this type of pullout resistance. You need some sort of drill rig to drill these into the ground. I believe there is a system where one helical auger can be used for short light poles (10 foot)instead of a concrete pole base.

 

[csd72]

jike,

Are you talking about a helical auger or a screw pile? I believe they are two different things.

 

[MiketheEngineer]

You might look into Post Frame Construction methods - "Pole Barns".

Go to

http://www.NFBA.org

to get started

 

[JStephen]

If your completed pole has to comply with any particular building or construction code, you migth check to see if it actually requires some kind of concrete foundation.

 

[apsix]

Screw piles have been used, not quite a root type system but still a non-concrete footing.

http://www.instant.com.au/a_light.asp

 

[jike]

csd72:

I was refering to helical anchors similar to Chance Anchors.

http://www.abchance.com/about/about.html

 

[COEngineeer]

Yeah, helical pier is your best bet for poles. I think it might end up being cheaper at the end and a lot quicker than concrete pole foundation.

 

[grandcasita]

Are you considering the need for corrosion protection of the buried steel?

You could possibly bury one or more thin, reinforced mats fairly deep, using the slab as your root system. If you like, the steel would provide the strength, with minimal concrete cover for protection and to create a cohesive mat, like the tiny feeder roots seen in plants. You cannot achieve your root effect without considering that root networks are almost solid.

You could also use a soil nail/auger cast/micro pile foundation placed diagonally into the soil in radials from your center.

In any case, I think you would want two levels at the pole to provide sufficient "back span" for the 50 ft.

You might also consider two intersecting, vertical vanes which extend from near the surface to a depth sufficient to resist overturning. There would obviously need to be sufficient resistance for gravity, as well.

 

[csd72]

50 foot is not that tall, a single helical pier(screw pile) will do the jon and will be the most economical.

The contractor will be able to get it designed for you.

 

[Gitakaren]

I've done some research on a screw pile, and it looks like a fantastic alternative to concrete foundations. It also fulfills the criteria of fast and easy installation, as well as having minimal work area.

I'll be bringing this idea up with my boss. Thanks for all the replies.

 

[btrueblood]

Yeah, helical piles work great. Until they hit a rock.

 

[transmissiontowers]

Did you mean a 50 foot above ground height for the pole? With only that much sticking up, you could use a direct embedded pole design. The general rule of thumb is to embed the pole 10% plus 2 feet depending on the soil so a 60 foot long pole would have 52' above ground and 8' below ground. The pole manufacturers coat the base with a material to prevent corrosion and a hole is excavated, the pole placed and plumbed, and the annular space filled with crushed limestone.

We use spun cast concrete poles direct embedded to mount antennas and they cost less than 1/2 of a comparable steel pole and we use no concrete in the foundation unless the soil is extremely poor. In very poor soil, a steel casing can be driven and the inside excavated and the direct embedded pole can be put in and back filled with limestone.