Pole Foundation (Drilled Shaft) 1

[ab21]

Hi,

I have a project where I need to design a utility steel pole foundation with large moment (free standing). I know they are mainly directly embedded but want to see what else we could do. From what I read some have designed drilled shafts or pads for these poles, does anyone have any guidelines or calculations on how to do this? Is shaft design same as pile design because they are mainly designed for axial load and not huge moments.

Moment on the footing is going to be very high, in the order of 2000kNm.

Thanks a lot

 

[Lomarandil]

Yes, you want a drilled shaft foundation.

The theory is the same as any deep foundation (e.g. piles), but you're right that we usually use the word pile in cases with primarily axial load.

As far as guidelines, you can start with NAVFAC DM-7, the AASHTO guide to signs and luminaries, or maybe there might have been something in the RUS? Not sure about that last one. Without knowing more specifics of your application, it's hard to give much more than generic guidance.

----
just call me Lo.

 

[MIStructE_IRE]

Thats a huge moment! How tall is the pole?!

 

[ab21]

Lomarandil
I will try to get those guidelines but in this instance would I design a single pile with 2000kNm moment? Thats huge for a pile.
Ive read that for steel pole its prefered to use drilled shafts rather than direct buried but I dont see the benefit of it except that it eliminates possobility of corrosion? Presumably the pile would have to be the same depth as if pole was to be direct buried?
What other information do you need?

MIstructE IRE
The pole is 35m tall and has to take tension load from conductors. The site is particularly bad and has 3m of fill that provide next to nothing in passive resistance.

 

[SlideRuleEra]

I know they (steel poles) are mainly directly embedded...

...they (steel poles) are mainly designed for axial load and not huge moments

In poor soil, not really.

Moment on the footing is going to be very high, in the order of 2000kNm.

Large drilled piers:

See "ASCE 48-11 "Design of Steel Transmission Pole Structures".

Several foundation types are allowed, such as:

1) Drilled shaft foundation with anchor bolts
2) Direct-embedded foundation
3) Embedded casing foundation
4) Rock anchors

http://www.SlideRuleEra.net

 

[JLNJ]

L-Pile is a good option. There are other rational methods out there.

Check out this old US Steel guide I've attached.

 

[dauwerda]

Most (if not all) self supporting dead end or high angle transmission poles that I have seen all utilize some type of foundation and are not directly embedded. Typically the foundation is a drilled pier. The diameter of the pier will be determined by the anchor cage diameter. Brom's method is one way to design the required depth (due to overturning moment) with hand calcs. If you have access L-pile is a great software for performing drilled pier design

 

[ab21]

I will read the document sent. Thanks all for your contributions.

The questions I have in my mind at the moment is what is the difference between using a drilled pier and direct embedding the pole? Unless you increase the diameter of the pile the required depth would be the same, and then you would have to pay for a piling contractor too. The diameter of the pole is 1.2m and if we were to use direct burial, the hole diameter would be 1.5m backfilled with concrete for protection.
For the purpose of calculating the depth required, could I use diameter of the hole (1.5m)? If I were to increase the hole diameter would I have to consider load transfer to concrete as well (the problem becomes structural too)?

While, we are discussing this topic, if I were to use a pad foundation (I cant in this case because of depth of fill layer) but if I could then would the design be similar to any simple pad design but with large moment (in this case 2000kNm)? Because if so that would be one pad with lots of reinforcement.

Thanks

 

[BridgeSmith]

We would never consider direct embedment of a steel pole. By the time you auger the hole, and fill it with concrete, what have you really saved? You'll spend alot more on the fabrication of an additional 7 or 10 meters (maybe 12 or more) than a base plate, anchor bolts, and reinforcing steel. Plus, it's much easier to get the pole plumb; I'm not sure how you'd manage that with it embedded. Not to mention if it's embedded, it's very permanent.

 

[dauwerda]

HotRod10, most tangent transmission poles are in fact direct embed, depending on soil type they may or may not use concrete as backfill. The embedded portion often is coated with Corrocoat

https://www.corrocoat.com/product/corrocoat/

(or an equivalent) and the ground line will have an extra 3/16 or 1/4" plate around the pole all to help prevent the pole shaft itself from corroding. Poles are priced per pound, a 4'x4'x3" base plate will weigh 1960 lbs. an extra 12' (assume 100' pole using embed rule of thumb of 10%+2' - I am not a proponent of using this for design but for this example it will work) of 1/4 thick shaft will weigh about 1578 lbs plus a 1/4 bearing plate at the base weighing 163 lbs for a total of 1741 lbs which is less than the weight of the base plate for anchor rods. Plus, as you mentioned, there is still the cost of the rebar and anchors which is not insignificant.

ab21,
The important thing to note here is that it is the tangent poles that use direct embed as they see much less overturning moment as the line tension is balanced on both sides. The only moment is caused by wind on the poles and conductors (not insignificant but much less than poles that resist line tension). As HotRod noted, the direct embed simply isn't as reliable or as easy to adjust plumb (or even raked). This is why for the dead end and high angle poles that resist conductor tension (can range from 10 to 60 kips per conductor) you will see the more robust (more reliable) concrete drilled piers getting installed.

If a direct embed tangent pole foundation fails this may result in a leaning pole that can be repaired without an actual outage happening. If a dead end foundation fails a few miles of line may get pulled down as the tangent poles end up taking the line tension load...much more catastrophic.

Yes, these can also be installed on spread footings and the design is the same as any spread footing with moment. Electric generating windmills are often installed on spread foundations, yes its a lot of rebar and concrete. Here is a good video showing the construction of one.

https://www.youtube.com/watch?v=Wr3O0D1TJn8

Drilled pier foundations also give you the ability to start construction before the poles are received. Depending on the market (this is the US), lead times for tapered tubular poles are at best 10 weeks and I've seen them up to 35-40 weeks. You can typically get anchors in 6 weeks or less so you can get all of the foundation work done while waiting for poles to be manufactured and delivered.

 

[BridgeSmith]

Thanks for the info, dauwerda. I guess I can see a cost advantage if you can use the 10% + 2'. I can't imagine that working for us. We design the foundations for our 120' highmast light towers using L-Pile or All-pile and the the least embedment I've ever used is 17', and I've had installations that required up to 30' of embedment. Those only have a ring of lights at the top, and the drilled shaft foundation is twice the diameter of the pole.

For our span wire traffic signal installations, we use directly embedded timber poles with an embedment depth of 1/4 to 1/3 of the height of the span wire attachment.

All that to say, for the installation the OP is proposing, with a significant lateral force applied at the top, I would expect required direct embedment of 30% or more of the clear height. With that much embedment, I think the cost difference would be much smaller, possibly even swinging in favor of the concrete pier foundation.

 

[dauwerda]

Totally agree HotRod. Also if the OP already knows the diameter of the pole then I would think the height is established as well. If it was designed to go on a concrete foundation I'm not sure how you would switch to direct embed and still maintain the height/clearances that would be needed.

 

[JLNJ]

Utilities seem to be OK with failures (severely leaning or downed poles after a storm) which would make most structural engineers cringe.

Use the 10% plus 2' rule only if your client doesn't care if your pole leans or falls over in a strong wind.

 

[ab21]

dauwerda and HotRod10

Thanks for your contributions, not doing 10% of height + 0.6m, based on Brinch Hansen method the embeddment depth should be about 8m. I think I will do a cost estimate for using piers as well. This pole only has 5 degree angle on it so works more like a tangent than a strain pole and the client doesnt want more reliability from its strain structures. They've also used direct buried poles for strain pole previously and the poles I see on site which are direct buried look plumb to me.
I know the height of the pole needs to be 35m above ground and our max auger size is 1.5m hence I know max pole diameter can be 1.2m so I know they will accept the design but the question really interested me because overturning moment should be the same regardless whether you use piers or direct bury the pole hence the depth should be the same too. I just wondered why would someone design piles then get a contractor to install them when they could easily and quickly bury the pole into the ground using the same machinery (crane and EWP) that would be needed to install the pole.

Suppose I wanted to use concrete pole, would you still recommend using piers because the direct buried part of the pole should act like a pier anyway?

 

[dauwerda]

As I kind of hit on before, one of the main issues with direct embed of steel poles is corrosion. Using drilled piers instead gets rid of this issue. As you just mentioned, using a concrete pole is another way to deal with this. I believe some of the manufacturers have even started supplying hybrid poles that have a concrete bottom section and steel top section(s) to help address just this situation.

http://www.valmontutility.com/products-solutions/transmission-poles/hybrid-poles

I guess it really comes down to reliability issues which may be increased or decreased depending on soil and environmental conditions.

 

[ab21]

Thank you all. If I were to design a pile foundation, structurally can I design it like a beam (because the soil will restrain it against buckling) or does it have to be designed like a column?

 

[BridgeSmith]

Generally, even steel h-piles fully embedded in soil are considered braced against buckling, not that axial capacity is much of a concern for pole foundations, anyway. Drilled shaft foundations for poles, especially poles supporting span wires, are typically well under 5% of their axial capacity.

That said, they're not really designed as beams, either. There are several methods for calculating moments, shears, and required embedment depths for deep foundations subjected to large moments, most of which use some type P-y curve analysis (non-linear springs). L-Pile and All-Pile are fairly common programs using this method. Be aware, the largest moment will occur somewhere below the ground surface and will be somewhat larger than the moment at the groundline.

 

[wiktor]

Try this spreadsheet - I think that exactly what you need

http://geotech.eng.usf.edu/Shaft123.html

 

[ab21]

Sorry for my stupid question but I havent designed piles before (I have designed pile caps but pile design was given to the contractor). I know maximum moment will be 2000kNm, I've used Brinch Hansen method and I know embedment depth needs to be 8m. The question is how can I design it structurally, I thought maybe I could treat it like a beam? But then I've seen piles have different, spiral, reinforcement.

I've looked at L-Pile I dont think I can get a free trial version? I can only see Demonstration mode and I am told I cannot use it to do analysis.

 

[skeletron]

For a concrete pile: get your applied forces (axial, shear, moment, torsion, etc). Then design the pile like a concrete column. Set up a spreadsheet to plot the P-M interaction and see where your design lies to decide whether you need more/less reinforcement, more/less diameter, etc. The soil effectively braces the pile below a certain point.

Honestly, the hardest part is getting the internal forces because of the different soil layers. It's doable using hand methods or iterative design tools, but getting a pile program can really help speed things up and give you an accurate picture for deflection results. I would invest in one if you are doing more than two or three of these piles.