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

 

[BridgeSmith]

Structurally, the drilled shaft is designed as a column, as skeletron said. The hard part is that it's not fixed at the groundline. The resistance to bending develops gradually along the side of the shaft, making determination of the maximum moment accurately by hand very difficult. The simple hand methods for the most part ignore or very conservatively estimate the effect of the soil along the shaft, making those methods typically very conservative with regard to the maximum moment.

 

[ab21]

Thank you all

 

[Settingsun]

ab21's case is a 35m tall pile which will be 8m into the ground, including 3m of weak ground. It's not possible to be very conservative with respect to maximum bending moment under these circumstances.

 

[BridgeSmith]

So, steve49, are you saying that even if you make the most conservative assumption of the point of fixity possible and no restraint above it, the moment will not be "very conservative"? If so, for this particular case, I would agree with you. Any conservatism would be small, since the depth difference between where it starts to be restrained and where it needs to be fully fixed to provide stability is only a few meters.

That highlights another concern (especially in light of the 3m of weak material, which I missed somehow), which wasn't really asked by the OP, but warrants a cautionary statement. I'm not familiar with the analysis method you mentioned, ab21. For similar poles, foundation diameters, and embedment materials, with much smaller lateral loading (the aforementioned 120' high mast light towers), our analyses have required similar or greater embedment depths. If I was designing this, I would be suspicious of the 8m embedment, and would want to verify it using a different method.

 

[spats]

You can't even begin to evaluate the drilled shaft without detailed soil boring information. When I do foundations for high mast light poles, a soil boring is taken at every light pole location.

You might not want to spend the $950, but CivilTech has an excellent program for analysis called AllPile.

 

[BridgeSmith]

I'm not sure I'd call it "excellent", but Allpile is good and what we use the most. I'd consider LPile to be a superior program.

 

[skeletron]

I still can't get over how many spelling mistakes are in ALLPILE's program. Really obvious things. I like the soil properties options (maybe LPILE has the same). It's basic and calculates what you need, but I am regretting not taking LPILE for a demo run before making the ALLPILE purchase.

 

[BridgeSmith]

The real problem with AllPile, is that it's not a standalone program. It's an interface for FHWA's ancient and unsupported COM624P analysis engine, so every question for their tech support results in the same answer - "it's the analysis engine and we have nothing to do with that".

 

[Settingsun]

HotRod10, I'm taking the 8m embedment at face value. Maybe the fill is over rock or hard clay. But agree with a double-check.

ab21, have you applied a safety factor to the Brinch Hansen result? It's a limit equilibrium method so gives ultimate capacity, not allowable. Also, is 2000kNm the bending moment at ground level? The maximum moment will be larger than at ground level.

 

[ab21]

steveh49

Bending moment is 3m below ground level (thats what Ive assumed). Its ultimate moment. I've applied 0.7 reduction factor on soil strength when using Brinch Hansen.

Come to think of it, when designing based plate pole, are we assuming that base plate connection to the pile is moment connection? I assume the pile is allowed to rotate isnt rigid?

 

[BridgeSmith]

The connection of the pole to the foundation is a full moment connection. With adequate embedment to achieve fixity of the drilled shaft, the drilled shaft is a cantilever column. If the shaft does not have adequate embedment, Allpile flags that it "rotates as a rigid body" indicating that the deflection will be excessive and the analysis is not valid. Using the moment you gave, assuming a value for shear, and good soil properties, the 8m embedment works in Allpile, albeit with a little over 60mm of deflection at the top of the foundation. The max moment was around 2300kNm . Again, that was with my WAG for the shear and assuming 2000kNm at the top of the shaft.

 

[ab21]

Thank you all for your input, you have been very helpful.

 

[Agent666]

I don't know if this is of any use, I've never used it but I came across it the other day and bookmarked it for a future use/look.

https://github.com/NHERI-SimCenter/PileGroupTool

https://simcenter.designsafe-ci.org/learning-tools/pile-group-tool/

Least its free and open source to give yourself a feel for what might be required.

Don't forget about the fact that your pile will be cracked and have much less stiffness than the full moment of inertia, especially under those huge loads, so deflections and rotation might be .

Also any rotation at the top of the pile needs to be factored into the deflection occurring 35m up at the top. If you have some target deflection limits for the pole, the foundation rotation should also definitely be factored in.

Not sure where you ended up with embedded pole or pole bolted to pile. But also consider the construction aspects. Embedding the pole is quite a pain on site, how do you hold it in place, how do you keep it centralised. My vote is for either a shortish stub cast in to which you make a steel to steel connection/splice, or just embed bolts for a ring type baseplate and use a Reinforced concrete pile.

 

[ab21]

Is deflection an issue for design of transmission structures? I've seen some clients limit deflection under everyday case but I thought transmission design was mainly based on ultimate limit state? and vertical loads for poles are not huge so shouldnt get too much P-delta effect. I take it deflection we are talking about here is under maximum wind and tension.

The project is at feasibility stage so Im sure it will change when we get more information, survey, access road info etc, I was wondering how I would design it later and what benefits could be gained from using piles.

 

[dauwerda]

I am used to seeing 8% of pole height as being the upper deflection limit but will sometimes be less than this if excessive deflection will cause clearence or right of way issues. Will often see 2% or 4% specified for the "normal, everyday" load cases that have no ice or wind and average temperatures. Other options for controlling deflection can be to camber or rake the pole, I've never seen that done with a direct embed pole however.
Most specs that I see require a second order analysis.

 

[ab21]

I use 5% of pole height although I dont think the client cares much about deflection.

Thanks

 

[Agent666]

So for 35m pole, that's 1.75m. That sounds pretty excessive at ht/20?

 

[dauwerda]

Agent666, for poles that have equipment on them (such as disconnect switches which have moving parts and need to align properly) it is typical to limit deflection to H/100. However, for transmission poles that are only supporting conductors, large defections are typically not a concern as the conductors don't care if the top of the pole moves. i.e. there are no serviceability issues.

Also, dead end structures and large angle structures which are constantly loaded under the wire tension will usually have stricter deflection limits (or require a camber). This is really only for aesthetics.

 

[Agent666]

I'd suspect that at ht/20 you'd have wind induced vibration issues, and hence possibly fatigue issues. As long as those types of things factor into the normal design process and are addressed, the total magnitude of the deflection wouldn't matter so much from considering 'visual' aspects of serviceability. It just sounds like a massive deflection, but if the numbers work out then does it really matter. The way I talk myself into these higher deflections sometimes is the argument/test that no ones going to be out in the hurricane observing the pole thinking that its deflecting a little further than sounds comfortable, if it ain't seen it ain't a problem, out of sight out of mind.

Though one has to wonder if the conductor loading changes at all with such a fundamentally large change in the geometry of the system (I guess not given the magnitude of the conductor span would be orders of magnitude further than this deflection so maybe not).

If you had any microwave links or similar on the tower, then I guess this is also a case where you might require more consideration of deflection/rotation so these remain aligned. Don't think the ab21 has actually noted what the pole is for or where this moment is coming from apart form saying deflections aren't cared about unless I'm mistaken.

 

[JLNJ]

Hey HotRod

With 60 mm of deflection at the top of the foundation (which seems like a lot to me), does the pier begin to "plow" and therefore increase the subsequent deflections under cyclic loading? I know that the soil at the top is normally ignored, but the elasticity of the deeper soil must bear some of the brunt of the cyclic effect.

(I have no idea if this is actually the case, but it might explain why power poles go down in a storm.)