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]

If the pole is subjected to cyclic loading of the magnitude that produces 60mm of deflection, it will actually end up deflecting more. I assumed the loading mentioned by the OP was a design loading condition which the pole would not be subjected to regularly.

The interesting thing about span wire poles is that when it deflects more, it relieves some of the tension in the wire and the force on the pole. It's the difference between the tension on a tightrope vs. that of a slackline.

 

[ab21]

The pole doesnt have any antennas or other equipment on it and I dont think the client cares about deflection since its not in their design requirements but I know 1.75m is a big number for deflection. The pole is a dead end structure so it will have sustained tension but the moment value I gave was for ultimate wind.

I am using PLS pole/cadd to get deflection which I dont think consider any foundation movement (I think it assumes foundation is fixed). Just out of interest is 60mm deflection at top of foundation too much (because it should be 325mm at top of 35m pole)? I wasnt even going to consider foundation movement until it was mentioned here!

 

[Agent666]

It's the rotation that's required, and this wasn't mentioned. translate this base rotation to a deflection at the top of the pole and add your 1.75m to this.

To play devil's advocate, I guess owner might not know they need to be concerned/specify anything in this respect? If its not mentioned isn't it prudent to ask if he has any requirements in this space?

Because its no good for anyone (especially you) if he comes to you and states he's not happy with the end result if it bends over like a wet noodle under just the cable tension.

 

[dauwerda]

It's not so much lateral deflection of the foundation that will affect the overall pole deflection, it is the foundation rotation. My experience is that foundation movement is typically ignored for defection calculations but it is considered for strength calculations. Many utility specs that I see call for a 1 degree rotation to be considered when checking the strength of the pole. PLS pole does allow you to add spring/stiffness constants to the foundation fixity, for lateral, vertical, and rotation.

 

[ab21]

Thanks all, you have been very helpful