Why do wood utility poles fail above the groundline?
Why do wood utility poles fail above the groundline?
(OP)
Wood utility poles generally fail 5 or 6 feet above the ground line. Why is that when the moment will be greatest at the ground line?






RE: Why do wood utility poles fail above the groundline?
RE: Why do wood utility poles fail above the groundline?
RE: Why do wood utility poles fail above the groundline?
Concrete piling just rotated. The best coastal solutions seemed to be 8"x24 reinforced block piers (perpendicular to the coast and parallel to the surge direction) on spread footings about 6' deep. - Masonry blow-out panels between if the lower area was enclosed.
RE: Why do wood utility poles fail above the groundline?
RE: Why do wood utility poles fail above the groundline?
RE: Why do wood utility poles fail above the groundline?
The failure doesn't occur at the point of maximum moment, but at the point of maximum bending stress, which in a tapered section is going to be somewhat above grade.
RE: Why do wood utility poles fail above the groundline?
You'd think you could just grasp a bundle of it tightly at one end, and snap it off clean between two closely spaced fists.
It doesn't work that way; you have to move one hand away from the 'ground', and then it fails a couple of bundle diameters 'up', and never a nice clean break. Just like when a wooden pole snaps.
The spaghetti flying all over the kitchen usually convinces the wife that she doesn't really need my help...
Mike Halloran
Pembroke Pines, FL, USA
RE: Why do wood utility poles fail above the groundline?
As stated above, maximum moment doesn't mean maximum stress.
Refer to attached for Broms'theory.
RE: Why do wood utility poles fail above the groundline?
RE: Why do wood utility poles fail above the groundline?
I agree with EIT's first comment about rot - the most likely culprit.
The typical wood piles have their taper such that the top is a larger diameter than the bottom (tip)... although I've see a few telephone poles that I'd exclude from this.
Combine this with the fact that the max moment (unless there is a collar) IS below the ground surface-at the point of fixity, you would expect it to fail below the ground surface because of a higher moment and a smaller cross-section.
Therefore, I'd conclude that the material is, for some reason, weaker at the location of failure. I would guess that there is typically a small amount of wicking of groundwater to a certain height which would give a continual wetting and drying of the wood with changes in groundwater level, therefore causing the more rapid decay.
cms
RE: Why do wood utility poles fail above the groundline?
Seriously though, for a tapered pole, with the section varying dramatically along the length of the pole, it only stands to reason that it will fail at the point where the section becomes insufficient for the applied moment.
I also agree with StructuralEIT about the rot issue here. Seen a lot of that with marine pile that always fail in the area of the waterline region of the pile.
Also, with linemen and treated poles, the spikes they use may, with time, penetrate the region of pressure treating, allowiong rot to start. Stands to reason that this area would be toward the base of the pole where more activity occurs and the moment is greatest.
Mike McCann
MMC Engineering