How do Embedded Poles Overcome Bending Moment? 1

[ScratchyFilm]

I've been looking at examples of large utility poles being embedded at 10% height of pole +2 feet. I'm curious how the bending moment due to wind is actually being overcome in making these judgments? Is it the passive and active earth pressures acting on the embedded portion of the pole that is counteracting the bending moment due to the wind at the ground line?

Thanks!

 

[1503-44]

Yes, it is exactly that.

 

[JedClampett]

In the IBC, Section 1807.3, there's an ancient formula giving resistance to lateral loads for embedded items. It's pretty mysterious, but it works. I'm sure the utility companies once had someone figure it out per those formulae. Apparently, 10% of the height plus 2'-0" works.
Side note, I once took a Red Vector Course on Utility Poles and it was tougher and more technical than I thought it would be.

 

[1503-44]

Wind, snow, ice, corner turns, end of run poles, temperature change, wire loadings, clearance calcs, back stays, cable design, cable connectors, rock bolts can all conspire to make it a bit more difficult.

 

[dauwerda]

Th 10% +2' is a rule of thumb that is often less than conservative (depends on actual loads and soil types). You still see it getting applied by some utilities for the the design of distribution poles, but almost all transmission poles (what I would call "large") utilize an actual design.

 

[1503-44]

That rule supposedly (I hope) is only to set an initial depth to start the analysis of soil pressure, max moment for [pre][/pre]pole design.

 

[dauwerda]

Unfortunately, there are thousands of utility poles installed all over the US based on that rule of thumb. So when you drive down the road and see utility poles leaning much more than one would think appropriate, you know that that is one possible reason.

 

[1503-44]

Leaning is OK. And very few actually falling over.

 

[Agent666]

Lookup the Broms method for cohesive soils, you should find plenty of advice.

It simply converts the moment into two soil reactions based on your soil properties and calculates a minimum embedment depth to support that moment. (sort of like a propped cantilever in a way). There are also formulations for short/intermediate/long piles where for longer piles you might yield the pile prior to mobilising the full soil reaction possible.

For cohesionless soils a similar calculation exists, however it's usually based on a passive resistance.

 

[Tomfh]

Is it the passive and active earth pressures acting on the embedded portion of the pole that is counteracting the bending moment due to the wind at the ground line?

That's the idea.