How to analize the lateral forces in a soil due to a pole? 7

[mtroche]

Searching in my soil mechanic books I didn't find any information and formula about how to analize the forces that the wind or any other lateral force acting on the pole causes the pole itself to drift. I am trying to analize the lateral forces in the soil due to the forces acting on a transmission pole.

This is the scene. I have a pole installed in a particular soil and there are power lines connected to this pole; so, there are three forces acting in this pole causing the pole to drift. These forces are: wind load, the power lines weight, and the tension of the lines. The only elements resisting the drift of the pole are any guy installed to the pole and the soil itself.

The problem is that I want to take in consideration the resistance of the soil against the drift of the pole, but I can't find any information or formula on how to analize the resistance of the soil to the pole drift. Any help will be very appreciated.

 

[JWB46]

Hi mtroche,
You really need access to a laterally loaded pile program like LPILE. I think there are free versions available on the web somewhere if you search. If you get stuck come back and I'll find out the suppliers name. The theory is given in various references in API RP2A.
Cood luck!

 

[Qshake]

You can get a good idea of the analysis and also the industry standard code used on light poles, liminare foundations etc by getting your hands on the AASHTO reference: AASHTO Standard Specifications for Structural Supports for Highway Signs, Luminares, and Traffic Signals.

This includes the soil mechanics analysis and actual metal support design (poles) both in easy to use nomographs.

 

[JedClampett]

There are a bunch of posts on this topic in the "Structural Engineering Other Topics" Thread.

 

[wel]

Why not do a stability calculation of the pole about the pivoting point ? That is calculate the overturning moments due to the forces and then calculate the resisting moments due to the pole footing allowing for a saftey factor. Probably conservative design and using a little more concrete but it works?

 

[CISGeotechnical]

Hi Mtroche:

I have worked with Billboard signs foundations as the Geotechnical Consultant and has made research looking for calculating the depth of embedment required to overcome the lateral loads imposed by winds. If you want I can send you a paper in pdf format that helped me a lot getting into the analysis.

Repply if you still need some info on this. CIS Geotechnical
jrodriguez@centralindustrialpr.com

 

[henerythe8th]

I came here to ask a question on this topic, I hope that you don't mind me butting in/providing information/asking a question regarding this provided information...

Yeah, yeah get on with it you say!

The UBC has some information regarding this topic in the foundation section. I am sorry but I don't have the code in front of me and can't remember the actual section...

My query is that in solving for the "depth of embedment necessary to resist lateral load" --maybe not a direct quote, but close--another (the same variable) appears again.

A co-worker believes that the two (d)'s are in fact the same d and that you have to solve the equation iteratively.

I believe that the d's are different and if notated appropriately in the code they would have different subscripts or something.

A bit more info if you are still interested:
There are two variables (d)..
One of them is associated with an equation and is described as "depth of embedment necessary to resist lateral force, P"-now referred to as d-sub-1--again, not really a quote, but...
the other is described as "depth of embedment used to determine S-sub-1"-now referred to as d-sub-2

The way that I read this is d-sub-2 is the actual depth of embedment (but not to exceed 12 feet)
and d-sub-1 is the "depth of embedment necessary to resist lateral force, P"

Can anyone please clarify?

Hey CISGeothechnical, I would really appreciate a copy of that info, if you wouldn't mind.

 

[CISGeotechnical]

TO: henerythe8th

You have a good point in the UBC 1997 code, section 1806.8. You need to iterate to use the depth of embedment that the formula yields in the allowable lateral soil pressure computed. That's a good starting point.

However, the UBC values for lateral soil pressure are a "cookbook" and I prefer to use my knowledge in Soil Mechanics and use the formulas for lateral soil pressure. In that way you can go to the basics of Soil Mechanics and feel confident with the results.

I have a .pdf copy of the paper I mentioned, which is very good to understand how this procedure works out. However, in reality there is seldom homogeneous soils. Therefore, I developed my own spreadsheet to calculate lateral soil pressure based on a subsoil investigation and estimated soil parameters. Anyone can feel free to contact me to my e-mail and I would send both the paper and the spreadsheet for the sake of the profession/practice.

Just send me an e-mail and I would forward the info. My e-mail should show in this page. CIS Geotechnical
jrodriguez@centralindustrialpr.com

 

[ergon]

The Brinch-Hanson method can be found at

http://www.usda.gov/rus/electric/bulletins.htm

The RUS Bulletin number is 1724E-205 "Embedment depths for concrete and steel poles".

 

[Togel]

Not so much an engineering response but a practical one. Nonetheless, the added expense of oversizing embedment and footng size for power poles will always be less than the potential cost incurred by a failure.

 

[Focht3]

I don't normally add to such an old thread, but this is an important exception.

ergon's suggestion to use Brinch Hansen's method is a very bad idea. If you download the USDA paper and look at Figure 1, look at the soil pressures at the pivot point shown as D_R - they do not diminish as they approach the pivot point. This is not physically possible!

Brinch Hansen's work is useful for a first estimate of embedment - using conservative soil properties and a minimum FOS of 4 or more. It is not suitable for final design, and is particularly dangerous for foundations with length to diameter ratios of less than 10. This includes almost all direct embed transmission poles and H-frame structures.