Light Pole Foundation over Pipe 1

[LBengineer]

Hello all,

I have been looking into a problem my company has been having with installation of light pole foundations over Reinforced Concrete Pipes. We have a standard foundation design for the pipe being directly over the reinforced concrete pipe but obviously that isn't always the case. In the cases we are having problems, the pipe is significantly offset from the center of the foundation but not significantly enough to slip a precast shaft foundation by. for this case we usually install a shallow footing that is poured on top of the RFC pipe and includs rebar to distribute the load around the pipe. the idea that would make sense to me would be an eccentrically loaded shallow foundation with rebar to distribute that load away from the RFC pipe but i am not really sure where to start. I am looking for any readings that any one has used with distributing load within an eccentrically loaded foundation, if anyone has any ideas besides distributing loads in an eccentrically loaded foundation I would love to hear.

Note: in a majority of the cases, we are restricted by construction in the median of a roadway so we cannot move the lightpole out of the way of the pipe.

If you need any more information besides what I have provided, I would be happy to provide as much information as I have.

 

[SlideRuleEra]

Post a sketch of your proposal, preferable drawn to scale.

 

[LBengineer]

Hopefully this helps.

The drawing is to scale but some measurements are left out (listed below). The drawing is this specific case with the standard foundation.

Width of footing: 5.2'

Height of footing: 2.25'

Internal Diameter of pipe: 2.5'

External Diameter of pipe: .52'

Diameter of Pedestal: 2.166'

 

[SlideRuleEra]

LBengineer - Edit: Sorry, I now see this is your proposal (my mistake). Have a suggestion, will be back later.

Thanks, that does help, but we need to see your proposal. You mention that you want to "...distribute that load away from the RFC pipe but i am not really sure where to start".

Is "away" = 1 foot? ...or maybe 5 feet?

I would start by clearly defining exactly what you want to do (a reasonably accurate scaled sketch drawn by hand is fine). Speaking in vague generalities (like away from the pipe) gets you (or, at least me) nowhere.

 

[LBengineer]

SlideRuleEra: I Quickly threw together a sketch of an idea, but this is without any reference. If there are any readings that you could point me too to better detail and understand this type of rebar work that would be greatly appreciated.

To Remove my vagueness if the sketch below does not help, I would like to use slanted rebar to distribute the loads that are currently being put onto the reinforced concrete pipe (existing). Distributing this load around the pipe would be the idea (as the original design without the offset is meant to do) but the offset puts a wrench into this because the loads wouldn't be evenly distributed across the foundation potentially causing problems.

Thank you again for the help and hopefully this gives you the information that you need.

NOTE: the first drawing was not my proposal, it was a drawing that I had done previously to show how the offset effects the standard design that is used when the foundation is directly over the pipe.

 

[SlideRuleEra]

LBengineer - I see your detailed proposal now. That's how to start (when you don't know how to start)... Define the problem, first draft does not have to work. The process of "defining the problem" will give you ideas for an improved second draft.

Here's my suggestion: Forget about trying to make an eccentrically loaded footing work. Instead, design a trapezoidal slab with no eccentricity. Centroid of the trapezoid coincides with light pole load. (There is no "rule" that slabs have to be rectangular). Maybe this will work for your application, maybe not... just my first draft.

Note: Trapezoidal footing are often used to eliminate eccentricity in combined footing, such as, putting a column at a property line:

 

[1503-44]

From a piping perspective, I'd prefer a higher bottom of footing, perhaps wider for any needed overturning stability. We tend to not like things bearing directly on, or in direct contact with, the pipe and often leave a small annular space for a 4" layer of Styrofoam to go around (or above) the pipe. For future ref, gas and oil pipelines need a min 12" clearence, so no way we'd let you do that.

Also be sure the weight of slab doesn't actually increase the load on the pipe more than it would be without the footing installed (at a higher elevation).

Any possibility of a slab spanning across to sonotubes on either side, or just one side???

Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[LBengineer]

SlideRuleEra: The trapezoidal foundation is a great idea to help with the unproportional distribution of loads, I will definitely be looking more into that.

1503-44: I completely understand why we shouldn't be adding any weight or load to the pipe. In truth these poles still may be removed from the project. If they aren't removed the design of this will go through plenty of hands to actually be approved. I will make sure to keep those recommendation in mind as I am moving forward. the 4" layer of Styrofoam is a detail that will be included.

 

[1503-44]

I was also thinking that a bottom of footing at least going down to the pipe centerline, if possible, might help in minimising load actually applied to the top arc of the pipe as well, but if you use a foam there, that's probably good enough.

Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[SlideRuleEra]

LBengineer - Glad the trapezoid concept may help. Three other issues:

1) You asked about applicable reference material. Since the project directly effects existing concrete pipe, would be a good idea to learn more about concrete pipe (unless you have already done so). The American Concrete Pipe Association makes this easy with their (free) "Concrete Pipe Design Manual". All of it useful but Chapter 4, "Loads & Supporting Strengths" may be of most interest to you.

Comparison of concrete pipe with other pipe materials, such as ductile iron, HDPE, steel, PVC, etc. is only useful up to a point. Reinforced concrete pipe (RCP) is the "toughest" of all... that's why we used it with limited cover under heavily loaded railroad tracks.

2) I completely agree with the proposed foundation not contacting the pipe (note that is what my sketch shows). But, when designing to minimize or eliminate eccentricity be sure to take bearing strength of compressible material between foundation and pipe into account.

What can not be done is to design the foundation for minimum eccentricity (on soil), then arbitrarily place compressible material under part of the foundation... and expect the foundation to retain minimum eccentricity.

3) When designing foundations, keep the design as simple as practical for constructabilty reasons. In this case, if a flat slab is suitable... stop there.

 

[LBengineer]

Thank you for the insight, I will make sure to consider all of this information. This was a lot of help.