Foundation for Sign Pole 1

[Abdel22]

Hello,

i m designing a foundation for a sign pole(drilled shaft). i found the embedment length. anyone know which formula to use to calcs the necessary reinforcement.
i looked at some old design, they use # 5 Vertical with # 4 @ 12" without any clacs

anyone with some reference on how to design sign foundation.

thank you

 

[phamENG]

That depends on how it'll be constructed. Are you pouring the concrete and coming back and bolting to the top of it? Then it's essentially a column (round concrete section under axial and flexural loads)- ACI 318 will be your guide.

Are you dropping the sign post in the hole and then pouring concrete around it? Then you don't need much more than some rolled WWR to keep it from cracking apart.

 

[Abdel22]

i m dropping the sign post in hole at min required depth then pouring concrete.

what the reason of calculating pressure at 1/3 of depth?

 

[phamENG]

What's your reference?

 

[Abdel22]

LRFD specifications for structures,..........

 

[BridgeSmith]

LRFD specifications for structures

There's several of those, from AASHTO (sign structure spec & bridge design spec), ACI, and others.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Abdel22]

i m using AASHTO. This my first time designing a sign pole.
trying to figure out, how to design it.

 

[phamENG]

I'm not familiar with embedded foundations per AASHTO. IBC I can help you with, but BridgeSmith and other bridge guys will be the ones to discuss AASHTO.

 

[r13]

i found the embedment length.

what the reason of calculating pressure at 1/3 of depth?

I am puzzled. Usually one would need to calculate soil pressures to determine embedment depth, ...

 

[Abdel22]

I USE Broms' equation to calculate embedement length for cohesive soils.



i was looking at some calcs and they present final result show pressure at 1/3 of the depth.

this why i ask?
i m just trying to grasp the design of sign pole foundation

 

[r13]

I assume you have used the equation for cohesionless soil, for which the reactive stress distribution is linear varying with depth (triangle stress distribution), and the reaction is located at L/3 from the base of embedment. The reaction is equal to the applied horizontal force.

 

[BridgeSmith]

Brom's works if you're only concerned with stability and not concerned with limits on deflection. In cohesive soil, if your permanent load has significant eccentricity, you'll want to look at the amount of long-term deflection from creep under the sustained moment due to eccentricity. If that's the case and you want to limit the deflection, you'll probably want to run it in a P-y curve analysis program, such as Allpile or LPile to see how much movement to anticipate.

I'm not sure what example you're using for the 1/3 of the depth, but I can tell you that for a pole foundation, the maximum moment will occur somewhat below the soil embedment level. From my experience, a depth of 1/3 of the required embedment depth would generally be a decent approximation for the location of the maximum moment. The magnitude of the maximum moment could be roughly approximated assuming it to be unrestrained above that point.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[dik]

Some 'scribbles'...

 

[BridgeSmith]

The weight looks to be fairly well centered over the foundation, so my concern with creep deflection is probably not warranted.

I haven't really used Brom's method, so I don't really have a feel for how it compares to the more detailed methods. I'll leave that to those more familiar with it.



Rod Smith, P.E., The artist formerly known as HotRod10

 

[jdonville]

Abdel22,

We try hard and have a bunch of experience. However, your primary resource should be your supervisor and colleagues.

 

[r13]

My impression is you have assumed non-cohesive soil in determine the depth, however, it could be far from the fact. Do you have soil report, or anything can support your assumption? The behaviors are quite different for different soil types.

 

[Abdel22]

No i dont have soil report. just assume soil type 5. ( CBC 2016 TABLE 1806.2)

Rod, can you please give some reference about more detailed methods.

i couldnt access your file dik.

thank you guys

 

[dik]

AbdeL22: You need a program called SMath... like MathCAD, but free... excellent program

https://en.smath.com/view/SMathStudio/summary

Dik

 

[r13]

Ok. Got it. It is clayey soil with C = 130 psf. So you have used Brom's method for cohesive soil. I guess it should be fine then.

 

[BridgeSmith]

The method we typically use is a 'P-y curve' analysis, which involves modeling the soil as a series of non-linear springs, each governed by a force-displacement curve. It's an analysis best done with one of the stand-alone pile/drilled shaft analysis programs (Allpile, LPile) or a similar module within one of the larger structural analysis software suites.

Rod Smith, P.E., The artist formerly known as HotRod10