Design of light pole footing 2

[rpand4]

Hi,

I am new to this forum and I am a graduate.

I have been asked to calculate the footing reinforcement for a 15 m high light pole. The footing is to be 700 mm in diameter and is to be reinforced. The footing is to be poured in situ in the ground.

To do this I made a model of the light pole in Space Gass and inputted my loads wind loads etc to help me generate the design ULS loading. I got the worst case loading as follow:

N*= 6kN M*=120kNm V*= 15kN

These are the loads that the top of the pile footing will experience due to the loading from the light pole.

The light pole footing is going to be constructed in several different soil types. So I sent the above loads to our geotech engineer. He carried out some analysis for different ground models and essentially came back to me with the following table:

In his model he tells me to use the highlighted red bending moment and shear force values for designing the reo. We intend to use the same steel reo design for all the pile footings. In another table he also gave me all the pile lengths for different soil conditions.

What I intend to do is:

1) Use the BM he provided and the axial load I calculated above and plot these on my interaction diagram to see if my reinforced concrete pile can handle these loads.

But here is my question:

What do I do with the shear force load he has given me? I am kind of lost. Is it for me to do design the Shear ligatures? I have no idea how to design shear reinforcement for the pile. Could someone please shed some light?

Any help is much appreciated.

Regards

 

[Lomarandil]

First off, well done involving a geotech and recognizing that the at-grade reactions are not the maximum forces in your foundation! Most graduates don't catch that right away.

You now need to design your "footing" (which would be more accurately termed a 'drilled shaft foundation') to carry that shear, just like any other structural member. In your case, it will behave like some combination of a beam and a column (although the axial load is so low, you can consider it a beam for all practical purposes).

The good news is, a 700mm diameter foundation is plenty big for a shear load of 100kN. You'll find that you can carry that load entirely with the concrete cross section (phi*Vc).

If the load was higher (or diameter smaller), you would specify hoops of a certain bar size and spacing just like a beam stirrup to add to the shear capacity of the entire section (Vn = Vc + Vs)

In this case, your horizontal hoops (ties/stirrups) only really need to keep the main bar reinforcing in place during the concrete pour, and provide adequate confinement to the anchors between the light poles and shaft. (Most codes require two or three hoops in the very top of the shaft for this reason). Some engineers will specify hoops that meet minimum shear steel requirements regardless.

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just call me Lo.

 

[KootK]

Check this out: Link

It's no substitute for figuring stuff out on your own but, always, it's good to have a sense for what the answer should be before you start.