Longitudinal reinforcement for concrete pole footings

[mcr07]

I have a quick question to see what the structural community opinion is with regards to this:
This concrete pole is being installed on a Seismic Design Category D

We've designed this concrete pole based on the guidelines of the CBC 10, section 1807 for Embedded poles. A light pole is being attached to this pole.
We figured the depth, based on the equations on section 1807.3.2.
Based on the loads, the depth we found was only 4'-0". A square area of 48"x48" was assumed.

The question now is the minimum longitudinal reinforcement for this concrete pole.
I've discussed this with other engineers, and to my understanding, section 1810.3.9.4.2 shall apply based on the Seismic Design Category and the fact that Seismic Loads govern the design.
The minimum longitudinal reinforcement called out for this section is 0.005 of the Ag.

Any feedback for this? Reason I'm asking is because based on the sq area, the still is about 12" sq in.
Any exceptions that could apply to this?
Are there any other provisions for foundations of this type? It seems pretty straight forward, but the question arose because we've noticed other companies issuing less steel for an application like this.

Thanks!

 

[Michel60]

Good afternoon mcr07,

To me what you have really isn't a pier or pile foundation. The 0.005Ag is really a consideration of "felxural" reinforcement and behavior. A 4' cube of concrete just won't be bending that much. I'd say reinforce for minimum requirements of crack control in both the vertical and horizontal directions then check for additional anchor bolt reinforcement across the shear cone and call it good.

regards,
Michel

 

[mcr07]

Michel,

I understand that, but we need to consider something. The design for this foundation is considering lateral bearing formulas, not typical vertical bearing pressures. And since it's also taking axial, it should be considered as a column. Therefore, similar requirements as for columns should apply. However, i agree with you that strength wise, the 4' cube of concrete will be ok. It's just that the only minimum requirements that really comply with this foundation, are the ones noted on the CBC section described above. If you could quote me a code section that resembles the description of this element and has a less stringent requirement of reinforcing; i'll appreciate it.

 

[Michel60]

I do appreciate you want something to hang your hat on and i think the starting point here is to back away from the idea is that what you have is a pile foundation. It's more appropriately something of a spread footing. If you assumed nothing more than soil bearing and the mass of the footing to resist overturning would your system still work? Just how tall is your light pole?. Do you auto impacts to consider? Typically even if tall they're just not all that heavy and a 4' cube of concrete will generally stop a car...... If you prefer to go with something more of the embeded "pole footing" approach I'd suggest reducing the diameter/cross section and increase the embedment so you get less of a minimum reinforcing requirement.

regards,
Michel

 

[mcr07]

Thanks Michel,

Yes, if were to analyze this a spread footing, assuming soil bearing and mass of footing only, then the system would be OK for the loads. However, the layout for the reinforcement is different than just having a "cage". The rebar will be less, meeting min requirements of 0.0018 for temperature (which is good).
Now, i need to bring the other side of the coin, and the issue is that a previous drawing by another engineering firm, shows a "cage like" configuration with a 0.0020 steel ratio for the same cube size shape. Hence the dilemma.
I did suggest a more "pole like" approach, with a round pole of 24", but since the depth got more into the 6ft range, the design was shut down. Now I need to suggest the footing with mat reinforcing approach and see if the contractor will be ok with less steel, but different rebar placement.
Now, for the record thou, would you agree that there's no code related section that would describe a 0.0020 steel ration for a column like design?

Thanks for the suggestion.

 

[Michel60]

It's not too uncommon for the typical minimum structural slab reinforcement of 0.0020 (ACI 318-05 7.12.2.1) to find it's way into a footing calculation and be shown with hooked bars top and bottom. Alternatively some hooked verticals on the perimeter with lapped U-shaped ties all around would be just as reasonable an approach for reinforcing. The former probably provides better crack control and makes more sense as the plan dimension of the footing gets larger but I don't see it as absolutely necessary for the dimensions of what you are working with.

Also a 24"diam x 6' pier doesn't strike me as being out of bounds for a light pole in the 16'-20' range. But it might be a little suspect on something less than 12' pole.

regards,
Michel

 

[mcr07]

Michel,

I agree with you if the configuration we're talking about is a footing.
However, if you'd see a cage like configuration, would the longitudinal reinforcement then follow the 0.0020 criteria?
My initial reaction would be NO. ACI 318-08, 10.9.1 and 10.8.4 mention a 1% min steel that can be applied to 1/2 of the effective area Ag.

Yes, the 24" diam x 6' depth is for a 22ft pole. That's what we're discussing here.

Thanks again!

 

[FixedEarth]

The section 1810 deals with deep foundations. From a geotechnical standpoint, when the depth of foundation is equal to or less than the width of the foundation, it is classified as shallow foundation. Have you looked at it as an isolated footing loaded with a lateral force?

 

[mcr07]

Thanks FixedEarth,

That's good to know; i ran some numbers and yes; a regular footing analysis with the dimensions mentioned would work.
However, the problem is that the contractor wants to build a cage like reinforcing for this block of concrete because of a previous design.
So my concern is that the requirements in order to provide the reinforcing is not the same as a regular pad footing, but as a column like pole.
I'm gonna explain this to them the best I can.

 

[hawkaz]

I agree with Michel60- 24" diameter X 6' seems reasonable and is more standard (and it is 3X less concrete than a 4'X4'x4' cube). In a drilled pier like this, I use .005Ag.