IBC 2009 Equation 18-1 5

[ToadJones]

Anyone familiar with IBC Equation 18-1 for pole embedments?
My question is, in determining "S1", is it required to use the presumptive lateral load-bearing values of table 1806.2?
The allowable lateral load values of the table seem quite low.
Im not sure what the what the equation was in the older IBC editions.

 

[msquared48]

If your soils report shows higher, use that. Otherwise the table values. Note that the values can be doubled under certain conditions.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[ToadJones]

Mike-
They can be doubled?
Where is that noted?

 

[JedClampett]

If your only problem with the table values is that you're getting a foundation that seems unreasonably deep and/or wide, I wouldn't increase them to site specific values. If there's other reasons (increased loads on an existing footing, change in wind loads due to code, etc.), I might consider it. In other words, if it's just a matter of more concrete, just pour more concrete.
These are somewhat empirical values, meant to be used with that specific formula. They've been the same forever and I wouldn't stretch them too much.

 

[ToadJones]

Jed-
Have you used Equation 18-1 before?

 

[ToadJones]

I ask, because I have not.
I can see there is many ways to calculate pole embedments.
Is this equation applicable for a concrete pier/caisson for a utility pole?

 

[ToadJones]

also, the equation seems to be circular ...S1 requires knowing "d"...so I guess the process is iterative?

 

[MiketheEngineer]

Yes - it is iterative. Also see Eqn 18-2 and 18-3

Search these forums - this comes up from time to time.

 

[ToadJones]

I set up a small spreadsheet to make the iterations a little easier.
I looked at 18-2 and 18-3, and they don't apply to me.

Last question would be the code says d=depth of embedment in earth, but not over 12' for the purpose of calculating lateral pressure.

Does this mean that 1/3 of the depth is never more than 4 feet?

 

[JedClampett]

I've used it several times. I've also seen it used by engineers who do a lot of similar designs, like for light poles. You're correct that there are many ways to design pole embedments. But this one is in the code and as such has the weight of the law behind it.
When I'm doing one of these designs, it always seems way too conservative. But it's a very cheap installation (core hole in dirt, drop in reinforcing cage, pour concrete), so I just run the numbers and wait for the complaints.

 

[ToadJones]

Jed,
Makes me feel better about how giant this thing is.

I'll ask one more question for those with more experience on these things.
We are installing one of these utility pole foundations in an area that is excavated very deep at the present time...actually almost as deep as the pole footing needs to go.
We will be backfilling in very small lifts and tamping with excavator mounted tampers using engineered fill.
Would one expect the design of a foundation with such an installation to remain the same as one that was being drilled?

 

[msquared48]

Toad:

Look at Section 1804.3.1, the second paragraph, for the doubling of the lateral values.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[ToadJones]

I'm looking at 2009 Mike.
I'll have to do some reading.

 

[msquared48]

Also you asked:

"Last question would be the code says d=depth of embedment in earth, but not over 12' for the purpose of calculating lateral pressure. Does this mean that 1/3 of the depth is never more than 4 feet?"

For the nonconstrained condition, the answer is yes, but for constrained, no. If you have a power pole, if there is a large concrete slab present (and I do not meaqn AC concrete), you can use the constrained condition.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[ToadJones]

Thank you Mike-
in the 2009 Edition the doubling of loads is 1806.3.4 "Increase for Poles".

I am unconstrained, therefore my lateral pressure cannot be more than 4'x's the value in table 1806.2, correct?

 

[msquared48]

Yes.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[ToadJones]

Well while I am beating a dead horse here....
Since eqn. 18-1 only provides for a lateral force "P" and no moments, I am assuming that I can simply place my shear reaction at an height "h" such that it causes the same moment as my design reaction moment and, obviously, the shear.

 

[msquared48]

Yes. And ignore the top foot of soil for the lateral resistance.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[ToadJones]

Thank you again Mike.
I oh you a delicious ice cold beer of your choice.
I prefer IPA's but you may choose your favorite.

 

[ToadJones]

oh = owe...man it is definitely Friday.