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IBC 2009 Equation 18-1
5

IBC 2009 Equation 18-1

IBC 2009 Equation 18-1

(OP)
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.  

RE: IBC 2009 Equation 18-1

2
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

RE: IBC 2009 Equation 18-1

(OP)
Mike-
They can be doubled?
Where is that noted?  

RE: IBC 2009 Equation 18-1

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.

RE: IBC 2009 Equation 18-1

(OP)
Jed-
Have you used Equation 18-1 before?  

RE: IBC 2009 Equation 18-1

(OP)
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?   

RE: IBC 2009 Equation 18-1

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

RE: IBC 2009 Equation 18-1

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

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

RE: IBC 2009 Equation 18-1

(OP)
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?  

RE: IBC 2009 Equation 18-1

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.

RE: IBC 2009 Equation 18-1

(OP)
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?   

RE: IBC 2009 Equation 18-1

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

RE: IBC 2009 Equation 18-1

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

RE: IBC 2009 Equation 18-1

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

RE: IBC 2009 Equation 18-1

(OP)
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?
 

RE: IBC 2009 Equation 18-1

Yes.

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

RE: IBC 2009 Equation 18-1

(OP)
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.  

RE: IBC 2009 Equation 18-1

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

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

RE: IBC 2009 Equation 18-1

(OP)
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.  

RE: IBC 2009 Equation 18-1

(OP)
oh = owe...man it is definitely Friday.  

RE: IBC 2009 Equation 18-1

Salvator?

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

RE: IBC 2009 Equation 18-1

(OP)
Never had it...I'll have to try it.

RE: IBC 2009 Equation 18-1

IPA's are excellent.  From the traditional British brew that was made extra strong and extra hoppy to survive the long journey to her colonies in India.

Stone Ruination IPA is the best.

RE: IBC 2009 Equation 18-1

(OP)
Bobber-
To date, I have been afraid to try Ruination.
It simply scares the hell out of me.
I am afraid I'll never be able to taste any other beer again!
My favortie (for now) Sierra Nevada Torpedo Extra IPA.  

RE: IBC 2009 Equation 18-1

completely off subject, but mmmm Torpedo... have some in my fridge right now.

RE: IBC 2009 Equation 18-1

Torpedos will submarine you.

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

RE: IBC 2009 Equation 18-1

(OP)
Torpedo is bomb....

I think we need to get Mike up there for the Tipster of month or week or whatever...guy is totally willing to help research nasty code provisions for all!!!
Mike- you are appreciated.   

RE: IBC 2009 Equation 18-1

3
He's a good guy, Toad, but the only way he is going to be top Tipster is if you click on the bottom left.

RE: IBC 2009 Equation 18-1

A couple of other points on using the flag pole formula (Equation 18-1)

1.    Determine where your passive pressure starts.  Usually the geotech will give this to you.  (We have had cases where the passive started about 10 feet below the ground level!)

2.    The actual moment on the pole/caisson/pier will be somewhere below the start of the passive.   Some published papers use .34d (d = calculated depth).  Therefore M = P x (centroid of wind force + .35depth).

3.    If you are placing the pole in the fill, then you should probably have a geotech provide you with the passive pressure and starting distance.

If you really want to get into this, look at Section 13.10 Embedment of Lightly Loaded Small Poles and Posts, (page 13-8) of the AASHTO Highway Signs, Luminaires and Traffic Signals (LTS-4-13).

A small spreadsheet is the way to go.  

When I worked in London, Watneys pale ale was my usual order.  

RE: IBC 2009 Equation 18-1

(OP)
oldrunner-
do you know of anywhere I could get a hold of one of these papers?  

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