IBC Section 1805.7 for post foundations

[dmoench]

I'm apologize if this question has been asked before but I couldn't find it in the archives.

I am dealing with post foundations and I fully understand and use equations 18-1 and 18-2 and 18-3. I recently worked with an engineer who increases the values of his A term for equation 18-1 by referring to 1804.3.1

The equation for A is 2.34P/S1*b, he makes the case that based on 1804.3.1 he can add 2 in the denominator based on the last statement in 1804.3.1 and he says he can add another depth term in the denominator based on the first statement of 1804.3.1. This amounts to the equation changing to A=2.34P/2*S1*b*depth of embedment. Now personally I feel neither the 2 nor the depth of embedment terms he has added are allowable if you are merely calculating lateral pressure terms. 1804.3.1 seems to be addressing the lateral pressure component of sliding and not the lateral pressure the post can be designed to. Just putting in the terms and solving for units will make the case to get rid of the additional depth term.

An e.g. may be helpful. I am only interested in the depth of embedment required and we will only do the first part of the equation solving for A.

Suppose we have a 8'-0" post with a load of 3000 lbs applied to it. Assume S1=150psf/ft and arbitrary attempted depth of embedment will be 6'-0", 12" round post. Table 1804.2 allows a 4/3 increase due to WL combination loads so lets use it.

With my understanding of the IBC in this area the equation becomes; A=2.34(3000)/(150*6'/3)*1.33*(1)=17.6 ft.

My friends interpretation becomes;
A=2.34(3000)/(150*6'/3)*1.33*(1)*2*6'=1.47 and no units

What are you all doing out there for this equation?

Thanks

 

[msquared48]

This subject has been beat to death in the forums here, but the value of S1 can be doubled if the structure can tolerate a ground motion of 1/2" at the finish grade line. If not, don't double the value.

Also, these equations are itterative, having to try two or three trial solutions to bracket the final solution.

Just look up "pole Foundation" in the search engine here.

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

 

[dmoench]

Mike,

I appreciate the comment but the fundamental problem I see with that is that the 2 factor falls under the section Lateral Sliding resitance (1804.3.1) while I'm interested in bearing values per Table 1804.2 I undertand the iterative process and that was not my question. The question is the appropriateness of using 2 or my friends additional depth term in calculating the depth based on allowable foundation lateral pressures.

Thanks

 

[msquared48]

The "appropriateness" of using the extravalue of 2, doubling S1, comes into whether your structure can tolerate the 0.5" of movement, or sliding, in order to DEVELOP the higher lateral bearing pressure.

You will have to make that call.

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

 

[dmoench]

I guess I'm not being clear about my question;

First assumption - This buidling with definitely move more than 1/2" over time.


the lateral pressure increase of a factor of 2 falls under the section 1804.3 which is titled Lateral Sliding Resistance. I felt this 2 factor should fall inside 1804.2 where we are actually talking about Lateral bearing values. This is what drives the final depth of embedment of the post per the equation for A. As I understand sliding resistance in footing design it is a separate calculation from calculating bearing allowables. Now I realize that bearing works with sliding friction to give an overall resitance to movement but is it appropriate to use the 2 factor for the design of the embedment of this post as opposed to the sliding?

I hope that it clear.

 

[ToadJones]

Not quite sure what is going on here....
In IBC 2009, all of this falls under section 1806 "Presumptive Load Bearing Values of Soils"...more specifically 1806.3 which includes
1806.1 Combined resistance (sliding, lateral bearing)
1806.2 Lateral Sliding
1806.3 Increase for Pole Depth
1806.4 Increase for Poles
Increase for Poles
"Isolated pole for uses such as flagpoles or signs and poles used to support buildings that are not adversely affected by 1/2" motion at the ground surface dude to short-term lateral loads shall be permitted to be designed using lateral bearing pressures equal to two times the tabular values"

 

[dmoench]

ToadJones,

I only have the 2006 IBC. Would there be a copyright infringement for you to PDF the section you are referring to from the 2009?

Thanks

 

[ToadJones]

Not sure?

 

[MiketheEngineer]

NO - can your building move 1/2'' RIGHT now under the anticipated wind/seismic loads.

If it is a simple pole barn covering hay or tractors or horses - sure - no problem

If it is supporting a brick veneer - no way.

 

[dmoench]

MiketheEngineer,

Steel truss building with fabric roof. Very similar to Cover-All structures before they went bankrupt. Under wind they do move, seen it before.

 

[DaveAtkins]

I agree that IBC 1804.3 is poorly worded. They use the term "lateral sliding" to refer to lateral sliding plus lateral bearing, and then to just lateral sliding, in the same sentence!

Anyway, your colleague is correct. S1 can be doubled if 1/2" movement is tolerable, AND S1 increases linearly for every foot of additional post embedment. However--there is another limit to increasing S1, found in Section 1805.7.2.1. This limits you to increasing S1 by a factor of 4.

So the most you can increase S1 is by a factor of 8.

DaveAtkins

 

[ToadJones]

Also, not sure about 2006, but in 2009 your depth "d" for purposes of computing lateral pressure cannot be more than 12'.

 

[dmoench]

Dave,

I appreciate that.

So the equation for my example becomes;

A=[2.34*(3000)/[(150*6'/3 * 2 * 1.33 *(1')]=8.80ft.

Note I don't have a 6'/3 term and another 6' term like my friend had in his example. Am I correct in your interpretation?

 

[DaveAtkins]

Yes.

DaveAtkins

 

[msquared48]

Dave et al:

What you are speaking of is the non-constrained condition based on 1/3 of the maximum of 12 feet or 4 feet, correct?

However, if the pole depth is greater than 12 feet, 4X the maximum is still applicable and the equation still applies. It's just conservative if you look at the resisting pressure diagram applied.

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

 

[DaveAtkins]

That is how I look at it.

DaveAtkins