Angle Measurement 4

[msquared48]

Are there any design guidelines for the quantification of an angle of rotation for a concrete pole foundation buried in soil, acting as a pole footing for a steel structure above?

I have a cantilever structure to design where the lateral deflection has to be carefully controlled, and need to put a finger on the degree of base rotation needed to develop the lateral soil pressure used.

The height of the structure above soil is 31 feet, and the depth of burial is 9 to 10 feet for a 24" diameter drilled foundation.

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

 

[dik]

I would adjust the pole by means of 'levelling nuts' and not have it dependent on the installed concrete base. Provide a slope on the top of the base to shed water and use galvanised anchor rods...

Dik

 

[msquared48]

OK, but that is not the deflection that I am getting at.

I need to know the rotation that the foundation will have to rotate to generate the design passive pressure for the pole footing. For every degree that the foundation rotates out of plumb, the tip of the pole moves by .54", in addition to the deflection of the pole structure above. I need to limit the sum of the two within certain guidelines.

To limit it, I may need to go with a greater diameter, or greater depth of footing.

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

 

[ToadJones]

Mike-
the structure you are describing really has me wondering what it actually is?
You describe a flag pole type column, but it sounds like it is part of a frame.
I am eagerly awaiting some answers to this!
I have had similar concerns in the past.

 

[ToadJones]

I am wondering if FEA software could be used?
If you knew the pivot point of the pole foundation (seems to be many approaches to pole foundation design) and could assign some spring constant to the soil then rotating the footing some angle should yield a distance the spring would be compressed at any point and give you a passive pressure, no?

 

[BAretired]

It is a beam on an elastic foundation. You cannot calculate rotation unless you know the modulus of subgrade reaction.

BA

 

[SkiisAndBikes]

I have a bit of info from my sign structure book by Benjamin Jones (2006).

The simplified formula's in this book for a non-constrained pole type footing are based on a detailed review paper by M.T. Davisson and Shamsher Prakash. Perhaps if you can track down this paper, more detailed info than the simplified info I am providing may be available.

The simplified formula in the book (based on UBC) allows the UBC tabulated allowable lateral soil pressures to be doubled, IF the structure is not adversely affected by a 0.5 inch lateral motion at ground level. To me, this value provides an upper limit for the lateral deflection if using their formula. I have never doubled the allowable values so have assumed lateral deflections less than 0.5". They also assume the pole rotates around a point at approximately 2/3 the footing depth. A 0.5" lateral deflection a ground level for a 9'6" deep footing is approx. 0.38 degrees rotation.

 

[msquared48]

Toad:

It is basically a two pole sign structure, without the signage, and very limited in motion. It is for the FAA. Sorry, but that is the limit of what I can say at this point...

Skiis:

I am aware of the UBC 2X increase and associated deflection, but need to get the equations to justify as the deflections are functionally critical. Cannot be a wave of the hand.

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

 

[msquared48]

Skiis:

A 0.5" deflection at the ground level over a 9' pole depth, rotating at the 2/3 point of embedment (6') translates into a 31'/6' = 3.2" deflection at the tip of the poles. This is not tolerable for the project. I have not doubled the soil pressure values, so figure 1.6", a little more than the current deflection of the steel poles. I think I will have to do something more to limit the soil-related deflection, as mentioned, by increasing the diameter and lowering the depth.

I may also have to consider adding a concrete grade lateral bearing beam between the poles to limit the deflection further.

Fattdad... , BigH..., Any input?



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

 

[DWHA]

Just a idea, maybe not even a good one but...
Could you place the concrete footing with wick drains (if needed based on soil type) under the footing. Then place a surcharge load on the footing for some period of time. Then remove the surcharge load and put up the pole.

This method would compress the soil and develope the passive soil strength, which would help to limit any movement.

But, based on construction limitations, this may not be feasible.

 

[rb1957]

it sounds like you're asking of ideas to shore up an embedment ... since you've got data and calcs.

what sort of load is causing the pole to deflect 0.5" at ground level ? Your analysis looks like enforced displacement, without any bending of the pole. if you made the embedment a bigger diameter (or deeper) it'd be more rigid (but that would help an enforced deflection. if you supported the embedment with something stiffer than the existing soil (packed gravel ?, i don't know) that also wouldn't help an enforced displacement.

how about guy wires ?

 

[ToadJones]

Mike-
I have done substation structures similar to what you are describing, I think.
I looked into using two cantilevered poles and did not like the foundations I was coming up with. I instead went with an "A" frame structure on spread footings. Any chance this can work?

 

[dik]

My understanding of laterally loaded piles is that the top boundary conditions are nearly impossible to model and that this is key any angular or lateral displacement. If you can 'guaranty' the soil is consistent to the top, I have a *.pdf copy of an 'ancient' Reese and Matlock paper that you can use to calculate displacements.

Dik

 

[Den32]

What about doing an LPile analysis or similar? I believe it provides pile rotation/deflection based on the provided loads.

If you don't have this software, a local Geotech Engineer should have it and if you provide him with the boring logs and design forces, can likely give you the output.

How does running a grade beam between the caissons help with deflection? Worst case deflection would be in the opposite direction (out of plane from the sign).

What about doing a 2-pile cap with straight or battered piles? Connected with a grade beam between to brace weak-axis? Forces would resolve into tension/compression and a Geotech can provide anticipated pile movements

 

[msquared48]

dik:

I don't have a soil log yet, but will have to demand one after considering this. I seriously doubt that the soil is uniform, but I'll see. Thanks for the offer. I'll let you know the results.

Den32:

I'll see if I can get ahold of Lpile. FYI, the major load is wind transverse to the two steel poles. The .5" deflection at the top is based on the note in the old UBC's allowing doubling of the allowable lateral soil pressures if the deflection of 0.5" can be tolerated. That is based, apparently, on the reference Toad mentioned. It is not in the IBC to my knowledge.

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

 

[ToadJones]

Mike-
From IBC 2009
Section 1806.3.4 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"

Tabular values in this case being those in table 1806.2 of the 2009 Code "Presumptive Load-Bearing Values"

 

[ToadJones]

...the code, however, does not use the term "dude" as I have in this case.
dude = due

 

[msquared48]

Thanks. At least some things do not change.

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

 

[dcarr82775]

Lpile is the easiest way to go, but if not available the NAVFAC Design Manual (Sect 7.3 I think) has charts that allow you to calculate rotation at the surface of the pier given some soil, loading, and pier parameters. My experience is that the charts predict things reasonably well.

 

[dhengr]

Mike:
You said conc. filled drilled piers about 9 or 10' into the ground. With the soil conditions unknown and still suspect at the surface unless you do something to be sure that no body tampers with the soil around the piers. Maybe your grade beam btwn. piers to improve soil pressure area, or a slab on grade to confine soil at the tops of the piers. You need lateral reactions near the top of the piers to limit their lateral movement, or deflection about the soil reaction point about 6' below the surface. What would happen if you ran two screw or auger anchors or grouted anchors, at about 45°, down from the tops of the two piers, so you could post tension them after the piers were cast. Much as you might tie back temporary lagging and whalers for a deep found. excavation. This might eliminate some of the beam on elastic found. and soil properties uncertainties.