Laterally Loaded Pile Criteria 5

[scofie]

For some reason, I cannot seem to get a definitive answer to this issue, so I am petitioning the good readers here to help.

For a laterally loaded pile, what is the maximum lateral displacement limit (in inches) and what Factor of Safety should one apply to that limit? Specifically, I am concerned with pile foundations for monopoles.

It kills me to see all this reference material that solely focuses on determining the applied moment along the pile based on p-y curves, etc. when typically the flexural resistance of the pile is inconsequential because it is so large. Yet these same references say things like one must also limit the deflection of the top of the pile but give zero values for what this limit should be? Also, shouldn't there be a limit on the amount of lateral pressure the pile applies to the various soil layers?

I find many engineers that design these foundations will use LPile or whatever software,determine the maximum pile moment, design the reinforced concrete per ACI 318, but then inexplicably they will either totally ignore the soil or they will use a UBC equation to determine the required length of pile that, in my opinion, is totally not applicable to most piles of this type (i.e. large diameter, long length).

Please, somebody restore sanity here!

 

[btangzh]

Based on a field survey of some bridges in Canada by Bulzuk et al., I have undertaken a pilot study on the top displacement control of piles in industrial mills and given the criteria as follows:
a)when Sv<50mm and Sh<25mm , the displacements almost have no harm to upper framework, in which Sv denotes the settlement of pile foundation and Sh is the lateral top displacement of pile foundation.
b)When 50<Sv<100mm or25mm< Sh<50mm,the movements of pile foundations will bring some damage to the superstructure;
c)If Sv>100mm, the settlement difference should be paid much attention, while if Sh>50mm ,the second stress in the superstructure caused by the lateral displacement of pile foundations will be serious.Both of them may induce the distress of the superstructure.
I realized that the criteria should attach much importance to the type of superstructure. For instance, the control criteria for bridge, industrial mills and monopole should be different because each type of structure possesses its own tolerance capacity for second stress or displacement of its foundation.
Therefore, The criteria above is just a coarse result. Any comments will be welcome.

 

[QSE]

Hi Scofie,

The answer depends on the intended use of the monopole.

The &quot;acceptable&quot; deflection limit under severe loads on most transmission line pole is about 10% of its height (Suspension typically). I have seen specs with 2.0 deg rotation at ground level for these loads.

Poles used in telecommunication are a different story, particularly those that carry Microwave antennas. Specs for &quot;acceptable&quot; movement of these antennas are typically 0.5 degree tilt. This means very little movement to the pole foundation. Here lies the &quot;problem&quot;. So much precision using sophicated computer for structural analysis/deflection and yet so &quot;unsure&quot; with regard to soil mechanics and foundation rotation/deflection.

Lpile does show deflection at ground level.

 

[btangzh]

Using the order of 10% of the pole height as the acceptable displacement, I think, has some limitation, especially for longer monopole. As known, for general multi-story residential building, the permissible inclination is merely about 6¡ë according to people¡¯s psychological tolerance capacity.

In QSE¡¯s posting, the 2.0 or 0.5 degree rotation or tilt, may be not the criteria for the top movement of the pile foundation but the tilt limitation for the monopole.
Generally, the measured degree takes the top of pile foundation as the benchmark, that is, disregarding the absolute movement at the top of pile foundation.

If there are errors in the comments, any correction will be appreciated.

 

[QSE]

http://www.civiltech.com/

Here is a new software for pile under lateral loads (allpile). I downloaded the demo and found the input and output great. I am not trying to push this product but there so few good ones out there on this topic and I welcome the competition to LPile.

The downloaded demo have several examples and please try example 12. It is a 36 inch diameter pile in dense sand loaded with 200 kips at ground level.

The computed deflection at ground level is 0.85 inch. Comments please, deflection seems so small for 200 kips.

 

[dgb]

1. Codes I know best are Indian Standards. Indian code on the subject says &quot;The safe lateral load on a pile is that corresponding to a deflection of 5 mm at ground level&quot;. The reason for this spec is not given by them.
2. The allowable deflection can be different for diff structures as already pointed out by the earlier replies. My own experience shows that for piles with a long unsupported length, such as in jetties or bridges, a deflection of the top of pile up to 80 to 150 mm is often tolerated. However, where the structure supports sensitive topside features such as cryogenic pipelines, etc. limits set are more stringent, for example 25 mm. Even there bellows, etc. can be provided to build in a greater tolerance.
So it seems that a joint effort between the topside designer & the pile designer could be made to allow for a practically achievable max deflection.
3. Bowles, in his chapter on subgrade modulus, gives the maxm hori/vert soil movement allowable to be 25.4 mm i.e. 1 inch. again without giving reasons.
It is indeed a gray area as scofie says - one only has his own judgement as a guide.
dgb

 

[scofie]

Thanks for the comments.

So I conclude that there is no particular lateral deflection limit required in all cases.

However, I still am concerned with the issue of lateral pressure on the soil layers. For the design of a deadman anchor for a guy wire, one considers the lateral pressure applied to the soil as a passive pressure condition and this pressure is limited with some factor of safety on the soils bearing pressure capacity, correct? Then why is this not also checked for piles? Or maybe it is and I just have not found this in the literature?

 

[QSE]

Hi Scofie,

I beg to differ with your conclusion. Allowable deflection of piles at ground level is related to whatever they are supporting

If the piles are supporting
1-Buildings, Telecommunication poles, and pipelines (small allowable deflection)
2-Transmission line poles (larger deflection can be tolerated)

Say can you comment on example 12 from &quot;Allpile&quot;
(200 kips and 0.85 inch deflection etc)

 

[dgb]

Scofie,
(First, I clarify that I don't have real p-y type lateral behaviour software with me in India. Here we tend to use lower-end software like STAAD which gives only linear springs in the available version.)
If you are using a program that uses p-y type load-deformation characteristics of soil, then the curves will flatten as the deformation increases and a limit similar to &quot;passive&quot; pressure will be imposed automatically. It may not be the same limit though. This I guess is because the &quot;yield stress&quot; of p-y curves comes earlier than the passive pressure value due to a good depth of soil overburden usually present. In cases where you have a long unsupported length (in air/water) suddenly followed by a hard/very dense stratum, you should be more careful.
[While using STAAD, I manually remove springs and replace them with a calculated passive force, where the spring reaction exceeds it. Painful, but that's (my) life.]
In any case, I think Bowles (who also uses linear springs)allows a deflection of 25 mm in his derivation of spring constants.
-- dgb

 

[scofie]

dgb, thanks for the comment. I guess I am finally seeing the big picture. The key is that one must analyze the pile with ultimate load conditions rather than service load conditions. I would guess that many engineers performing these calculations miss that key point and run the lateral load analysis at service load conditions. They get the applied moment on the pile due to service loads from LPile or AllPile and factor it up by, say, 1.6, to get the ultimate moment, then proceed with their r/f concrete design.. which would be the wrong approach, correct?

So am I right that one should run the lateral load analysis using a p-y curve based program (such as COM624) and ultimate load conditions and see if the solution converges. If it does converge, this means that the p-y curves have not &quot;flat lined&quot; and the soil layers can resist the ultimate loads?

If my statements are correct, then we have a load factor as a margin of safety, but what about a soil resistance factor? It seems that we have not reduced the soil properties (i.e. a phi factor on material strength)?

 

[btangzh]

Except the superstructures impose a control on the displacement at the top of the piles, the type of piles with different torlerance against cracking also have requests on the top displacement. Generally, the RC piles have a lateral displacement limit of 1/4~1/2&quot;, while steel piles will withstand a larger value.

 

[scofie]

btangzh, your post highlights my very fustration. You make the statement &quot;Generally, the RC piles have a lateral displacement limit of 1/4 to 1/2 inch, while steel piles will withstand a larger value.&quot; Why is this and how do you know? A reinforced concrete element is designed for strength based on concrete compressive strain limit and rebar yield and rupture strain limit. There is a deflection of a given member at its ultimate strength, but that is not a constant 1/4 to 1/2 inch. So why is this the number you state? I would guess it is because you have read somewhere what I have read, which is a general, vague stipulation of this lateral deflection limit for piles, with zero rational, explained reason.

I understand that certain superstructures have certain sesitivities to deformations of the foundation, but why is it that this industry has not quantified these values in a much more detailed and rationale basis. This is the year 2003 for pete's sake! The allowable deflections of steel, concrete, timber and masonry structures are all clearly defined in their respective codes and standards.

Geotechnical engineering is too vague, yet again...

 

[geonerd]

It's not that geotechnical engineering is too vague...it's just cutting edge.

 

[Focht3]

I am troubled by your statement,

Geotechnical engineering is too vague, yet again...

Please remember that soils were made by God, not man. We must deal with them as we find them. It is definitely not &quot;cookbook engineering.&quot;

Regarding the use of p-y curves for design:


My first comment is that this should generally be left to:

a. A geotechnical engineer with a thorough understanding of p-y curve design and structural engineering, or

b. A structural engineer with a thorough understanding of p-y curve design and geotechnical engineering.

Lateral load analyses are neither simple nor direct. The choice of undrained shear strength, e₅₀, initial secant modulus - are all important and require experience and judgement. geonerd is right - p-y curve analysis is on the cutting edge of geotechnical engineering. P-y curve design is performance based - you are modelling expected performance, not calculating some &quot;allowable&quot; value. LRFD does not apply here! It requires a different mind set. In the p-y scheme of things, partial factors of safety only serve to screw things up royally!

Having said all that, I will give you a few comments and pointers.

1. The values of p_u are essentially ultimate bearing capacity values.

2. It is not unusual to have a local factor of safety in lateral &quot;bearing capacity&quot; of less than 2. (This occurs at the ground surface.)

3. Rotation, not groundline deflection, really controls the design of most drilled piers for supporting monopole communications towers (using 0.5 degree and 0.5 inch limits) - if the soil parameters were chosen well.

4.btangzh's 'rules of thumb' for allowable groundline deflection of RC piles are not design limits - they are based on his experience. You should use the calculated moments to determine when and where the concrete will crack.

5. Run multiple analyses - as a minimum at the service load level, ultimate load level and twice the ultimate load level (or factored ultimate load level if it is greater.) If you get numerical instability at the highest load level, add embedment until you get stable - and reasonable - behavior.

 

[scofie]

Focht3, thanks for the reply, comments and suggestions.

With all of the pile foundations in service throughout the world, don't you find it a bit troubling that this basic pile lateral load analysis stuff is cutting edge?

I can tell you that lack of a clear method of analysis with well defined limit states criteria is activly producing one certain result ==> Confusion in the industry.

I am no rocket scientist I confess, but I venture to say that if I cannot decifer this mess, many many other engineers cannot. And yes, I mean both structural and geotechnical engineers. While I can appreciate the need for developed expertise in this area, maybe you will conceed that there are too many areas to be an expert in all and there are too many pile foundations to be designed than can be handled by the one or two dozen experts in this field.

BTW, that is why being able to post on this forum is a very helpful thing to many engineers. Advice from experts such as yourself help cut down the breadth of the problem at hand.

 

[Focht3]

scofie:

You are correct: there are too many pile foundations being built around the world to be handled by a few dozen experts. (Thanks for including me in that select few - even if I don't belong.) But there are hundreds, perhaps even thousands, of geotechnical and structural engineers around the world with the requisite education and experience. You just have to look for them. Where are you located? Perhaps I can point you to someone nearby that can help you.

I have had the good fortune to be personally acquainted with most of the real experts in this area - Lymon Reese was the second reader on my Master's report. In fact, my report dealt with laterally loaded group piles and used data collected by Shell Oil Company for a group test in Harvey, Louisiana. (Thank you, Shell!) I can tell you that the development of p-y curves was neither simple nor obvious; and it took three decades - thanks to the financial sponsorship of the major oil companies - to develop a useful set of p-y curve definitions. And curve development continues. This method was made possible by the development of equipment capable of automatically measuring and logging dozens of strain gages simultaneously - a capability that was a direct result of America's space program. It took the space race of the 50's and 60's - literally - to make this design technique possible. So don't be too harsh in your criticism of the &quot;cutting edge&quot; nature of this design technique.

Does the industry confusion bother me? Sure, it does! It really disturbs me to know that engineers are using LPILE (or COM624) to do pile design using inappropriate input and assumptions. That is one of the reasons that I respond to message threads like yours - I really do want to be of help in clarifying key issues. But I do not have the time (or patience) to serve as a tutor. Piling has been used for centuries; what is different now is that the p-y technique provides a rational, performance based pile design method. It's more &quot;efficient.&quot;

The major confusion has absolutely nothing to do with the design technique itself. It has to do with the design limits i.e. how much groundline deflection and/or foundation rotation is considered acceptable. The electric transmission industry (through EPRI) has made some strides in developing design guidelines. No such group exists for the telecommunications business. I am not surprised that confusion exists in this environment (I have encountered it, too.) Blame it on companies that do not support the participation of their engineers in professional societies, and allow - even encourage - non-engineers to make engineering decisions.

I urge you to engage in continuing education on this subject - especially if you do much laterally loaded pile design. Try to attend a course at the University of Texas at Austin. Austin is a beautiful, friendly place - and UT is where the expertise in p-y curves is concentrated.

Cheers!

 

[doresh]

Hello,

As a foundation consultant for over 150 sites of telecommunication poles I can say that deflection is matter of influence on the performance of the antenna. The antena designer can and must give you the amount of deflection and or movement allowed in order to still operate within theire rollerance.
According to TIA/EIA Standard (Structural Standard for steel Antenna Towers and Antenna Supporting Structures, the radio frequancy signal level degradation shall be within 10dB.
That need to give allowable deflection that should be received from the radio designer.

 

[Focht3]

doresh:

I've done quite a few towers (monopole, self-support, guyed) myself, as well as wind turbines in San Gorgonio Pass (monopole in decomposed & fresh granite - near Palm Springs, California) and electric transmission towers (direct embeds, H-frame, lattice towers). But my experience (and yours, too) and the TIA/EIA Standard have nothing to do with a simple geometry problem.

As I said before, groundline deflection means almost nothing:

Assume a monopole tower (with the antenna at the very top) is supported by a drilled pier with an embedment of 20 feet. Assuming the embedment is sufficient, the shaft will typically rotate about a point below the ground surface that is roughly 2/3 of the pier embedment depth; in this case, about 13 feet. A 2 inch groundline deflection (1.5 inch due to rotation and 0.5 inch due to horizontal translation) is only 16 percent of the total horizontal displacement at the top of a 100 foot monopole tower that has rotated 0.5 degree and translated 0.5 inch horizontally. In other words, 84 percent of the deflection at the level of the antenna - compared to the deflection at the groundline - is due to foundation/tower rotation, not foundation translation or groundline deflection. And 96 percent of the total deflection at the antenna level is due to the rotational effect - not horizontal deflection.

Make sense?

 

[Focht3]

doresh:

I've done quite a few towers (monopole, self-support, guyed) myself, as well as wind turbines in San Gorgonio Pass (monopole in decomposed & fresh granite - near Palm Springs, California) and electric transmission towers (direct embeds, H-frame, lattice towers). But my experience (and yours, too) and the TIA/EIA Standard have nothing to do with a simple geometry problem.

As I said before, groundline deflection means almost nothing:

Assume a monopole tower (with the antenna at the very top) is supported by a drilled pier with an embedment of 20 feet. Assuming the embedment is sufficient, the shaft will typically rotate about a point below the ground surface that is roughly 2/3 of the pier embedment depth; in this case, about 13 feet. A 2 inch groundline deflection (1.5 inch due to rotation and 0.5 inch due to horizontal translation) is only 16 percent of the total horizontal displacement at the top of a 100 foot monopole tower that has rotated 0.5 degree and translated 0.5 inch horizontally. In other words, 84 percent of the deflection at the level of the antenna - compared to the deflection at the groundline - is due to foundation/tower rotation, not foundation translation or groundline deflection. And 96 percent of the total deflection at the antenna level is due to the rotational effect - not horizontal translation.

Make sense?