Tower Foundation Design
Tower Foundation Design
(OP)
I am considering a monopole tower foundation design. The height is about 25 m. I would appreciate a couple of design references/manuals/texts that I can use for design. I am familiar with NAVFAC and wonder if NAVFAC has any chapter on tower foundation design. Thanks





RE: Tower Foundation Design
RE: Tower Foundation Design
Nothing wrong with a drilled shaft, however.
f-d
¡papá gordo ain't no madre flaca!
RE: Tower Foundation Design
RE: Tower Foundation Design
My question is more about the geotechnical parameters that the structural engineer need to design the foundation. Apart from bearing capacity at the toe of the pier. Thanks.
RE: Tower Foundation Design
Bearing capacity at the toe is unlikely to be helpful.
RE: Tower Foundation Design
Another method is what apsix suggested, combine side friction with end bearing capacity.
My favorite method is the one proposed by Teng in 1969. It takes into account the 3 loadings (lateral, axial and moment) + SPT or Cohesion, depending on the soil condition. Then allowance can be made for ignoring the resistance of the upper Y feet for instance.
RE: Tower Foundation Design
Why do you believe 'Bearing capacity at the toe is unlikely to be helpful'?
The pier in mind is 4m deep in till materials (SPT of 45+) and seems that all load may be transferred to the end of the pier.
Also could you elaborate on the issue of the passive resistance pf the soil? Is it due to the lateral forces, which mobilizes the skin friction? In that case only half of the perimeter of the pier is under passive pressure.
RE: Tower Foundation Design
Think of the pier as a pencil. You can hold the "pencil" between your two hands, and even if your hand was resting on a table, it is the side support that will do most of the pencil holding.
Brom had a method, Teng had another approach and I think brinch Hansen yet had another methof of analysing piers and lateral loads.
The passive resistance is Kp x Y x D, where Kp is the passive earth pressure coefficient, Y is the soil unit weight and D is the embedded portion of the pier. So if we draw the Free Body Diagram with a lateral load coming from the left side, then we draw a triangle on the right side. The base of the triangle is then equal to 3x Dia x Kp x y x D according to Teng's method of piers in Sand.
Looks like you are very interested in this topic. So try to purchase both texts below for more info:
"Design of Foundations for Buildings" by Johnson & Kavanagh, 1968 and
"Foundation Design" Teng, 1962
RE: Tower Foundation Design
RE: Tower Foundation Design
RE: Tower Foundation Design
RE: Tower Foundation Design
Given Kp, the "potential" passive pressure is determined by Kp*Gamma*A*Cp (citing Brinch-Hansen), where
Kp=tan^2(45+phi/2)
Gamma=unit weight (you get to determine whether to use moist or bouyant)
A=surface area of the embedded pier
Cp= arching coefficient, typically taken as phi/10
Problem with this "formula" is it represents the maximum load that the pier can provide to the soil without shear failure. In the free body diagram, you may not get a chance to mobilize all this shear strength as you have to have equilibrium in the x direction as well as moment equilibrium.
I agree that equilibrium in the Z (or Y) direction is likely moot.
f-d
¡papá gordo ain't no madre flaca!
RE: Tower Foundation Design
It will probably act more like a spread footing than a pier, but passive resistance could also be utilised.
In that case bearing capacity will be important.
You may find that bored piers aren't the best soluton here.
RE: Tower Foundation Design
RE: Tower Foundation Design
A rule of thumb for utility poles is that 1/7th of the length should be in the ground. On that basis, you would need a pier 25m/7 = 3.6m. However, they accept that they may have to rework a few poles here and there so you probably need a little more. There are several programs that give you the design requirement for the pier, I can't tell you which because they came along after I escaped from that business.
I couldn't find the paper I wanted, but this one is close to what we used to do.
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Along the way, I found this list of free "books" on the subject.
http://ww
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Tower Foundation Design
The diameter of your pier may be a function of the placement of the reinforcing steel cage, the anchor bolts location plus the concrete clear clearance outside of the rebar. Be careful of trying to get anchor bolt templates inside of the cage. You might be looking at a 1.5m diameter footing.
Haven't done any monopole in a few years but probably designed a few hundred in the past.
RE: Tower Foundation Design
Actually I also agree that passive pressure is the normal means of taking out the moment in cases like this.
My comments are based on a limiting depth of 4m.
I could be wrong but I thought that it was a little shallow for a 25m tower.
RE: Tower Foundation Design
RE: Tower Foundation Design
4m appears to be shallow for your project. Maybe just set the depth to 4m and widen the footing until the preliminary formula works using the passive allowables. Then check the bearing pressure with overturning ignoring the passive pressure. Might end up just a big glob of concrete.
RE: Tower Foundation Design
I fully agree, that shouldn't be forgotten when dealing with wind, seawaves, earthquakes and man-made vibrations.
RE: Tower Foundation Design
RE: Tower Foundation Design
RE: Tower Foundation Design
The attached paper "Design of Laterlly Loaded Piles and Caissons in a Layered Soil", by Naik and Peyrot may be of some help. They have a pretty good example problem using their method for a drilled caisson foundation for a large utility tower.
RE: Tower Foundation Design
I also have a question for bluefoot and Mccoy:
How do you consider cyclic loading in the design? Do you reduce the soil strength parameters (how) or something else?
RE: Tower Foundation Design
This can cause a larger diameter pile to have a lower (theoretical) lateral capacity than a smaller diameter one.
RE: Tower Foundation Design
a) The discarding of 1.5 m from the top, is it exactly a Brom's requirement? Thought it was allowance for depth of drilling which may be 0.6 to 1 m usually.
b) Also have seen amny references from Brom. One likes to read his own article but do not know where to find it.
c) How do you evaluate Das version of the Brom's theory.
RE: Tower Foundation Design
You may act on the parameters by reducing them, in 2 ways taht I know
1) rigorous 6 costly= dynamic lab tests smulating the design frequency, so that you'd have a phi_dynamic, c_dynamic, Su_dynamic....
2)Literature correlations = not always satisfactory
Or you might use a winkler-type model.
This way, you wuold consider a static horizontal (also vertical) reaction and a dynamic reaction.
A classical source for dynamic reaction is Gazetas, 1991
RE: Tower Foundation Design
There are two different things: ignoring the upper 1 m or 1.5 m or ignoring 1.5 x diameters (as apsix mentioned). Ignoring the upper 1m or 1.5 m is typically coming from issues such as soil disturbance due to frost-thaw action or any other phenomenon that you may have to reduce the soil resistance in the upper portion. On the other hand, I have not encountered anywhere the requirement for ignoring the 1.5 x diam
Mccoy, thanks for the info. I personally think for regular tower (e.g. tall light pole, telecommunication, etc), reducing the parameters from literature is good enough. For a more complicate tower load (e.g., wind turbine), the dynamic lab test is the way to go. However, I am not familiar with the winkler-type model or I may have studied it long time ago and completely forgotten about it. I would check it out.
RE: Tower Foundation Design
The paper I uploaded discusses Brom's original paper, as well as others, then extends Brom's work to include multi-layered soils. It also lists many references you could follow up on if you wished.
RE: Tower Foundation Design
Somehow I can not locate the file you have uploaded. How can I access to it?
RE: Tower Foundation Design
The advice re 1.5xdiameter is for Cohesive soils only and is stated in the California DoT FALSEWORK MANUAL (amongst others):
"LATERAL LOADING IN COHESIVE SOILS
The ultimate soil resistance for piles in cohesive soils
increases to some maximum value at approximately 3 pile
diameters below the ground surface then remains fairly constant at greater depth. Literature suggests using a soil
distribution of zero between ground surface and a depth of 1.5 times the pile diameter (1.5d) and then using a value of 9 times the undrained shear strength for the remainder of
the pile depth."
I understand that it's from Broms' original paper on the subject.
I don't know Das's version.
RE: Tower Foundation Design
If you click on the link in my first post, it should take you to another page, click on that link and it will download the paper in .pdf format. I just tried it and it does work.
RE: Tower Foundation Design
the text from California DoT (as you quoted) does not appear referring to Brom's method. I might be missing something in here.
RE: Tower Foundation Design
RE: Tower Foundation Design
From another source (AS2159-1978):
"the value of Nc in equation ... is generally taken as 9, but where the ratio of the depth to the diameter of the base is less than about 4, progressive reduction in the value of Nc to 5.6 at the surface will be necessary."
Do you have any basis for your assertion?
RE: Tower Foundation Design
You and I may be talking about the same issue. Allow me to elaborate and see if we are at the same page or not. This dialogue started when someone earlier in this thread mentioned in using Brom's method, we have to ignore the upper 1.5 x dia of pile. My point is when someone is using Brom's method, there is no need to "ignore" the upper portion of the pile unless there are other reasons for soil to lose the strength (e.g., frost/thaw, erosion, etc). The loss of lateral resistance in the upper portion of the pile is already included in the original Brom's method.
Here is the way I am looking at it: Brom's Method which was originally presented in graphical form provides Ultimate Lateral Resistance of the piles based on several variables such as e/L, Su, etc. One of the most important variables in his graphs is L/d (embedment length/diameter). In his graphs, if the L/d is low, the associated factor for Ultimate Lateral Resistance would be reduced extremely. For example, for a pile with e=0, at L/d=8, the factor is about 20 and for a L/d of 4, it is 5 and at L/d of 2, it is practically zero. As you can see, any lost associated with the low lateral resistance in the upper portion is already included in the graphs and if we use Brom's method/graphs and also decide to ignore the lateral resistance of the upper portion of the pile, we are deducting it twice (Brom already done that in his graphs). All we need to do is to get the associate L/d ratio (e.g. L/d=4) and other variables (e.g., e/L, etc) and use the graphs and obtain the Ultimate Resistance factors (everything is already calculated even low resistance in the upper portion).
Put all of that in perspective, when I design a pile/caisson subject to lateral load, first I use Brom's method/graphs to obtain ultimate lateral resistance and then if the pile/caisson is subject to freeze/thaw cycle, I ignore the resistance for the upper portion of the pile which is subject to frost action. This may not make any considerable difference at all for a caisson or pile whit diameter of 750 mm (18") or larger. However, if we are talking about a pile with diameter of 300 mm (12"), the way I approach it would make a noticeable difference for short piles as there are reduction in the lateral resistance in the upper 2Xdia. of the pile (already incorporated in the Brom's method) and then in addition to that, I ignore the lateral resistance for the upper portion of the pile (say 1.2m depth-frost penetration depth).
I hope this could clarify my opinion. I am not correct, I greatly appreciate everyone input.
RE: Tower Foundation Design
We do agree, we're just approaching it from a different perspective.
If using Broms' graphs the ignored 1.5 x dia is already included. Your approach is correct.
I've developed a spreadsheet using Broms' theory; one of the inputs is depth of pile to be ignored, which I usually set to 1.5d.
Sorry about any confusion.
RE: Tower Foundation Design
Thanks for the reply. I am happy that the issue is cleared. As I noted before, generally, I directly use the graphs provided by Brom; however, your spreadsheet idea is appealing also. Particularly, if you need to ignore more than 1.5 X dia. for the reasons I mentioned before (e.g. frost depth). The spreadsheet sound easier to work with for this situation
RE: Tower Foundation Design
The software is easy to learn/use, but generally will require a geotechnical report done unless you want to make some assumptions.
Or if you have STAAD you can use STAAD foundation. I'm not so familiar with that program.
Cheers!