Tower Foundation Design 2

[GeoGrouting]

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

 

[Ron]

The easiest approach to a monopole foundation is to use a drilled shaft. Conventional design, vertical load not really very high, but moment at the top of the shaft can be significant, depending on wind load.

 

[fattdad]

Once you know the overturning momenet, designing a gravity mass to resist overturning is no big deal. You can make a concrete footing and put a few feet of backfill over the top of it too.

Nothing wrong with a drilled shaft, however.

f-d

¡papá gordo ain’t no madre flaca!

 

[JedClampett]

I've linked to this before, but I still think it's a great tool. FDOT has a MathCad program to design tower light poles, from top to foundation. The anchor bolt design is a little hinky, but there's nothing in there that says you can't take out the portions you need:

http://www.dot.state.fl.us/structures/proglib.shtm

 

[GeoGrouting]

The foundation has to be a pier foundation.

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.

 

[apsix]

Undrained cohesion or phi, depending on your soil. I use these with Broms theory.

Bearing capacity at the toe is unlikely to be helpful.

 

[FixedEarth]

There are few approaches to the geotechnical capacity of piers. One method is to use the passive resistance of the soils. This ranges from 150 to about 400 psf/ft.

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.

 

[GeoGrouting]

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.

 

[FixedEarth]

I think first question for apsix.

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

 

[fabianjm]

Pier, bored or driven pile you have to take care with Uplift and lateral load for tower so Uplift analytically computed based on Meyerhof's Formula also for lateral load and vertical load. You can try Brom's also for lateral load

 

[fabianjm]

Meyerhof's etc formula for computing the Ultimate Bearing Capacity (UBC) of Pile based on Standart Penetration Test (SPT)data will be giving various value of UBC also I was tried to compare with PDA and O'Cell result so which is better to use?

 

[GeoGrouting]

The pencil example is fine but for a pier of 4 m long it seems the length is not long enough for a bored pier to act as a pencil between the fingers. Is there a critical length (or indeed embedment length/Dia ratio)that beyond which the pier is analogous to the pencil.

 

[fattdad]

I just want to clear up the passive pressure "calculation" presented by "fixedearth."

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!

 

[apsix]

A 25m monopole with a 4m deep bored pier is going to require a large diameter pier.
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.

 

[GeoGrouting]

Re the last message that bored pier is not a good solution, could you give me an alternative suggestion? Thank you.

 

[paddingtongreen]

I disagree with apsix, passive pressure is the normal means of taking out the moment in cases like this.

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.

http://www.cti.uconn.edu/chwrp/pdfs/jhr73-71_73-1.pdf

Along the way, I found this list of free "books" on the subject.

http://www.pdfgeni.com/book/pole-foundation-design-pdf.html

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[oldrunner]

First: You do need a geotechnical report. For estimating purposes you can use the generic formula in the IBC foundation chapter, substituting a trial depth until it closely matches the calculated depth. For this trial example you need the passive pressure provided by the geotech. He also has to tell you where the passive pressure starts below the ground surface. It could be that the passive pressure starts at 2 meters or more depending upon the site.

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.

 

[apsix]

paddingtongreen

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.

 

[Bluefoot]

Tower foundation may be designed by conventional methods. Considerations should be given to cyclic loading conditions. Bryne and Houlsby (2003) in their paper "Foundations for offshore wind turbins" give a simpflied design method to monopole foundations. This may be helpful to model your tower foundation.

 

[oldrunner]

Chapter 13 Foundation Design of the Highway Signs, Luminaries and Traffic Signals (AASHTO LTS-4)provides guidance for poles. In the US, monopole design is per the ANSI/TIA-222-G (or later) but it does not provide actual formulas similar to the AASHTO recomendations, but does provide some guidance in Section 9. Older editions of the EIA/TIA code did have some formulas that one could utilize.

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.