Foundation of a Tower - Moment in two directions - Overturning control 2

[GeoNik]

Hello everyone,

Consider a spread foundation, supporting a tower used for powerlines. The foundation is rectangular, and I have two moments acting simultaneously parallel to X and Y directions. (i.e. the resultant moment is diagonal, not parallel to the breadth or length of foundation).

I have two questions:

- How can I check if there is risk of overtunring, without computer modeling? Is there any handy equation (similar to what we use for single moment case), showing that the vertical load locates inside the central 1/3 zone of foundation?

- Do you use or know any specific Code for designing foundation of powerline towers?

Thanks in advance for your time and attention.

Nik

 

[Ron]

To do without simultaneous orthogonal analysis, check each condition independently, take the worst and size your foundation for that, then backcheck to make sure it covers the other conditions.

You will need to check the x,y and diagonal conditions, independently. If your foundation is sufficiently ridgid so that bending is neglible, the analysis is fairly straightforward. If bending is there, it's a little more complicated, but not greatly so.

 

[Ron]

..pushed button too quickly!

Don't forget to consider the unbalanced line condition and also the consideration that overturning usually doesn't occur inline with the lines.

 

[oldrunner]

On page 117 of the Handbook of Concrete Enginering there is a chart of values for forces parallel to the diagonal of a square footing which is handy. It will provide you with the bearing pressures and the area being loaded if there is uplift.

 

[GeoNik]

Hi Ron and thank you so mich for your reply.

Would you please explain more what did you mean by "the unbalanced line condition"?

 

[transmissiontowers]

I design the towers but not the foundations, and I hear the term FAD thrown around the coffee bar. On most normal towers the foundation load is mostly uplift or compression with a relatively small shear load for each of the 4 legs. Transmission poles, OTOH, are mostly moment with small axial and shear. In either case we use drilled piers and not square foundations because our loads are pretty high and the soil around here is not great.

_____________________________________
I have been called "A storehouse of worthless information" many times.

 

[DaveAtkins]

I use the chart for biaxial moment on a footing in "Foundation Design" by Teng.

DaveAtkins

 

[civilperson]

(Weight / Area) +/- (Moment about X-axis)(Y Distance to point)/(Moment of Inertia about X-axis) +/- (Moment about Y-axis)(X Distance to Point)/(Moment of Inertia about Y-axis)= Pressure. Two positives give greatest pressure and two negatives give greatest uplift. Check the corners to see maximums/minimums. The diamond shape kern in the the center one sixth of the rectangular foundation is the location of the "e" (M/W) for no uplift. Stability is similar to one way moments.

 

[Ron]

An unbalanced line condition occurs when you have irregular tower spacing or the next successive towers in the line in each direction are different distances apart.

 

[DaveAtkins]

civilperson,

I agree with your approach, except with a footing, there cannot be any uplift. If outside the kern limit, then
P/A +/- Mc/I +/- Mc/I does not apply anymore.

DaveAtkins

 

[Ron]

As the kern limit is approached, P/A increases. If the bearing capacity of the soil is not reached, true "overturning" occurs; however, there's a point where P/A will always exceed the bearing capacity, thus vertical deflection into the soil will occur just before overturning occurs.

 

[oldrunner]

Is that really true? You mean that if I have a allowable bearing value of 3000 psf, that as soon as the tower's footings corner reaches this bearing value, that the soil will sink?

What happened to my factor of safety (of the allowble bearing capacity)?

A little tongue-in-cheek tonight.

 

[miecz]

I have this wonderful system of formulas for calculating bearing pressure for this condition. The system was given to me (hand calcs) by an engineer (deceased) from Poland, and it matches results from AASHTO 4.4.7.1.1.1C. I'm looking for a published reference, which I believe is German or Polish. The system delineates five zones under the spread footing, and provides a bearing pressure formula depending on which zone the resultant lies.

 

[Ron]

oldrunner...I was talking about the "real" bearing capacity, not the one we use! It won't happen at your 3000 psf example but will happen at some point (probably just past that FS we use!!)