Annular Baseplate analysis

[Antnyt23]

I was looking at an example of a monopole tower analysis that is openly available on google at:

http://www.usedtowers.com/MONOPOLES/117 IDAHO/9911256-133 calcs.pdf

I am trying to better understand where all values come from however am having some trouble understanding how the one item is calculated.

on page 9 the plate moment is calculated as 273.6 in-kip....

I thought I had read somewhere that the actual plate moment of an annular plate could be calculated as the plate being a cantilever beam. So I thought it would be the (Bolt circle-OD of the pole/2)*tension in the bolt

the tension force in the bolt is 184k((30.6875in-25.5in)/2)=477.25kip-in

I have a feeling i'm completely misunderstanding something and was hoping for some clarification on the correct way to calculate this

 

[JStephen]

In the calculation, it says "Base Plate Calculation (according to TIA-222-G)". That would be the place to start looking.

I can think of several different ways to approach this, and as it appears the design method is somewhat codified, the only way to duplicate somebody else's answer would be to find exactly what that codified method is.

The moment you're calculating would be the moment about a line of bending tangent to the pole, and implies a corresponding width at the bend line. Perhaps the method in question is calculating the moment about a point nearer or at the center, or the moment about a radial line.

Note that the plastic section modulus would be 1"xWx0.5"x2 = W, which implies a 13.35" wide section, and it is not immediately obvious where the width is 13.35" given those numbers.

 

[Antnyt23]

I believe TIA just provides certain design factors not much actually information on analysis.

 

[JoshPlumSE]

The folks over at Tower Numerics who wrote the TnxTower program (formerly RISATower, formerly ERI Tower) have a good paper on their website related to just about all things base plate for monopoles.

Design of Monopole Bases, by Dan Horn

It's the most complete treatment of this subject that I have seen. And, I'm not just saying that because I'm with RISA. If it doesn't have the information you want, then the references section will probably show you what articles you want to obtain.

 

[JedClampett]

Diameter of the pole is 25.5 inches. pi * 25.5= 80.1 inches. There are six bolts, so 80.1/6=13.35 inches.
As far as the moment, it looks like it's calculated from the inside face of the hole. So (30.6875-2.25-25.5)/2=1.47 inches. 184 kips times 1.47 inches equals 270.5 in kips.

 

[Antnyt23]

Thanks Josh... I have looked at the Dan Horn paper and agree it is very helpful. I do use TNX Tower and RISA I was thinking about contacting Peter and seeing if he has some hand calcs for what exactly is used in the TNX baseplate calculation. Wasn't sure if it incorporates the full paper into the program or what.

Jed,

I follow you so the 13.35 is similarly known as the beff calculation correct? My mistake I was thinking beff=(base plate diameter)*pi/(# of bolts) but it looks like it is pole diameter in lieu of the base plate diameter.

Similarly the equation I'm familiar with to calculate Mmax = Pten*((Bolt Circle Diameter - Pole Diameter)/2), where they used the Mmax = Pten*((bolt circle - bolt diameter - pole diameter)/2).

My understanding would be that after they had calculated their Mpl of 273.6in-kip = Mmax they would us it in the equation fact = 6Mmax/(beff*((plate thick)^2)) which would yield = 30.374ksi then fallow = fy*0.75 = 37.5ksi.

But this is just for the tension side correct it is also supposed to be calculated for the compression side.