The base moment applied to a foundation from a telecom self-supporting tower (lattice) 1

[gmoney731]

Hello,

I found a sample "tower material take-off" sheet from TNX Tower - a structural analysis software geared towards telecom towers.

One thing I am always confused about is the base moment - see attached PDF.

https://files.engineering.com/getfi...78b-48eb-bd2b-639aee275f0d&file=Tower_MTO.pdf

How do they get this value? Is it downward/uplift force multiplied by a moment arm? Or is it something else?

I am using a similar software, and it gives me the down and up reactions per leg, but it doesn't give me the base moment, and I don't know an easy way to calculate it.

 

[azcats]

That's the total overturning moment at the base of the tower. I would think they use the moment to calculate the leg forces - not the other way around.

If that's a 3 sided tower with face width of 9', the moment arm would be around 7.8' for T or C at one corner leg. That would get you ~164 kips of T/C not considering dead load which seems like it's pretty close. (1281492/7.8 + 10523/3 = 167.8 kips is really close to the max reported force of 167.9 kips)

tnx looks at these from lots of different directions though, so sometimes it's difficult to reconcile individual reactions.

Trying to work backwards into a moment from leg reactions would be possible, depending what reactions you actually have.

 

[gmoney731]

Thank you very much, azcats - your explanation makes a lot of sense and sounds right.

My issue was that I was using the face width of the tower for the moment arm, not the perpendicular bisector length from one corner to the center of the face width (the 7.8' value that you mentioned).

 

[rb1957]

if it's a triangle base, wouldn't you orient to moment (the wing direction) so that one leg is on the NA and only two legs carry the load ?

I guess you can show this is critical (or not) by applying a moment through the centroid of the triangular base, assume plane sections remain plane (load is proportional to distance from NA), and clock the moment through several directions. Assuming an equilateral triangle, I'd point the moment vector through the middle of one side and clock +-30 degrees, and calc the reactions at the three attachments.

another day in paradise, or is paradise one day closer ?