TnxTower Program Anomaly

[msquared48]

I originally posted this in the Transmission Tower forum but received no responses. I will red flag the Transmission forum post.

I ran a 59 foot, 12 sided monopole with a circular stiffened baseplate, and 18 anchor bolts with the stiffeners between every other bolt. The tower rating was 102.2 % with the baseplate design controlling the rating. This is the actual existing tower condition.

I ran the same thing again with a plane baseplate of the same thickness and no stiffeners, and obtained a tower rating of 61.3%. This seems inconsistent... However...

Could the program be looking at the effect of a 12 sided tower with only 9 stiffeners and the induced stress on the tower walls since some of the stiffeners are between the tower bend points in the 12 sided figure rather than at the bend?

I ran the program again with the number of stiffeners doubled, and the rating dropped to 51.1%, half that with half the number of stiffeners, and less than the unstiffened plate.

This makes no logical sense to me. What is driving the solution?

FYI: TnxTower was formerly RISA Tower, but is now a separate company.

Mike McCann, PE, SE (WA)

 

[msquared48]

Well, I think that I have found my answer on page 77 of this publication.

http://www.towernx.com/downloads/Technical_Manual_MP_BasePL.pdf

Where it states that the gusset is designed ignoring any contribution of the base plate. The gusset acts as a vertical plate only, and the gusset imparting a bending stress on the pole wall.

Looks like the analysis is very conservative for the ease of analysis to me. A finite element approach would be better here...

Mike McCann, PE, SE (WA)

 

[JoshPlumSE]

Monopole bases are tricky. It's been a few year since I've looked in detail at them. But, I would have pointed you towards that same reference. Or, towards the TowerNumerics tech support e-mail address.

I've thought for awhile that Tower Numerics should develop a specific program (similar to RISABase) targeted mainly towards FEM solutions for monopole base plates. It would be very useful for the tower industry (and vertical vessel design for heavy industrial projects). It is a bit of a specialty application, and Peter does have a lot on his plate. But, you should send him that request anyway. Those sorts of requests have a lot more weight coming from real engineers than they have coming from another FEM jockey.

 

[msquared48]

FEM is exactly what I suggested to him yesterday. Great minds think alike.

Mike McCann, PE, SE (WA)