TIA-222-G-2 Polygonal Monopole Design

[danismyname]

I am currently designing a monopole based on the TIA-222-G addendum 2 code. It's an 18-sided polygonal mono, and according to Table 4-8 of the code the effective yield stress can be higher (27% more) than the material's specified yield stress. This seems counter intuitive, and considering a circular design (in the same code) does not allow for this increase I don't feel comfortable allowing the increase in effective stress. Additionally, in the previous version of the TIA-222-G code, the effective stress for polygonal towers was at maximum equal to the material's yield strength. I have not been able to find a commentary or explanation of this change.

Is anyone able to provide some insight on this topic for me or link to a commentary?

 

[oldrunner]

If I have done my arithmetic correctly, using the 18 sided formula for w/t ratios multiplied by the square of Fy/E between 1.17 and 2.14, the maximum increase over Fy is about 1.06. Sometime in the past I remember reading about some increase in the allowable stress due to strain hardening in the bending process during fabrication. Note note 2 in Table 4-8 - where you wouldn't get any increase for a member greater than 18 sides.

 

[danismyname]

The values you used is for the original code without addendum 2. Addendum 2 changes the ratio lower limit to 0.759 with a corresponding equation F'y = 1.27 Fy.

 

[oldrunner]

I'm not quite sure what the ratio lower limit refers to. I only have Addendum 1 to work with at this time. In the new addendum, is the maximum w/t still 2.14(E/Fy)^.5? (Note 1).

What is your w/t ratio and what is the yield of your steel?

 

[ATSE]

Are you using the plastic section (Z) or the elastic section (S) in conjunction with your design effective yield?

 

[msquared48]

Are you using "Tnx Tower" for your design program?

If so, you could call the help line and ask the question. Peter is very helpful.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[JoshPlumSE]

Danismyname -

Yes, that definitely seems odd to me. I found some meeting minutes from a 2006 committee meeting which said the following:

Compact Polygonal Shapes and Effective Stress Equations, Table 4-8:

Compact polygonal shapes should be treated in a similar manner as compact circular shapes and be able to develop their full plastic moment capacity. This is supported by the test data used for the equations in table 4-8. The additional strength capacity was not used by ASCE 48 ias the strength levels in ASCE48 were intended to be limited to first yield to avoid permanent deformation under the NESC loadign conditions (50 year return extreme wind condition). this is not the case with the TIA-222-G standard (200 to 1000 year returns) as demonstrated for circular shapes. The ASCE 48 equations should be extended in the compact regions limited to a nominal strength equal to the yield strength times the plastic section modulus "Z".

That seems to indicate that this is really related to the efective yield stress for flexure (to be used in section 4.7.4). But, does it also apply to compressive strength? Section 4.5.4.1 of addendum 2 seems to be missing the reference to table 4-8 for polygonal tubes!

My belief is that this was done because you don't check compressive strength for monopoles.... only individual members in a lattice type pole. The idea with monopoles is that you rely on your 2nd order analysis to give you amplified moments (due to the presence of axial force) and then you check the monopole's flexural capacity.

 

[danismyname]

oldrunner:
The in the previous addendum, the ratio's lower limit was 1.17 with a corresponding F'y = Fy whereas in addendum 2, the lower limit and F'y equation is as referenced above. Fy = 65ksi, and the w/t ratio is approx. 19.5 with a calculated F'y = 78.5ksi.

ATSE:
The section modulus is not used in the development of the effective yield stress. I will be using the plastic modulus in subsequent calculations

Josh Plum:
That's an interesting discussion on the table although it may be a discussion for addendum 1 which was published in 2007, Addendum 2 was published 2010. It may be along the idea of the wind load application and the return periods, but it just seems odd that the effective stress would be increased rather than the return period lowered. But even with this discussion, I still don't understand how using Addendum 2 I can have a acceptable design while using Addendum 1 I can have an over-stressed design with the same material properties and loading. Compressive strength is minor compared to flexure ( <5% ) which applies to both addenda as does the 2nd order analysis.

My problem comes to this: either use addendum 1 values and not use an cost efficient tower, or use addendum 2 and have a design that "follows the code" but may be unsafe. This comes down to not knowing the details of how or why the revision was made and thus I can't judge which is better, although I tend to ere on the safe side.

 

[ATSE]

Dan,
Do not use the higher F'y with plastic section Z. Unconservative. See section 4.7.3.
Using the higher F'y with S gives you an equivalent plastic moment capacity. That is, instead of using Fy and Z (like conventional steel design), you use F'y and S. In the end, it's the same design strength (even if non-intuitive).

 

[JoshPlumSE]

I don't believe Table 4-8 wast changed with Addendum 1. Therefore, that discussion has to be related to Addendum 2. And, that discussion is totally related to flexure.

If you look at the Z/S ratio (plastic section modulus divided by elastic section modulus) for a thin pipe, that is approximately equal to 1.30.

Section 4.7.3 (flexure of polygonal tubular members) has the following:
Mn = F'y * S.

Which would result in a much lower flexural capacity than the equivalent round pipe (which is based on Z, not S). Therefore, it seems pretty clear to me that this is what the Addendum 2 changes to table 4-8 were trying to address.

To me, these changes should not be used with the axial compressive strength of the monopole (section 4.8.2) where Pn = F'y*A (note that the axial strength of the tubular pole structure is NOT based on section 4.5.4). In section 4.8.2, I would instead use the F'y from the original G standard. Or, maybe I would use the value from addendum 2 divided by 1.27 so that the axial capacity never exceeds Fy*A.