Round Wood Pole - Combined Stress

[JM743405]

I am new to this forum so if I am violating any rules or protocol, please let me know.

I am working on a round wood pole (Distribution/Telephone Pole) and have been asked by the client to analyze it per the 2012 NDS. In section 3.9.2 there is guidance on how to analyze the combined stresses, although I am finding that the term that accounts for the moment magnification (1-fc/FcE) is coming out as zero since my FcE1 value is less than fc, and per Section 3.9.2, there is a limit which states that fc < FcE1. So in this case the second term of the equation results in an error since you can't divide by zero. Part of the problem is that the unbraced length (Le) is very large resulting in a very slender element.

What does this mean? Should this be considered a failure? Is there another equation I should be using for this situation? Any thoughts would be helpful. Thanks!

 

[KootK]

I would think that it means the following:

1) You've made a calculation error somewhere OR;

2) Your compressive stress actually exceeds the elastic Euler buckling stress.

If it's #2, you need to resize the pole.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[JM743405]

Koot,

I noticed that when I reduce the unbraced length, the calculations work out, so I don't think there is an issue with the calculations. I had a feeling it was the that we are exceeding the buckling stress, but wanted to make sure I wasn't missing something. Like they say, you don't know what you don't know.

Thanks!

-JM

 

[KootK]

You're most welcome JM. Frankly, I'm surprised that there would be so much axial load on a pole. Is there heavy equipment up there or does it just result from carrying iced up lines etc?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[JM743405]

It is actually guyed with 6 different guy cables which results in the high axial loads.

-JM

 

[26sig]

Can you use the guy cable locations as brace points to reduce your unbraced length?

 

[JM743405]

Sig,

Thanks for the thought, although I have already accounted for that. The pole is ~48' tall, and the largest unbraced length is 29'-6". The end goal for me is to modify the pole so that it passes the structural checks. I am thinking of adding additional guy cables to create more bracing points (aka reduce the unbraced length). That should solve the issues I am having with it. Thanks!

-JM

 

[dik]

Wood utility poles often have much higher allowable flexural stess levels... might start with the pole supplier.

Dik