H-frame Transmission Pole

[mumuk]

Hello,

I’m doing calculation on H-frame transmission poles. The loading is according to:
- Guidelines for Electrical Transmission Line Structural Loading”, ASCE Manuals and Report on Engineering Practice No. 74, 1991.
- National Electrical Safety Code”, NESC,C2-1993.

The pole is of 12-¾, the arm is 8-5/8 and the brace is L150x150x12. All material is mild steel.

For normal loading, the stress is below allowable stress, but for unbalanced load the stress is more than allowable stress. However, the existing line of the same height and loading works just fine.

In the approach, I assume that the support is fixed. I know it is not correct since in reality the pole can move slightly.

The question:
a. What is wrong in my approach?
b. Will changing the point of fixity lower than ground surface reduce the stress?
c. How do I introduce the lateral earth pressure in my model (to change the modulus elasticity of the soil into spring support)

 

[SperlingPE]

Contact Hughes Brothers Inc, they have been designing and building H-frames transmission structures for years. There is a fairly standardized way of designing H-frames. I believe the method is detailed in an ASCE publication. The HBI engineers will also be able to answer your questions.

 

[MikeDB]

I don't follow the sizes of your members or know what "normal" and "unbalanced" loads you are refering to, but here is a stab at answering your questions.
a. There may not be anything wrong with your approach.
b. Lowering the point of fixity will increase pole stress above ground.
c. A program like Lpile or COM64 might help with the spring support.

Now then, I suggest you keep life simple, you are building an H-frame not a swiss watch. If the unbalanced load you are referring to is a longitudinal load, that may be what is causing the overstress as H-frame are strong in the transverse direction, but weak in the longitudinal direction (whcih can lead to cascading failures). The "existing line" may be doing quite well as the shield wires and phase conductors that are attached to it will provide some longitudinal strength. In real life, as the structure deflects from the unbalanced longitudinal load, the attached wires pick up more load and equilibrium is restored. If you don't mind this inter-dependency the attached cables can be modeled to yield results closer to what is really happening.

 

[Focht3]

While LPILE or COM624 would handle the soil-structure interaction quite well, in my experience the real issue for H-frames is uplift/compression on the legs. The lateral behavior isn't much of a concern, normally.



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"