Structural Analysis 1

[YanSain]

Hello
I am fresh student engineer, and I was given a mast lightning post to calculate. Can someone help me?
The post has different diameter from the top to the bottom. The arm is fringed to the top of the mast and also the diameter of the arm is different from the two ends. How can I conceive the solution to that exercise??
thanks in advance for your answer

 

[driftLimiter]

First step is basically a required for any structural analysis problem. Draw a free body diagram, indicate the geometry of the structure, the loading, and the section/material properties.

Next step, define what values are needed for the solution.

Perhaps peak shear, axial, bending stresses, peak displacement, Foundation Reactions. Are you trying to design the pole and foundations? Define what provisions (codes) govern.

Now using the knowledge you have obtained, and resources you have access to you start chipping away at each of these. shear, bending, axial, and reactions should be pretty easy for this structure.

For the displacement it is a bit trickier to solve but certainly impossible if you don't properly go through the steps I outlined.

My suggestion to you is to make a good free body diagram of your structure including loads. Do your best to solve it out and post it back here for additional feedback.

 

[BridgeSmith]

For tapered poles (and arms), there's the incremental approach (breaking the pole up into short sections, each with a different moment of inertia, area, loading, etc.), and then there's the calculus approach. I've only done the incremental approach, using Excel; thought of solving a triple integral to get the deflection makes my brain hurt.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Agent666]

This may be of some use if you're going down the route of needing an equation to describe the change in moment of inertia that you can then integrate as BridgeSmith noted.

https://engineervsheep.com/2019/moment-of-inertia-1/

However, I'd expect if you divided the section into several vertical sections, and take the properties as being constant/averaged over that length you will get a good estimate. You should test how accurate your solution might be by halving the length of your sections and see if it converges on a solution, or see if the relative differences get within some margin you're happy with, otherwise keep dividing the length.

Your wind code should have some equations to work out the wind drag on the element.

Also if you make too slender and have high stresses then fatigue should be a consideration you look at. For these types of structures, this can often be the governing load case and it is one people miss assessing at all most often.

Also, don't forget the best place to start may be to sit down with someone else at work who is familiar with these types of structures and pick their brain, they should be able to mentor you through it and provide sufficient guidance.

https://engineervsheep.com

 

[BridgeSmith]

Also if you make too slender and have high stresses then fatigue should be a consideration you look at. For these types of structures, this can often be the governing load case and it is one people miss assessing at all most often.

Yep, the design for pretty much all of our poles is governed by fatigue at the welded connections, despite using a 120 mph design wind speed for the strength design. All the failures of high mast light towers we've seen were due to cracking at the base plate to pole welds. Oddly enough, it was from harmonic oscillation perpendicular to the wind direction, a phenomenon known as galloping.

Rod Smith, P.E., The artist formerly known as HotRod10