It has been raining in Iowa…a lot. It has been so consistently raining that it seemed appropriate to purchase a rain gauge for the back yard. Growing up in the Midwest, nothing is really more important than not only knowing it rained last night, but exactly how much it rained! My wife and I have been laughing about our childhood memories of rainfall measurement.
Our childhood memories are very similar. It rains. Our families and extended families gather for coffee or lunch the following day. Someone asks how much it rained last night. The answers typically sound something like this: “fifty hundredths,” or “twenty-five hundredths,” or “an inch and seventy-five hundredths.”
I must have been already channeling my inner engineer, because I always wondered why they didn’t simply say “a half-inch,” or “a quarter-inch,” or “an inch and three-quarters.” Yes, I was a confused child.
This is apparently not a widespread Midwest peculiarity, but it was common in both of our families in northeast Nebraska. Of course, growing up in Nebraska, measurable rainfall was not all that common, so I guess it was worthy news when it occurred.
So what is my point? I’m glad you asked. I’m always glad when you ask!
In today’s engineering environment, we have computational tools that allow almost limitless computational precision. We can design transmission lines dimensionally to hundredths or thousandths of a foot. In the same time it previously took to hand-calculate a structural load tree with two load cases, design software can now evaluate any number of different load cases in far less time than it took to write this paragraph. And we can have the answers in thousandths.
I hate to break this to you, but line design is really not rocket science. I suggest that using the previously mentioned precision is, well, silly. To support this claim, consider just a few examples of inherent variability in line design and construction that more than offsets our desire to slice the design bologna really thin.
You want the poles 310.25 ft. apart? How many contractors do you know who can do that? I know many good contractors, but they cannot hit that kind of precision. Can they set the pole exactly 12.5 ft. deep? It might be pretty close, but it could easily be 12.45 ft. deep; who really knows? Are the horizontal post insulators aligned exactly ninety degrees from the conductors on a tangent pole? Nope. Is the pole actually tangent, or is there a small angle?
I agree, we don’t always use a wood pole. Sometimes we use a steel pole with a drilled-pier reinforced concrete foundation. Is the foundation hole exactly eight feet in diameter for the entire depth? Is it actually a hole that is 27.5 feet deep? Is every truck of concrete exactly 4,000 psi? Is the concrete cover between reinforcement and the earth exactly three inches? Everywhere? Do the soil properties used for the foundation design exactly match the soil that was actually tested in a boring a half-mile up the road?
Does your pole design assume all vertical loads are applied directly through the vertical axis of the pole? Is the pole raked, or is it slightly crooked? Is it precambered? If it is guyed, does the pole stay exactly vertical after conductors and guys are installed?
Is your southern yellow pine pole designed for 8,000 psi ultimate fiber stress? Do you know how many southern pine poles have exactly 8.000 psi capacity? Probably none. That is why the overload capacity factors for wood poles are a relatively large number.
When the contractor sags the conductor, is the temperature exactly 70 degrees? Is the conductor actually at “initial” state? Is the conductor the same temperature as the ambient air temperature?
By now I hope you see my point. We can assume all the precision we want in design, but in reality the ability to actually achieve that precision is fleeting, at best. The good news is that thousands of miles of precisely designed lines are reliably in service in spite of being installed imprecisely.
This is my 2.007 cents-worth for today.