Transmission towers, can you shed lights?

I only check this forum occasionally so here goes. Theoretically, the equations in PLS-Tower are based on a uniform cross section over the entire member length. If you have local corrosion, I guess you could conservatively input the corroded section properties as if the entire member had reduced properties. Once upon a time I did a spreadsheet to calculate the angle properties given the 2 leg lengths and the thickness based on the equations in the Red AISC manual(forgot the year)or the old Blue manual.

I was modeling a tower that had a few metric angles and I had to create the properties in English units. The properties are not that tough to program and put them in a row so you can just paste them into the XXXX.ANG table. You just need to come up with a name for the corroded angle so you can pick it.

The PLS-Tower program will calculate the allowable loads based on the smaller area but remember that this will probably under predict the actual allowables. It just depends how conservative you want to be. The reality is that we assume we know the loads on the structure to 5 decimal places because the program calculates it. The same with the wire tension. If we are off by quite a bit on the loads, then the results will be off. The compression equations and tension equations were developed based on testing many years ago and they tend to be conservative. When you pile up all the conservative assumptions and run the tower, you get a sense that you are in trouble (and you may be) but you might want to do a full scale test to reinforce the assumptions made on the loads.

The problem is the wind on the structure. In my part of the world, the hurricanes can blow up to 140 mph and unless you put a full size tower in a wind tunnel and rotate it so you get an oblique wind to find the maximum leg loads, you cannot determine the correct wind on the tower.

Sorry for the long answer to a short question, but we tend to rely on computers to give us the answer to our problems without understanding what goes into all the assumptions. Have you considered the wind drag on a lattice tower and the difference between the bolt heads in the wind stream and the nut and threads in the wind? Are coarse threads worse than fine threads? Something to ponder.

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I have been called "A storehouse of worthless information" many times.

So assume you want to check the capacity of an exisiting tower for whatever reason. You go out to the field and you find out these steel angles in dark brown color. You could certainly grind the surface to near white and use a caliper to measure thickness as long as this portion of the tower is reachable.
My question is what if the upper sections of the tower looks corroded but not that serious, what would do? Commission the technician to climb tower and measure? When you model the vintage tower, how would you factor this in? I mean the steel angles look dark red but you can't estimate the level of corrosion?

A climbing inspection, followed by cleaning and painting would be my recommendation. If you want a qualitative answer, you would alter the member properties to account for the rusting and run an analysis on the reduced sections. If the tower analysis shows the %used as less than 100% for your maximum load case, you could let the tower stay and come back and measure it in a few years based on the corrosion rate. If the analysis shows the tower is over 100%, then it should be replaced.

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I have been called "A storehouse of worthless information" many times.