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## Re: [TowerTalk] FW: Sources for mast material

 To: , Re: [TowerTalk] FW: Sources for mast material "Michael Fox \(K5MEF\)" Thu, 7 Aug 2008 12:54:07 -0700 mailto:towertalk@contesting.com>
 ``` > 1) What is the meaning of the "wind speed" parameter. The dialog box says "maximum wind speed expected". In my county, basic wind speed is 70 mph fastest mile, 85 mph 3-second gust. So I figured the max instantaneous is probably higher. So I figured 90 or 100 would be a good number. But it really depends on what the meaning of this number is in the MARC tool (fastest mile, 3-second gust, other). Can you define? We used the formulas and specs from Dave Leeson's (W6NL, ex-W6QHS) book "Physical Design of Yagi Antennas." Maybe there's more of an explanation there. The over-riding spec is your county windspeed rating from the TIA-222 windspeed chart (available from http://www.championradio.com/windspeed.php). There are several ways of arriving at a figure. I don't have a copy of the above book. Perhaps someone who has a copy knows the answer to this. The windspeed shown on championradio.com for my county is the fastest mile windspeed (70 mph fastest mile). But the current TIA-222 spec now uses the 3-second gust speed (85 mph 3sg). So, as mentioned above, the question is, to which "wind speed" does the WinMARC application refer: fastest mile, 3-second gust, or "maximum" (as the dialog box says)? Without knowing that, the number to enter for "wind speed" is unknown. 70 (fastest mile)? 85 (3 second gust)? 90 (more likely "maximum")? And, if the correct wind speed is unknown, then the resulting tensile strength calculation is meaningless. > 2) Also, when the calculated total is 12,290 in-lb, the lightest suggested mast type shown in the list is a 40,000 in-lb yield strength. It's seems like it's only displaying mast types that are at least 3X higher than the calculated total. Maybe you could shed some light on that too? No, that's because tubing has that as a minimum rating. As opposed to pipe which doesn't; it's around 35 kpsi and we don't include pipe as a generally suitable mast material. But the behavior of the WinMARC application does not agree with that description. For example, if I reduce the wind speed such that the calculated tensile strength is only 6151 in-lb, then the list of possible mast types also changes, with the lightest version being 25,000 yield strength "6063-T6 Aluminum pipe" - that's a 4X difference. A53 and A36 "Structural steel pipe" are also listed as suitable mast material. So, again, the application seems to be listing recommended mast types that are 3-4X higher in strength than the calculated load and the answers do include both tubing and pipe. The simple question is "why is it picking those types, rather than something closer to the calculated value"? If your design yield strength is 13 kpsi, then anything will work. That's why I suggested Schedule 40 or 80 water pipe or conduit. You're over-thinking this whole thing. Yes, this situation is relatively simple. And thanks for the straight and simple answer on the sched. 40. That helps a lot. But the problem is that the application is giving answers which differ significantly from what you're saying here. So I'm trying to understand why - and not getting very far. This isn't the only antenna mast I need to figure out. So I'm simply trying to understand the proper inputs to, and the meaning of the outputs from, the application so I can use it for other configurations. Thanks for all the helpful info. 73, Michael - K5MEF _______________________________________________ _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk ```
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