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[TowerTalk] Tower Falling Radius (long)

 To: [TowerTalk] Tower Falling Radius (long) sawyers@inav.net (Steve Sawyers n0yvy) Fri, 04 Apr 1997 22:26:08 -0600
 ```What we are sort of talking around is a Failure Mode and Effects Analysis. (This is also called the systematic application of Murphy's Law.) This is where you look at every part and do a "what if" analysis. Things like what if this breaks, or this is left out, or this particular form of overload is applied. Generally you have to look for the "weakest link in the chain" to see what breaks first, after it breaks, what happens next and so on. Different "what ifs" give different effects. This gets especially hard under complex loading situations like long flexible members in compression, supported laterally with less than rigid supports. (Fancy definition for what a tall tower looks like to a structual engineer). I agree that tall towers (600 feet or greater) generally fail due to initial buckling of one or more legs. Primary causes are someone working on the tower and damaging or improperly removing a lateral, vertical ice load, or unbalanced ice load which moves the effective center of the compressive force away from the center of the tower. Granted, sometimes high wind will complicate the ice loading and sometimes a straight wind will overload a tower or tornadic wind will twist a tower and start the collapse. But in most of the cases I have seen or read about, the lateral wind was less than half the design wind. Something else was the major contributor to the initial failure that brought the tower down. As a general case for all towers, I cannot say that they will all fail in a localized pile unless they are designed and built to do so. The question is what is the Failure Mode. If the Failure Mode is compressive collapse of the tower sections, then the falling radius is small. If the Failure Mode is a guy wire failure, then two things can happen. On taller towers, the tower can start to move lateraly until it either stabilizes, or some localized tower member and/or one or more legs reach their limit stress which initiates the vertical collapse. If the tower is shorter and/or more rigid, there is a possiblity that a guyed tower will fall straight out. Tall towers (600 feet an above) have multiple guy levels and are really rather flexible. The towers sections are usually handling their maximum compressive load. I agree with Rod W5HVV when he said: "Isn't it entirely feasible (and even probable) that the tower would then essentially collapse straight or nearly straight down? Why would the tower, given that form of failure parameter, "lean over" and crash some distance away when there were all those upper level guys to keep it from doing so until after the momentum was essentially straight down?" There is so little force to start the tower moving laterally as compared to the huge inertia of the tower and even the guy wires, that it is not probable to have tall towers fall straight out. On shorter and relatively stiffer towers, say Rohn 25 at 70 feet, the situation is different. It is loaded at about 25% of the maximum compressive section capabilities. (See Note 1 below). In this case if you had a failure of the guys from one side, the tower would have enough inherent strength to maintain its shape until it hit the ground. In this case, straight out. I have seen it happen where a tree took out a set guys and the tower fell over straight as a string. In this case the tower was stronger than the guys for the applied load. (Yes, I know that you shouldn't have to design for trees falling on your guylines - or should you? Most towers that I know of will take more wind or ice load than most trees that I know of.) Now that we have outlined the primary Failure Modes, what are the effects? That depends on what it falls on, and how much damage it does when it falls. In the case of the failed Rohn 25, it fell across a couple of back yards onto a standard 4 foot high chain link fence. It did not do that much damage as the tower buckled over the fence. In this case the weight per unit length of the objects coming down was less than a 4" diameter tree limb. On the other hand,if this had been a section of Rohn 65 and it had landed on a house, then it would have done lots of damage. In my case, I am planning 70 foot of Rohn 25 next to my daughter's bed room, my preferred failure mode on this tower is to fall straight out rather than collapse straight down into her bedroom. The guys will get designed to meet the wind and ice load for the installation, but the tower and its appertances will by sized to assure that the weak link is the guying, and not the column buckling of tower sections. For the 300 lbs. per section 114 foot tower that is going in 60 feet away, I want that one to fail straight down, no matter what the loading conditions. Thus I am going with double 5/16" top guys and 3/8" second and third level guys and huge concrete guy anchors. The column buckling will be the weak link on that one. For the 150 lbs per section 80 foot tower that is 100 feet from the house, I will go with a pretty much standard approach where I will balance the guy strength and the tower column strength to get the maximum wind load up. IMHO a Failure Modes and Effects Analysis is very neccessary. It lets me sleep a lot better. Sorry this was so long. I hope it was educational and gives you some other insights into tower installations. de n0yvy steve Steven H. Sawyers PE ARRL Volunteer Consulting Engineer Note 1: The tower base reaction from the Rohn literature for a 70 tower is only 3010 lbs in a 90 mph wind zone with no ice. In this case the bottom tower section is not loaded anywhere near it's 12,000 lb maximum compressive stress (this theoritical load is only applicable if the load is exactly centered on a section and there are no external bending moments). I support of the assertsion that 3010 is a small tower section load, I woul dlike to point out that Rohn does show a 190 foot tower having a load on the bottom section of 9870 lbs in the same wind conditions with 31 foot guy spacing. Dwg Ref C870488 R1 from my latest Rohn Consumer Products Catalog. -- FAQ on WWW: http://www.contesting.com/towertalkfaq.html Submissions: towertalk@contesting.com Administrative requests: towertalk-REQUEST@contesting.com Problems: owner-towertalk@contesting.com ```
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