THANKS TO WB5VZL FOR PROVIDING THIS INFORMATION TO THE KA9FOX WEB SITE! ----------------------------------------------------------------------- From n4zr@netcom.com Sat Sep 3 20:26:23 1994 Date: Sat, 3 Sep 1994 18:26:12 -0700 (PDT) From: "Peter G. Smith" Subject: Pier pin vs embedded base Here's the summary - it's even bugger than I thought -- 25 K! Following is a summary of comments received on the CQ- Contest Internet reflector and on the East Coast Megacluster on the topic of rigid versus pier-pin basing for Rohn and other similar towers. Thanks to N3RD, KM9P, NG0X, K9MA, K9FN, WA6SDM, N6VI/KH6, K0KR, K7GM, N3RS, W2UP, K5ZD/1, VE3CDX, K8LX, KC7V, K1KP, K4XU, Blckhole@AOL.COM, N3JT, K1HTV, NY3M, KE3Q, K3SKE, K1DG, W0UN and K1GW. In what follows, while I have tried to retain the full flavor of the originals, I have deleted liberally, chopped, merged and summarized, so I apologize in advance if anyone feels I have mangled his input unduly. In this corner ... supporters of the rigid base approach: ... we had a Rohn 25 tower with the first section embedded in concrete. This tower was taken down and replaced with Rohn 55. We ...decided to go with the pier pin and Rohn baseplate, as you describe. ... The first tower section is not rigid, and can't be climbed on safely without temporary guys. Since our first real guy went in at 35 feet, we had to progressively move a set of temporary rope guys up the tower. No big deal, just a pain in the neck and a lot of time spent. The final 130' Rohn 55 tower does not have the "rock solid" feel that we anticipated. We attribute that to the fact that the tower is not rigidly anchored at the bottom into concrete. We have every reason to expect that this new tower will satisfactorily support the antennas we plan for it, but the base plate and pin approach was a new one to us, too. (see also N3RS's comments on thee same installation, below) - N3RD ------- If I am breaking welds on the tower because of wind, I have more to worry about than the tower. I've lost all the antennas, proably a lot of trees have fallen on the house, tower, and everything else and I am trying to chainsaw my way back to civilization! - KM9P ------ Remember that Rohn 25 can be used up to 180 feet. Perhaps at that height it's really important to minimize stress on the base. On the other hand, at heights under 100 feet or so, there's only a fraction of the load on the base, which may explain why so many hams get away with embedding the first section. By the way, Rohn sells a 3 foot section, which is probably better to embed in the concrete than the bottom of a 10 foot section. - K9MA ------ I've seen very few 25/45 installations of any size that DON'T use embedded tower sections as the base. I guess we all think we're smarter than the factory engineers! I have six 90' Rohn 45 towers, all with embedded sections for bases. All have been up 16 to 18 years with pretty big antennas on them (Telrex 8/8 on 15, 2-el. 75m, etc.) with no structural failures. There is noticeable twist in the wind, the little torque bars don't prevent that. All this is NOT to say ignore the engineer's advice; they're probably right. Just wanted to pass along what I've seen. - N6VI/KH6 ------ I have only seen the embedded section routine for Rohn (and that is all I have ever used). I think at "ultimate wind load" than whether your tower flexes a bit at the bottom is the least of your concerns. The embedded section way is easy, gives better grounding connection with the ground, and is more likely to give you a straight tower than any other method. In addition, if you ever move, the short section you cut off can be used at the next QTH as the base. - K7GM ------ I have a Rohn 45 with tower embedded in concrete. With a guyed tower, there really is minimal lateral force at the base. It is all a downward force. Technically, I think it would be easier to true up the tower section. What I did was put 2 tower sections together and then put it in the hole. Then with temp. guys you make it vertical. With only 5 feet sticking out it is not easy. With the base plate/pin, you probably will be off vertical, but you can pull it straight with guy wires. In theory, I can't fault either base support. I think Rohn is pushing it a little with their explanation... - W2UP ------ ... in small installations it's much easier to plant the bottom in concrete, true it and stack from there. If your intended installation is so close to the design limits that flexing of the tower member components is even a remote possibility, you need more guys or a bigger tower. - K4XU ------ I like the pin idea, though getting the first sections up could be dicey. Also, if you ever have to use the tower in a self supporting configuration temporarily, such as when lifting an antenna into place, you may find the old system more convenient!!! - N3JT ------ A section in concrete provides you a sturdy starting point and you don't need temporary guys. It also gives a "rock solid" feel to the tower at and near ground level which is reassuring. The pier pin gives a disconcerting feel of movement, even when you've added some sections and guy wires, though that movement may in fact add to the mechanical integrity and longevity of the tower. - KE3Q ------ For most everyday tower construction, a short concrete section sunk in 3 1/2 to 4 feet of concrete is going to work just fine. It makes construction of the rest of the tower a lot easier as well, since you have a firm base to attach additional tower sections. With a pier pin, now you need to FIRMLY support and raise enough tower on the first lift to attach and secure the first level of guy wires. This is the part most people don't want to fool around with. You really can't build the tower up from a safety point of view. To use temporary rope guys is asking for problems and risk injury to someone. - K3SKE ------ The biggest problem in such an installation is getting the first 20 feet of tower installed. You need help - can't be done alone, compared to the anchored first section method. - K1DG And now, the pier pin camp is heard from: Before moving west, I lived in Minnesota, and we got the occasional "wrath of God" style T-storm. Any guyed towers that I saw in a crumpled heap seemed to have been killed by excess torque. From that standpoint, the pin makes sense, since the tower can rotate on the pin rather than build up torsional stress. - NG0X ------ In the broadcast biz (my field), I have seen lots of guyed towers...we have four of them here and all have single point bases that can pivot. By allowing the tower to swivel a bit in extreme condx, you allow the force to be shared by all the guys, rather than being concentrated at the point the base would otherwise enter the concrete. Our towers are Pi-Rod...a company that uses solid steel for the legs, rather than hollow tube...(I have one of their freestanding towers at 100') and they also recommend against securing the base in concrete, even though their style of construction might be able to take it. By the way, another strong reason to not bury a section in the concrete is that it is then much more difficult to control moisture in a hollow-leg tower. I have seen them rusted through so badly you would not dare climb 20' up. - K9FN ------ As I recall from the statics classes I took, the free pivot base is a solvable problem, the fixed base problem is not. At least all the problems we worked involved a pinned base. I believe the Rohn speak is "We can't figure the loads when the base is fixed in concrete, so don't do it." - WA6SDM ------ Unlike that of the pier pin, alignment of the half section is a wee bit critical and is nontrivial to accomplish. Plus, if you flub the alignment, you have a problem that can be solved only by demolition and beginning again from scratch. Imagine the expense, the effort, and the frustration. Avoid the risk. Use the pier-pin base. Some fellows try to correct a misalignment of the half-section base by using differential guy-cable tension on higher sections. Bad move. Yes, there is some play in the leg joints that can be taken up to correct for slight misalignment of a half-section base. Yes, really careful use of a carpenter's level will get one into the ballpark on alignment. But, why take the chance that the level itself is misaligned, and so on. Actually, I believe the majority of fellows use the half-section base. I believe that they do so for one or more of four bad reasons: (1) they do not know any better, (2) they are following the crowd, (3) they think that the half section makes it easier to get the tower started, given that, initially, no guys are available to stabilize the bottom several sections of the tower, and (4) they consider the half section to be generally more stable. As to reason (3), I use a set of temporary guys, just enough to stabilize the lower sections until enough of them are stacked to reach the level at which the first set of permanent guys is attached. This is not much work, and certainly represents less work and risk than is involved in "truing up" a half-section base. As to reason (4), many fellows fail to comprehend that the greatly predominant vector of force on the bottom section is straight down, with essentially no shear force being present. There is, in fact, so much down force that, with a big tower, the base would probably not move even if the pier pin wasn't there! K0KR ------ I am using the pier pin approach on a brand new Rohn 55 130' tower. It doesn't seem to be as stiff as a concrete base mounted tower, but it is surely a good way to put up the tower. It has only been up a short time, so I cannot say much about it beyond it is pretty strong. [A prominent contester with large towers/antennas] also has towers erected in this fashion and swears by them. - N3RS ------ I am using pier pin bases here on 2 towers (and used them at my W3 QTH). In addition to all the good stuff Rohn told you, they offer one big advantage to the ham on the move. They don't cost much, and when you get ready to move you only leave a little stub of pin sticking out of a buried block of concrete! Buy a new pin for the new QTH and you are back in business. The only down side is that it makes getting the first 30 feet of tower a little harder to put up. And after all, how often do you put up the first 30 feeet of your tower?! - K5ZD/3 ------ I have pin bases on all my towers and even my tv tower (commercial tv) has one and its 1005ft with an 8ft face and 5 1/2inch solid legs. If you have a choice go with the pin. - VE3CDX ------ Single point mounting is used on virtually all large commercial guyed towers for the reasons Rohn stated. However the Rohn implementation (flat plate on concrete) negates much of the theoretical advantage. I have been at the base of such an installation during high winds and observed the top of the tower (130' of 45 with 2 large beams on top) being twisted 10-15 degrees, while the base was still. - K8LX ------ I have used the pier pin method over the last 9 years with Rohn 45 and Rohn 55 and have never experienced a problem. We do get severe winds and thunderstorms here during the summer and the towers have withstood all. Currently I have 2 100 foot Rohn 55 towers up with beams spaced up and down each tower. The same goes for a 100 ft. Rohn 45 tower. Each uses the pier pin method. You just have to make sure the base is flat and the pier pin is vertical when embedded in concrete. It sure makes life easier when putting the towers up. - KC7V ------ I have the pier pin set up on my 70 foot Rohn 45 tower. No problems. The pin didn't end up exactly vertical, and the top surface of the base didn't end up exactly level. Still, no problems. - K1KP ------ I'm in the process right now with a 90' 45G. The pier pin method keeps the tower from twisting. Its sorta weird putting up the first couple of sections, though ........ - ? (I goofed) ------ I was thinking, if this twisting issue is real important, the guys with large rotating tower setups are in trouble, because it is common, in gusty wind condx, for the top and and bottom antennas to get swinging out of phase at times, thereby maxing out the twisting torque. Doesn't matter how the base is attached in that situation; and with your guy wires attached to slip rings, they offer no twist resistance. - K8LX ------ I went the route of the pier pin, with a homebrew base. Make sure that the base surface is perfectly flat and level. At first I was apprehensive about thinking of what might happen if a set of guys went. But if you do it right and use a good anchoring system and guy hardware your fears will be reduced. Its been about 4 years now, and the tower is really solid as a rock. - K1HTV ------ I was involved in erecting two 100 foot towers in one month. The first was put up with the usual section in concrete. I can tell you that to stand at the top of that tower and look down is a scary proposition. It twists and turns, because the owner was unable to have all the guys perfectly positioned at their anchoring points, and also because of a slight asymmetry in the alignment at the base. A small error at the base of a tower that size leads to a considerable misalignment at the top. I predict that a considerable torque is generated on that thing every time the wind blows. For that reason, I chose to put up my own 100 foot Rohn 45 following Rohn's suggested pier pin. As soon as we attached the lower set of guys, the bottom portion of the tower pivoted into precise alignment. It went up slick as a whistle. I asked a friend to bring a surveyor's instrument over later to check out how straight the tower was. The thing never varied more than 3/4" from bottom to top. When I look up it, or down, I have confidence that all forces are appropriately balanced. The pier pin tower is solid as a rock. I never have detected a shift in it after the first time, when it was equilibrating the forces from the guy wires. I should tell you that I have it guyed with three sets of 1/4" guys, with the Rohn torque arm. I don't know whether that made a difference, or not. The other tower didn't shift when I was on it, either, it was simply crooked, which would cause me to worry when the wind blows, were it mine. Hams in my club say that the main reason to avoid the pier pin is that the tower might fall over complete with a guy failure. That is, a 100 footer could damage objects 100 feet away. A tower in concrete, they say, will break in the middle, giving it a smller radius of potential damage. I don't know about this. The only tower I have any knowledge of going down was imbedded in concrete, and it broke at its base, falling on a house. - NY3M ------ The local tower guru likes pier pin installations, since they allow the tower to twist on the pin rather than torque itself apart. Seems like a reasonable, common sense idea. A section in concrete provides you a sturdy starting point and you don't need temporary guys. It also gives a "rock solid" feel to the tower at and near ground level which is reassuring. The pier pin gives a disconcerting feel of movement, even when you've added some sections and guy wires, though that movement may in fact add to the mechanical integrity and longevity of the tower. - KE3Q ------ I have had my tower installed with the base pin method for years. It has been no problem. Installation was quick. The pin alignment (with vertical) is not very critical. I put the first few sections together (2 or 3, can't remember) and tilted them up, guyed them and then built the rest. - K1GW ------ And finally, an agnostic (I think) with an interesting and relevant tale to tell: Most of my towers are "embedded" basing. I have done things both ways over the years on the professional jobs I have done. Embedded is cheap and easy or quick and dirty or whatever cliche you want to use, but can be problematic if the first section isn't done perfectly and if the guys aren't brought up to good (10% tensile) tension. Pier pins allow you to not worry about the base as much but it is always a hassle with the temporary guying. I guess, in summary, that the pier pin is somewhat better, since it allows the guys to do all of the work, both for horizontal loading (wind) and torque loading, since the tower doesn't have any inate strength in either the horizontal (ie it will fall down if unguyed) or the torsional (it just twists around the pin). If embedded and the guys are loose both the horizontal force and the torsional force are answered (in part) by the tower structure itself, so it is doing more work than just responding to the vertical forces due to the tower's weight and to the vertical component created by the guys due both to their tension and to the wind loading adding more vertical loading. It is just that the tower is pretty good in its own right and if the guys are properly tensioned and if torque stabilizers are used the tower can handle the loads when embedded. But if the tower's first section is not true then it must be pulled to vertical by the guys and there are stresses that can be unevenly distributed on the welds and structure. The Rohn pier pin mount for Rohn 45 and 25 isn't all that great, since the three legs come down to a flat plate with the pier pin in the center. If the concrete isn't exactly flat and level, one or more of the legs may set on the plate with no concrete underneath it. Then the vertical load is trying to shift the legs with respect to each other and also puts forces differentially on the legs causing stress on the welds. The only fool proof pier pin approach is with the tapered base section, where all three legs taper into a central point. So pier pins are theoretically a little bit better, but the Rohn 25/45/55 pier pin plates aren't really the right answer if the concrete isn't perfect. In that case you just as well use the embedded section and get it level. I have two levels mounted on a piece of aluminum angle that clamps around a leg and gives me an indication in both planes at the same time. Then I "shoot" the tower with a transit in both planes to confirm that it is truly vertical all of the way up. Maybe what I said at Dayton was my story about having a guy slip while installing Rohn 55. I had one of my own installations where I was using a concrete base of 36 by 36 by 48 deep on an embedded style installation. I had 40 ft of tower up and was tensioning the 5/16 (11,000 lbs tensile) to about 1,100 pounds when one of the Klein grips popped off. Instantly I had a vertical tower with only two guys each with 1,100 pounds tension. The tower oscillated enough for a second or two resulting in the concrete enlarging the hole in the dirt enough to put the better part of an inch gap between the concrete and the earth around it. The concrete remained in place so that the first tower section was still vertical but the two guys were pulling the 40ft up point about 2 ft off vertical resulting in a severely bowed tower. I released the tension on the other guys and inspected the tower very closely and saw no apparent damage. I reattached the guys, tamped the soil down around the base to eliminate the air space and proceeded to re-tension the guys. As I got to 1,100 pounds again the same Klein grip popped loose and I went through the same procedure and the inspection revealed no damage, which I credit to the quality of the Rohn production and to the design of the tower and the fact that all of the forces trying to break welds, etc were distributed over 40 ft of tower. Bottom line is that I have a healthy respect for Rohn tower, I threw away the (surplus) Klein grip and bought new ones, and the tower has been up at 200 ft for about 8 years, even though some jerk shot out a guy wire at the 160 ft level before I had installed the 200 ft guy wires, so once again the tower was being pulled with a pair of 1,100 pound guys 40 ft above the last solid guy point. But this time there was an additional 40 ft of tower above the point where the two remaining guys were trying to pull the tower over. Again, there was no damage and the guy who was shooting at the Golden Eagle that often roosts at the point where the guy attached has not been seen since. And I really didn't have anything to do with his disappearance, really! I am now installing two Rohn C towers. (24 inch on a face with 2 1/4 " dia legs and bolt on braces) Both of these will be pier pin towers for two reasons. They came with pier pins when I bought them surplus. And I will make them rotating towers and mount the tapered section on a bearing with a sprocket for chain drive. I guess, by definition, all rotating towers are pier pin towers at least from the bearing on up. - W0UN 73, Pete N4ZR@netcom.com "Better, faster,cheaper -- choose any two"