Strange Tower Base Connection
Strange Tower Base Connection
(OP)
This is the connection for a 60 foot tall guyed steel lattice tower in Eastern Oregon. So much for the easy part...
What I do not understand is the base configuration with the triangular steel beams and the tower legs sitting at one point of the triangle. There should be no moment at the base since this is a guyed tower, but I am reading otherwise due to the structural configuration, and the tower is technically set off center of the concrete footing. I have not seen it yet, but I wonder if this tower base was designed to develop the moment capacity of the steel lattice tower. At first I thought that the steel beams were an after thought to plumb the tower, but the shims look about the same thickness.
Anyone seen a base like this one? I note that the smaller tower in the background is also guyed and mounted off center of its concrete footing. Perhaps this is common?
What I do not understand is the base configuration with the triangular steel beams and the tower legs sitting at one point of the triangle. There should be no moment at the base since this is a guyed tower, but I am reading otherwise due to the structural configuration, and the tower is technically set off center of the concrete footing. I have not seen it yet, but I wonder if this tower base was designed to develop the moment capacity of the steel lattice tower. At first I thought that the steel beams were an after thought to plumb the tower, but the shims look about the same thickness.
Anyone seen a base like this one? I note that the smaller tower in the background is also guyed and mounted off center of its concrete footing. Perhaps this is common?
Mike McCann, PE, SE (WA)






RE: Strange Tower Base Connection
I’ve never seen or designed something like that before, so what do I know? But, try these ideas on for size. There is some bending moment in the three legged tower (a continuous beam-column) w.r.t. lateral loads, and supported at each of the guying levels. The tower doesn’t weigh to much, but accumulates some additional compression and bending at each guying level. On that small a tower base (foot print), the dimension btwn. legs, we can make the req’d. A.B. connection to stl. frame easier than to a relatively thin conc. footing, with close A.B. spacing at each leg. But then, we can spread these anchor loads and moments out through the triangular channel base frame, and have much lower A.B. forces, over larger areas, at each corner of this stl. frame; thus easier to install and make work in a fairly shallow footing. Also, attaching the tower base to the triangular stl. frame leads to a slightly softer base fixity (less stiff base fixity) than would be the case if the legs were bolted directly to the conc. ftg. Are these oriented in any way w.r.t. the prevailing winds? You’ve worked on some of these before, haven’t you? I thought you were looking for some software for designing these some time ago. Any of the above make any sense?
RE: Strange Tower Base Connection
RE: Strange Tower Base Connection
If you look at the adjacent tower behind this one, it looks to be of similar size but the tower is in the center.
That could be catchbasin next to this foundation, perhaps that prevented them from putting this concrete in the ideal location.
My best bet though is that they were re-using an existing concrete base. The bolt positions on the one on the far side looks to be the same, they just didn't re-use them there, but that one certainly is guyed.
Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin
RE: Strange Tower Base Connection
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RE: Strange Tower Base Connection
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RE: Strange Tower Base Connection
I wonder if the cable tray/bridge is taken to be a structural member, restraining lateral movement.
Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin
RE: Strange Tower Base Connection
RE: Strange Tower Base Connection
RE: Strange Tower Base Connection
I'm just guessing here but the base may also allow the tower to be erected on gravel or rock in remote areas where the base would be ballasted rather than anchored. It looks like a rather short tower.
RE: Strange Tower Base Connection
There are two guys to each leg at about the 50 foot level. I just did not post pictures of them.
Hokie:
It is possible, but why the eccentric mounting? Just makes no sense to me
Dhengr:
I will have to digest your post and thing about it.
I am waiting to get more information from another engineering report. May be something to glean from that.
Mike McCann, PE, SE (WA)
RE: Strange Tower Base Connection
RE: Strange Tower Base Connection
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RE: Strange Tower Base Connection
If the tower was assembled on the ground, and rotated up using the two of the legs of the W shape triangle at the base as a linear pivot point, then this would result. The load to the foundation is eccentric, but if the footing can take the eccentricity, it becomes a moot point.
May actually make towers of this geometry easier to erect rather than the tapered pinned base which is very unstable, but a true pin.
Mike McCann, PE, SE (WA)
RE: Strange Tower Base Connection
Mike McCann, PE, SE (WA)
RE: Strange Tower Base Connection
RE: Strange Tower Base Connection
RE: Strange Tower Base Connection
http://www.flysouth.org/base-atch.jpg
RE: Strange Tower Base Connection
RE: Strange Tower Base Connection
Looks like without the horizontal steel beams, the leg tension and base moment probably could not be developed here.
Mike McCann, PE, SE (WA)
RE: Strange Tower Base Connection
RE: Strange Tower Base Connection
I’ll be darned, you may have proved, or be proving what I was suggesting in my first post. You’re saying essentially what I was trying to say, maybe inelegantly. But, without the benefit of any loads, dimensions, etc., or the software to run an analysis on that tower, I had no idea of real numbers.
RE: Strange Tower Base Connection
the 2nd tower in the background ... base looks different, as though the single leg is inside the triangle (as oppoesed to this one where the single leg is nested into the corner.
lord knows what the triangle of steel is doing, other than complicating things !?
base moment ... why not say zero moment ? whatever moment develops there is small compared to the moment reacted by the guys and maybe local yielding would make it less ?
is the tower built in segments ? or one piece ?
another day in paradise, or is paradise one day closer ?
RE: Strange Tower Base Connection
The tower is built in three 20 foot segments that are bolted together. As for the moment, to say it is zero is not what is there. It affects the verticals and the webbing. If this were a tapered base, yes, but that is not the case.
Webbing is a very strange arrangement, too, as it is a staggered "Z" shape on each side, with the height of each "Z" at 1.8 feet, overlapped 50%, netting 0.9 feet between the horizontals. Never seen that before, and apparently, neither has TnxTower. May have to model this in RISA and import it.
Fun and games, right.
Mike McCann, PE, SE (WA)
RE: Strange Tower Base Connection
Mike McCann, PE, SE (WA)