Strange Tower Base Connection 1

[msquared48]

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?

Mike McCann, PE, SE (WA)

 

[dhengr]

Mike:
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?

 

[hokie66]

Maybe the triangular base frame is to assist in erection...give some stability until the guys are all in place?

 

[paddingtongreen]

I don't see any guys for that tower!
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

 

[KootK]

That is puzzling. Curious: in your estimation, does the footing work for axial loads as a standard eccentric footing without the benefit of moment connection to the tower? It's hard to imagine the need for it but it almost looks like someone's misguided attempt at a property line footing of sorts.

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[JAE]

I would suggest that hokie66's idea is most probable.


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[paddingtongreen]

For a change, I disagree with hokie66, I should be able to see a sign of a guy, and I cannot. Also, the tower is field bolted to the frame.

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

 

[bob96]

Triangular base with helicals or other deep foundation embedded into ground, to resist uplift, overturning...

 

[PicoStruc]

As hokie said, the shape is probably for counterweight during erection

 

[charliealphabravo]

I think the base was mainly part of the erection procedure to provide lateral stability as the tower was raised with a gin pole. In guyed towers this stability is provided by the lateral guys while the upstream guy does the heavy lifting. With no guys the triangular base would provide the stability needed to keep the tower rising in line with the erecting cable. It is actually pretty amazing how towers are raised with so little man power. I know there are 60M monopole guyed towers that can be erected by a couple of guys with only a gin pole and a come along.

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.

 

[msquared48]

Paddington:

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)

 

[charliealphabravo]

If this is a guyed tower then base moments and base loads may not be the controlling factor in the design. Guyed towers are often designed to be floating at the base with no moment or shear connection at all. This leads me back to erection being the primary purpose of the base configuration.

 

[SlideRuleEra]

Mike - Maybe nothing to do with structural design at all. Instead, perhaps a manufacturing convenience, one size triangular base "fits" all of their line of triangular towers, from small to much larger than the one in the photo.

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[msquared48]

OK. In thinking about this, I think that Hokie may be correct here.

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)

 

[msquared48]

Ran across this photo of how NOT to connect guy wires to the anchor rod.

Mike McCann, PE, SE (WA)

 

[JStephen]

My guess- they reused the foundation from an older tower (possibly larger tower) with wider footprint and used the structural base to reach existing anchor bolts. And the "Fixed" base was for erection purposes, so they could set the tower up, then attach the guy lines for wind loading.

 

[Archie264]

Was it erected by first fully assembling it horizontally then tipping it into place? If so that might explain the need for an eccentricity.

 

[Archie264]

...something on the order of this?

http://www.flysouth.org/base-atch.jpg

 

[Ron]

I'm with hokie66 on this one as well. Erection aid. Beams can be easily attached to concrete. Eccentricity might not be an issue in the design of the foundation. This is probably a very manual erection process intended for remote areas with little equipment.

 

[msquared48]

Ran the analysis on what I know and it is giving me about 10 Kip Feet (un-factored) of moment at the base with about 10K (un-factored)of axial DL + Guy forces. Still waiting on that other report from the other engineer.

Looks like without the horizontal steel beams, the leg tension and base moment probably could not be developed here.

Mike McCann, PE, SE (WA)