Self-Collapsing Towers 1

[271828]

I am searching for information on "self-collapsing" towers, especially monopoles. I've also heard the term "crumble zone approach." I'm not finding much information on this. What design standards or references talk about this?

I have been asked to weigh-in on the idea of putting a monopole tower closer to houses than the height of the tower. The question is whether this is acceptable if the tower is designed to fall downward rather than failing at the base and thus having the upper parts of the tower land on the houses.

What would a "crumble zone" look like for one of these? I'm imagining something like an RBS, but that doesn't make a lot of sense to me.

If anybody can point me in the right direction and/or offer information on any of this, I would greatly appreciate it.
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[BridgeSmith]

If you're using a steel pole, I think you'd just have to design for the higher factor of safety required for a structure that poses a significant hazard to lives, under your applicable building code.

Under AASHTO, we have a higher design wind speed for our high mast light towers (120 feet tall) than we do for regular light poles (30 feet or so).

 

[CANPRO]

Not sure how you could reliably predict how the pole was going to fall. How could you ensure the pole would fall straight down due to lateral loads?

 

[271828]

If I understand correctly, it is fairly common to design taller towers to fall mostly downward. As the tower falls, the load due to wind is much higher near the top due to the angular velocity of the falling tower. This causes reverse curvature. If the tower has some predefined weak points, hinges will form at those locations and the tower will kinda fold onto itself. Some can fold over several times.

It's hard to see how this could be reliably done with a monopole. Certainly some sections could be made much thinner, but 200 ft isn't very tall, so it's hard to imagine the wind pressures increasing quickly enough to cause the hinges to form. I'm thinking that the tower would have to fall pretty far -- in a manner like a tree falling -- before these would occur. It sounds like something that might work theoretically, but would be unreliable.

 

[BridgeSmith]

I think you're right, 271828, about the unreliability of getting a pole to hinge where you want it to. The only reliable way I can think of is to guy wire the top so that it would have to buckle somewhere in the middle.

Of course, anything along that line would seem to be designing it to fail. Much better in my mind to just design it stout enough that it won't fail. The cost of engineering analysis and modeling to design a system with controlled failure points would likely exceed the cost of just designing it to withstand a tornado (or buying the house that's within the fall radius).

 

[CANPRO]

271828, thank you for the explanation, that makes some sense...but I still wouldn't feel too comfortable living 150' away from a 200' tower.

 

[271828]

As I think about it more, I really don't believe a monopole tower could be made to fold over onto itself in the manner I described above. At the very least, I would have to see some calcs and experimental tests to believe it.

If anybody has info on this, I would greatly appreciate it.

 

[271828]

CANPRO, it's worse than that. The houses are at about H/2 away from the base of the tower. Some parts of the back yards where the kids play are much closer.

 

[btrueblood]

I can imagine a mechanical or electromechanical "fuse" that would allow, say the upper third of the tower to hinge if the loading (or deflection) went above some limit. But I'm not sure how trustworthy said device would be given the low probability of maintenance (and how would you test it in-situ?). It would need to be able to hinge in a fairly wide range of directions as well, unless you know the prevailing storm winds are from a limited direction.

 

[271828]

There's no special fuse built in.

Today, I found a little more information about this. For monopoles, the idea is to have the hinge form pretty far above the base so that the failed part just hangs down off the un-failed part below. Of course, this assumes the pole doesn't fail closer to the ground, including at the base.

 

[TLHS]

It seems easier to just design the thing to a reasonable factor of safety against failure to ensure public safety.

 

[KootK]

For monopoles, the idea is to have the hinge form pretty far above the base so that the failed part just hangs down off the un-failed part below

I was very much in the "no reliable way" camp until I read that. That actually seems pretty clever and somewhat reasonable to me. Essentially capacity design everything below the hinge like we sometimes do in seismic. I'd be paying extra close attention to base anchorage in that scenario as that's probably the part of the system with the most highly variable capacity.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[BUGGAR]

I like the seismic comparison. Strong column, weak beam concept. With the "weak" beam being the vertical top half of the stack.

 

[BridgeSmith]

The only time we use the hinge concept is for breakaway roadside signs, where they are hinged so that if a vehicle hits it, it breaks above the vehicle and the sign goes over the vehicle. We never attempt that approach for wind loading, but I suppose it could be done. Fatigue issues may be difficult to mitigate at the hinge point, and wind-induced oscillatory effects could be difficult to predict with a weaker point in the middle. You may have to investigate ways to maintain the stiffness at the hinge location while reducing the strength.

It might cost more to design than the material and fabrication would cost to make it very strong so that it has a high factor of safety.

 

[Compositepro]

One could use guy wires on only one side of the tower. Not for support, but only to control direction of failure.

 

[BridgeSmith]

Something else that just occurred to me: if you do go forward with the hinge idea, be sure to provide a flexible tie across between the sections at the hinge, so the top section doesn't separate from bottom completely. It would likely be a violent failure, so carefully detailing that link so that it is not severed would be critical.

That said, I still think there are far too many unknowns with regard to the structural dynamics of a tall, slender pole, with varying stiffness, exposed to wind. We have more than 30 years of design, testing, and monitoring of the high mast light towers we're using, and we are still having unanticipated problems than may require us to replace many of the ones we have up. None of them have hinge points in the middle. Wind loading induces millions of stress cycles. It doesn't take much of a stress riser to result in a crack under those conditions.

 

[271828]

KootK typed "I was very much in the 'no reliable way' camp until I read that."

I think reliable is the key term here. With seismic design, a prequalified system goes through a lot of research and testing. To determine if a design method is reliable, one needs data that shows what the system will do. I'm still looking for a research study on the hinge idea, but haven't found anything. There are many details that need to be determined, such as overstrength factors. Best I can tell, there is no industry standard method for this, so engineers are left to devise their own method.