Breakaway Tether Wire Clamp of for Sign / Traffic Signal 2

[KootK]

- I'm dealing with another engineer's work and that engineer is no longer available. I've no project documentation other than the drawings.

- The project is a sign at the entry to a retail plaza. It is not a traffic signal.

- The project includes the component below which nobody involved seems to know how to procure.

- Some GoogleFu has made it clear that we replicated the detail below from some US DOT.

So my questions are these:

1) If I wanted one of these breakaway clamps, where would I get one? Are they on the shelf at some traffic signals store? Are they custom fabricated? If i can just figure out a way for my client to reasonably procure one of these things, my work shall be done.

2) I'd like to figure out if these clamps are actually required for a non-signals / non-traffic project such as mine. To that end, what is the purpose of these things? My best guesses based on staring at it for a while:

a) Tether wire keeps signals from swinging excessively. Hence they are straight and substantially without sag.

b) Because tether wire has no sage, it can't really be designed for much load. If it's overload, it snaps.

c) If the wire snaps, it's supposed to slide out of the clamp for some reason? Other than this, I don't see what is "breakaway" about this.

3) Is the "armor rod" just meant to be protection for the wire where these clamps grab on to it? Does the tether wire slide freely within the armor rod?

 

[BridgeSmith]

That's far different than how we do our span wire signals. On ours, the span wire and tether wire run parallel (same sag for both) and the signals are in between.

Anyway, the only way I can see the clamp being 'breakaway' is if the signal rotates up towards horizontal under a strong wind, the pull on the clamp becomes more towards the opening between plates. The way it's configured, it's looks like it's basically a free-swinging signal.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[3DDave]

https://mdotcf.state.mi.us/public/tands/Details_Web/mdot_sig-305-b.pdf.

There is an emphasis that the tether cable have break-away elements, both that clamp and the tether ends and that the attachment through the light give way if someone manages to snag it.

I'd start with a call to Pelco Products, Inc. While they seem to have every part except those tether clamps they seem to be well involved, including several catalogs that are state-specific. They have the tri-stud hanger bodies, for example.

- Found something much like it:

https://www.pelcoinc.com/wp-content/uploads/2019/05/Traffic-Catalog-Sec-T2-Free-Swinging.pdf

page T2-20 SE-3055-ZN1

 

[TLHS]

I'm pretty confident that that clamp thing is a three bolt suspension clamp, or something that's basically the same but named differently. They have a groove in them that you place over the wire in question, then you bolt the two sides together to clamp onto the wire.

They're commonly used in overhead line scenarios for a couple of things. Most commonly, they're used to attach wire rope messenger cables to poles. You use the two outer bolt holes to clamp the thing together and the central hole is used for a through-bolt that runs through the pole.

You can see one here, but they're available from a lot of manufacturers:

https://www.macleannetworksolutions...al-pole-line-hardware/cable-suspension-clamps

What seems to be happening there is that you put the clamp on the secondary rope, you put the u-bolt around the signal suspension neck thing, and then when it swings, it contacts the u-bolt and is stabilized laterally.

I agree that I'm not sure what's breakaway about it. You'd have to be careful with the tensioning of the lower cable on a sign application with a larger wind area.

 

[TLHS]

https://mdotcf.state.mi.us/public/tands/Details_Web/mdot_sig-305-b.pdf

The breakaway part is on the ends of the lower cable. There's a fuse link in this similar drawing set (see sheets three and four)

They're referring to the bottom cable as the break away tether. The detail in question isn't a breakaway assembly for the tether. It's an assembly to attach the light to the break away tether.

 

[TLHS]

Oh, and given how armor rod is used, I'm pretty confident that the armor rod is just the hatched area here. My gut questions it being included and that even if there were an issue it'd just be cheaper to increase the size of the wire rope. I also don't know how I'd evaluate that, though. The original designer is presumably trying to deal with really localized bending at the edges of the clamp and the fatigue issue you get when you bend a wire in the same spot repeatedly.

 

[KootK]

Thank you all for your generous assistance.

My gut questions it being included and that even if there were an issue it'd just be cheaper to increase the size of the wire rope.

I agree. I could see the fatigue concerns and the need for breakaway being a much more significant issue above fast moving traffic. Over a pedestrian plaza, I feel like the risks associated with the tether wire breaking are much smaller. As such, I'm planning to forgo the armor rod and just go with whatever kind of nominal clamp I can get my hands on. You're input on this has been a great help in allowing me to get this thing off of my desk, truly.

 

[BridgeSmith]

Over a pedestrian plaza, I feel like the risks associated with the tether wire breaking are much smaller.

Even a 1/4" cable can cause injuries or substantial damage if it breaks under tension.

I think the breakaway part of the clamp is that it's made to separate from the cable if the signal is impacted or the tether wire is in danger of being overstressed from wind loading, and the armor rod is used to protect the cable from damage if the clamp does pull off.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[KootK]

Even a 1/4" cable can cause injuries or substantial damage if it breaks under tension.

This particular sign is composted of fourteen closely spaced letters attached to fourteen of these breakaway clamps 16' in the air. Were the tether to break, I kind of feel as though the safest thing for pedestrians and property near by would be for the clamps to not let go of the wire.

I think the breakaway part of the clamp is that it's made to separate from the cable if the signal is impacted or the tether wire is in danger of being overstressed from wind loading

"Separate from the cable" in the sense that the cable might pull through the clamp longitudinally? That is, truly, the only way that I can see the clamp reasonably "breaking away".

 

[BridgeSmith]

"Separate from the cable" in the sense that the cable might pull through the clamp longitudinally? That is, truly, the only way that I can see the clamp reasonably "breaking away".

I was thinking more of the stabilizer bar bending or breaking and allowing the cable to slip out. On other breakaway devices I've seen, aluminum seems to be the material of choice for the part that breaks and stainless steel for the other components.

This particular sign is composted of fourteen closely spaced letters attached to fourteen of these breakaway clamps 16' in the air. Were the tether to break, I kind of feel as though the safest thing for pedestrians and property near by would be for the clamps to not let go of the wire.

The most likely place for the tether wire to break would be at the attachment point at the end of the span (in a typical configuration, at the clamp), which could be ok, depending on how close the first and last letters are to it.

If the letters wouldn't damage anything, it seems what you'd want in an extreme wind is for the letters to separate from the tether wire so that it doesn't break, and the letters become free-swinging, which greatly reduces the stress on the span wire, so that it's not in danger of breaking, either.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[KootK]

Thanks for your thoughts on this Bridgesmith. I'd wondered if this thread might do better in the Bridge forum.

 

[Tomfh]

It’s a fuse isn’t it? So if a tall vehicle hits the traffic lights they knock the light off without pulling down wires/poles/etc?

 

[azcats]

I wonder how hard things like this are to find?

Early in my career I was mimicking an old job at the firm and there was some interior deflagration loading that was vented by the roof deck detaching from framing members by using fasteners that 'released' at some force. Apparently this fastener didn't exist. Ended up using premanufactured overpressure vents.

 

[KootK]

I wonder how hard things like this are to find?

They haven't been too easy for the folks working on this sign to find, that's for sure. Given that the detail appears in the current standards of a bunch of DOT's, they must be gettable somehow, in some markets. I almost wonder if there's some, magical term for them that I could google and find out that they're on the shelves at home depot etc. TLHS turning me on to "3 Hole Messenger Clamp" was a boon to my effort, even if it's not exactly the same thing.

 

[KootK]

It’s a fuse isn’t it? So if a tall vehicle hits the traffic lights they knock the light off without pulling down wires/poles/etc?

It may well be. I've been having a hard time sorting that and determining whether the "fuse" is these clamps, the attachment to the poles, or both. Although, if it's the tether clamps, it's somewhat of a moot point if I can't find the damn things anyhow.

 

[dauwerda]

Looks very similar to a Pelco product, part number SE-3055-ZN1. Find it on page 20 of this catalogue:

https://www.pelcoinc.com/wp-content/uploads/2019/05/Traffic-Catalog-Sec-T2-Free-Swinging.pdf

 

[3DDave]

What did Pelco say?

 

[KootK]

What did Pelco say?

I haven't called them as I went in another direction.

 

[KootK]

It turns out that the original, U-bolt detail didn't actually suit the project anyhow. The U-bolt is meant to wrap around the pipe from which the traffic signals hang. The nature of these suspended, neon letters is such that there's not pipe to grab that way really. The three bolt setup actually works better from this perspective even if it doesn't have any kind of breakaway rating.

@dauwerda: thanks for the link.

 

[TLHS]

On the subject of getting things off your desk, little detail heavy thing are interesting, but also the most annoying sorts of things when you’re also dealing with the business aspect. They look really simple to the client and the install cost is low so it’s hard to explain the engineering bill. But if they’re even a little bit outside of the typical and aren’t something you commonly do, the background research to figure out a basis to even start from can be significant and then you have to put way more thought into every bit because you don’t have 50 similar projects in your brain to draw from where you already worked through all the detailing issues.

It’s the situation where you have a $30k budget for a project and then have to try to figure out how to justify asking for 5k for some dumb little thing that got added as an extra late in the game and makes up 5% of the construction cost of the overall project. How much do you get to charge for figuring out how to do something novel vs how much should you eat yourself if you have to do fundamental research that someone else might not have to do.

Man, it’d probably be nice if I could somehow find a way to specifically be an ‘over-researched detail development engineer’ and actually bill that time out.