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Author Topic: Span Wire Signal Heads Installation  (Read 924 times)

roadman65

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Span Wire Signal Heads Installation
« on: January 04, 2022, 11:50:08 PM »

https://www.flickr.com/photos/54480415@N08/51799419581
This is SC who manages to deal with using one wire to hang more than one usual signal heads. IMO they do a wonderful job keeping the signal heads  level to the eye.

https://www.flickr.com/photos/54480415@N08/49686381898
This Is Florida. They too do a good job keeping the heads level to the eye.

However both signal installs used different means to achieve this.  One pulls the wire tight, while the other uses different length suspenders to maintain level mounting while let the cable wire hang loose so to speak.

Why canít we have unison on erecting a simple span wire for a traffic signal assembly.
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Sheryl Crowe

wanderer2575

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Re: Span Wire Signal Heads Installation
« Reply #1 on: January 05, 2022, 12:19:59 AM »

It's not that simple.  I'm not an engineer but I imagine the length of the span wire, the number of fixtures suspended from it, and the amount of weight and where those points of weight are suspended are all factors.  Your FL example appears to have a longer span wire and greater weight of fixtures than the SC example.  Pulling that FL wire tight with no arc would require that it be made of some lightweight space-age material, and that the poles be made of kryptonite and firmly attached to footings buried fifty feet into the ground.  Assuming that the physics would allow it at all.
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ErmineNotyours

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Re: Span Wire Signal Heads Installation
« Reply #2 on: January 05, 2022, 04:18:56 AM »

It's not that simple.  I'm not an engineer but I imagine the length of the span wire, the number of fixtures suspended from it, and the amount of weight and where those points of weight are suspended are all factors.  Your FL example appears to have a longer span wire and greater weight of fixtures than the SC example.  Pulling that FL wire tight with no arc would require that it be made of some lightweight space-age material, and that the poles be made of kryptonite and firmly attached to footings buried fifty feet into the ground.  Assuming that the physics would allow it at all.

The wire material wouldn't need to be that exotic, but it would need to continue past the poles at each end and be anchored to the ground, and would resemble a suspension bridge.
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roadfro

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Re: Span Wire Signal Heads Installation
« Reply #3 on: January 05, 2022, 11:45:28 AM »

The SC install is a box span, so you only have the signal heads for one direction on each wire. That results in fewer signal heads and less weight, so it's easier to have the span wire pulled tight.

The FL install is a cross span, so you have signal heads for two directions on each wire. That is more signal head weight, so the wire will sag more. This install also appears to be using higher poles than the SC one, so the extenders may be necessary on the outer signal heads to ensure the signal heads are not mounted too high.
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Roadfro - AARoads Pacific Southwest moderator since 2010, Nevada roadgeek since 1983.

Dirt Roads

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Re: Span Wire Signal Heads Installation
« Reply #4 on: January 05, 2022, 05:02:57 PM »

It's not that simple.  I'm not an engineer but I imagine the length of the span wire, the number of fixtures suspended from it, and the amount of weight and where those points of weight are suspended are all factors.  Your FL example appears to have a longer span wire and greater weight of fixtures than the SC example.  Pulling that FL wire tight with no arc would require that it be made of some lightweight space-age material, and that the poles be made of kryptonite and firmly attached to footings buried fifty feet into the ground.  Assuming that the physics would allow it at all.

Interesting topic.  In railroading and rail transit, the signalling and electrification designers are required to make these types of calculations.  I'm not an electrical engineer, but my argument that this type of engineering belongs with a civil engineer fell on deaf ears.  Oh, and the main reason I jumped in here is that the curve is not an arc or a partial ellipse (both of which I can calculate) but rather the aptly named "catenary".  And yes, we still ought to know the length along the catenary in order to purchase the correct length of cable.  But instead, we usually have to purchase an extra 30% of "slop" on big cable orders because nobody took the time to calculate the exact (which would allow a much less expensive 5% slop).

So the question for you DOT professionals:  Do any of your Signal Departments employ civil engineers (or even better, automated design software) for the design of signal placement on long sections of span wire?
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