Mechanical Vibration from Fibre Cable

Hi.

I'm waiting for the installation of BT FTTP which due to Covid has been delayed a couple of times and has turned into a 2 stage install with a current installation date at the end of March. OR turned up this week and installed the overhead cable from the pole to the house.

I woke a few times in the night last night hearing a low frequency rumble like there was a van engine ticking over outside the house. After a while and a wander around upstairs I realised that the noise seemed to be loudest in the bedroom below where the new cable is anchored on the house. Once it was light I got up and investigated further and noticed through the bedroom window that the new cable was vibrating violently in the wind. The cable is anchored on the facia board and the vibration is sounding through the roof woodwork I assume. I suspect that the cable vibration is due to the uneven profile of the hybrid cable as the other normal round cables from the pole were just waving about in the wind. According to Google Maps the cable span is about 54 metres and we are 500ft ASL and quite exposed.

I cannot imagine how this problem can be resolved but I'm wondering if anyone else has had the same and managed to do something? Does OR have any solutions that might help? I am wondering about some form of damper that could be added near the anchor. Any info appreciated.

Cheers - Andy (A light sleeper hoping for calm weather. )

Edited by Rhubarb (Sat 13-Feb-21 12:28:57)

Easy resolution would be to have the fixing point moved to solid brick/block work.

In reply to a post by MHC:

Easy resolution would be to have the fixing point moved to solid brick/block work.

Yep, that's one thing I thought of but the only problem I can think of is that to get sufficient height they'd have to anchor near the top of the brickwork and that might not give enough strength for the long run given that there'd be little weight on the upper bricks. It's a house with the soffits level with the top of the upstairs windows. It's a long span and the cable sags more than the 'normal' cables and runs partly over a road. Andy

I have a 55m metre drop too - so it should bot be an issue provided it is correctly tensioned. They should have taken the road into account when specifying the pole originally

In reply to a post by MHC:

I have a 55m metre drop too - so it should bot be an issue provided it is correctly tensioned. They should have taken the road into account when specifying the pole originally

I'll need to try to contact someone on Monday to see if anything can be done. There's another pole that's only 40 metres away that would allow anchoring on the gable end which would be a much stronger anchor point but I doubt they'd move to that. I'm hopefully more likey to get something done now as the installation is not yet completed. Andy

Tricky one.

I’d expect Openreach to accept that this is an issue that needs resolving though.

Using the other pole if possible seems better, you don’t want the cable to be too tightly tensioned, it should have quite a bit of movement in it.

The cable is anchored on the facia board

Really ? We have not been fixing stuff onto facia boards for years !

In reply to a post by Rhubarb:

a low frequency rumble like there was a van engine ticking over outside the house.

I'm interested to know exactly where the origin of the rumbling is coming from, do you think its from the facia board due to the stress on it (from the cable and its anchor) or from the whip of the cable?


Edit: I've reread your post and think I've go my answer

Edited by deleted (Sat 13-Feb-21 15:12:07)

Hi. Thanks all for the replies.

Firstly, re fixing to fascia boards. The houses where I live were built mid 1970s and all had wooden fascias when new. Every house around here has the phone cable anchored on the facia board. About 18 years ago I had the old facias removed and replaced with μpvc. The job was done by a good friend with a little help from me. I was conscious of what I considered to be a long run being supported by the facia (I'd felt the strain when we'd secured the bracket via a strong rope to an eyelet in the brick when the job was being done) so I fitted a piece of 3" x 3" across 2 trusses and fixed it with 2 strong brackets so it was flush behind the new facia. The existing BT bracket was refitted with new longer screws through the plastic into the wood to ensure a strong attachment. Maybe I did too good a job of it! A neighbour recently had an additional line fitted for WFH and that was fitted to the facia.

I'd be happy for it to be anchored to the brick but most of the strength in a brick wall comes from the weight above the bricks and not the mortar between them. Fixing the attachment just below the facia/soffit would likely result in the brick(s) coming loose. Further down would probably result in the cable being too low mid span where it is just over the road. I might be wrong.

Transfer to the other pole would be a nice idea but it would involve crossing a cable from my current pole to a neighbor's house. Also each pole had an 8 and 12 way CBT fitted which seems to roughly equal the number of cables from each pole, I assume these will all be reserved for existing provisions when the PSTN finishes in 2025.

Having messed about when the wind got up again this afternoon it doesn't take much to stop the vibration transferring to the house, I opened the nearest window and rested an aluminium walking pole on the cable about a foot from the anchor and partially closed the window jamming the pole. I suspect this put some pressure on the cable (there was no noticeable deformation) but it stopped the noise. Maybe I need a Stockbridge damper.

Cheers - Andy

Edited by Rhubarb (Sat 13-Feb-21 18:44:50)

I don't know if this is the correct size to allow you to move the cable to a brick fixing, but Openreach engineers should have access to things like this

https://www.comtecdirect.co.uk/product/dropwire-brac...

In reply to a post by MC31:

The cable is anchored on the facia board

Really ? We have not been fixing stuff onto facia boards for years !

Someone should've told the OR engineer that ran in my overhead FTTP two years ago! The (new) dropwire bracket is most certainly fixed to the fascia board.

In reply to a post by jpm:

I don't know if this is the correct size to allow you to move the cable to a brick fixing, but Openreach engineers should have access to things like this

https://www.comtecdirect.co.uk/product/dropwire-brac...

Thanks for that. A neighbour seems to have 2 of these on his house both fitted to the facia lifting the cable above the roofline. I was wondering what they were. I can't work out why they were fitted because the cables would have already been quite high due to the ground rising and the wires don't cross a road. Andy

Not wanting to cause any panic, but this morning before your post here, I came across:
https://www.youtube.com/watch?v=QLl9NCJvZjw

Just shows what wind, temperature and altitude can do!

I have noticed that our telephone drop-wire, re-sited by Openreach at my expense in 2019, sways a lot more in the wind than our previous one and our neighbours.

Cheers!

In reply to a post by Ancient_Mariner:

Not wanting to cause any panic, but this morning before your post here, I came across:
https://www.youtube.com/watch?v=QLl9NCJvZjw

Just shows what wind, temperature and altitude can do!

I have noticed that our telephone drop-wire, re-sited by Openreach at my expense in 2019, sways a lot more in the wind than our previous one and our neighbours.

Cheers!

That’s overcooking it a bit Clive. One’s a 400 KV transmission line 30+ years ago and the other is a bit of glass strung up on someone’s house with a bit of thrum on some windy days....

Post deleted by Rhubarb

In reply to a post by Pheasant:

...a bit of thrum on some windy days....

A superb description. Andy

The facia is acting as a soundboard, similar to any stringed musical instrument.

Probably doesn’t help that the OPs is plastique / UPVC fascia. Best to relocate it, when engineers return to 2nd fix / finish the job.

In reply to a post by Pheasant:

Probably doesn’t help that the OPs is plastique / UPVC fascia. Best to relocate it, when engineers return to 2nd fix / finish the job.

I understand your thinking but the current bracket is secured through the fascia onto a piece of 3" x 3" wood fitted between 2 trusses and attached to the trusses with 2 heavy duty galvanised brackets. The BT bracket is attached by No 12 x 2½ screws through the fascia and into the 3 x 3. I'd Imagine it's certainly more firmly located than many similar installations where the anchor is just screwed into the facia. As regards relocating the anchor there's really no where else to fix it. The house is the left of 2 semis and the pole is 50m hard to the right, the cable has to be secured to the front of the house and the highest point is the eaves. If the cable were to be any lower it would cross the windows and more importantly the neighbours windows.. There was no such problem with the previous cable. Andy

Does it cross a road? If not slightly slackening it and introducing a couple of twists might quieten it down

I think that the fascia is a red herring. There is no need to postulate a soundboard or the need for a stiffer fixing. As the OP says, the sound could simply be conducted through the fairly rigid materials of the building. (The air in the room might resonate, in which case there might be louder and quieter positions in the room.)

The thing to focus on is the source of the sound which is the violent vibration of the cable, which must be caused by the wind. I wouldn't know the mechanism, but there is presumably some consistent effect (e.g. the formation of vortices which exert a periodic suction on the cable). One needs to seek a way to disrupt this response (since you can't stop the wind). Ways of doing this depend on the vibration mode, which as it is visible may be transverse vibrations. Changing the tension in the cable might make a difference, as might fixing a weight to the cable some distance out from the fixing. Or mounting the fixing on a spring and dashpot (like a shock absorber) to take the energy out of the vibration.

It will be interesting to hear if there is a 'proper' solution.

In case anyone hasn't seen the Tacoma Narrows film showing an effect of wind-induced vibration, here is one version https://www.youtube.com/watch?v=j-zczJXSxnw

OK. Just tossing some ‘blue sky’ ideas out there...

Stockbridge dampers (tuned mass dampers):
https://www.pfisterer.com/fileadmin/pfisterer/downlo...

Anti galloping devices (for really long) power lines:
https://arproducts.org/wp-content/uploads/2021/01/AR...

I cannot imagine how this problem can be resolved but I'm wondering if anyone else has had the same and managed to do something? Does OR have any solutions that might help? I am wondering about some form of damper that could be added near the anchor. Any info appreciated.

The span is too taught. It needs to be loosened, ever so slightly.

Whereabouts in the country are you ?

Probably the cold snap hasn’t helped. It might have been tensioned on the high side and with the added drop in temps, it’s now too taught as you say. Add some wind at just the right speed and it’s humming.

In reply to a post by Zarjaz:

The span is too taught. It needs to be loosened, ever so slightly.

Possibly so but it's lower than the other non fibre cables - I appreciate it'll be heavier. It passes over the outside corner of a bend in the road.so I don't know how much slacker it could be. The other cables are just waving around in the wind, my cable is fidgeting about erratically.

In reply to a post by Zarjaz:

Whereabouts in the country are you ?

NE Wales

Andy

Edited by Rhubarb (Sun 14-Feb-21 10:25:49)

NE Wales

Ah, that’s an idea stuffed then.

It literally is the tension on the span, so if exactly right, the vibration caused by the wind makes it resonate like a guitar string.

I’d suggest contacting whoever you ordered via, and have them send Openreach back out to sort it.

In reply to a post by Zarjaz:

I’d suggest contacting whoever you ordered via, and have them send Openreach back out to sort it.

Yep. I'm going to get onto that tomorrow. The order's not completed yet (BT) so I hope I have a better chance of getting something done. Cheers - Andy

Quick edit: As an aside, can you tell me what the minimum height is for a drop cable over a road that could have 4.9 metre vehicles on it?

Edited by Rhubarb (Sun 14-Feb-21 10:25:01)

Minimum height for any drop wire going over a normal road would be 5.2m but 5.9m or higher is preferred.

To resolve the galloping of your drop wire, OR can add some twists to the drop wire, this is something that has to be done to the incoming fibre only drop wires. They say one twist for roughly every 10m of cable will do the job. This can be done easily from the pole end.

The reason for the vibration or galloping is the flat profile of the cable.

Hope this helps.

In reply to a post by TheInstaller:

Minimum height for any drop wire going over a normal road would be 5.2m but 5.9m or higher is preferred.

To resolve the galloping of your drop wire, OR can add some twists to the drop wire, this is something that has to be done to the incoming fibre only drop wires. They say one twist for roughly every 10m of cable will do the job. This can be done easily from the pole end.

The reason for the vibration or galloping is the flat profile of the cable.

Hope this helps.

Thanks for that.

Picking up on the 'galloping' comment, I can pretty much see down the cable from the window and in certain wind condition I can see waves forming down the line as well as seeing the profile change as the cable twists due to the irregular shape.

Andy

Edited by Rhubarb (Sun 14-Feb-21 11:38:09)

Apparently there's a whole branch of physics devoted to the study of what's known as "Aeolian vibration" and is of great interest to grid / transmission line operators.

Usually demonstrated in one one of two ways apparently; "gallop" or "flutter". Gallop also known as 'conductor dancing' according to Wikipedia and is a low frequency (about 1 Hz) and high in amplitude oscillation - hence the dancing bit. As opposed to flutter, which is also resonant but of high(er) frequency (around 10 Hz) and low amplitude.

Your issue sounds, pardon the pun, more like "flutter" - which in larger structures and long transmission lines, apparently can be cured by fitting so called mass dampers (I always think of the millennium bridge in London) or Stockbridge dampers.

Sounds like in your case some easing of the tension in the catenary/drop, or maintaining the return twists as noted above will have a similar dampening effect on the wire.

Good luck.