When I saw the thread title I thought it was going to be about the Forth Road Bridge, which already has a few snapped cables. But that has 11000 cables, this Clyde Arc one has about 20!
There should be a big factor of safety in the design, so should be a duff tie rod (hopefully not) or installation workmanship at fault. I wonder who's neck is on the block?!
snipurl.com/1xhr8
Edited by Rich 9-3 on 15/01/2008 at 13:19
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Maybe the people who made the tie-rods are the same firm that made the columns for the Tay Bridge....
[Did they fill up the flaws with beeswax too...?]
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When I saw the thread title I thought it was going to be about the Forth Road Bridge which already has a few snapped cables. But that has 11000 cables this Clyde Arc one has about 20!
Apparently they now have microphone sensors to 'listen' for snapped or snapping cables all over this bridge.
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Actually it is probaly the cable supplier that is cacking it !
It seems it is not "the cable" but the bolt & eye at the end of the cable. The actual cable is still intact (lying across the carriageway) - the casting of the end seems to have sheared and come awy from the bolt.
It has to be hoped that it is 1 failure and not the 1st of 14 failures.
Spares - they have 1 x spare - if they need more than 1 (say 14!) then the delivery time is said to be 2 months.
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"delivery time is said to be 2 months."
Not a B&Q job then!
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"delivery time is said to be 2 months." Not a B&Q job then!
Screwfix have them (they have everything)
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I assume its safe to say this thing is under a fair degree of tension. I would assume therefore that it had a lot of momentum when it let go and could easily have taken someones head off/ gone through a car windscreen/insert other serious personal injury.
Someone is lucky they are not looking at a manslaughter charge (or whatever funny murder charge thing they have up there)
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Someone is lucky they are not looking at a manslaughter charge (or whatever funny murder charge thing they have up there)
Please; this is Glasgow.... They call it "Mudder" up there [don't you ever watch Taggart] - though they don't really do funny murders; must be a religious thing, fun's not allowed.
Anyway; it would likely only have been a motorist that would have been killed - and we don't matter unless there's money to be made.
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I assume its safe to say this thing is under a fair degree of tension. I would assume therefore that it had a lot of momentum when it let go and could easily have taken someones head off/ gone through a car windscreen/insert other serious personal injury.
"The cable" that fell is measured in several tonnes (several thousand Kilograms)
Yes it would have caused serious injury at anytime other than the middle of the night - especially a wet Monday night in January.
After it snapped (like an explosion) the other cables vibrated like strings of a musical instrument!
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There was a news article on one of the regional news programmes in whilst I was in Wales about the problem with cables on the old Severn crossing threatening to snap.
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There was a news article on one of the regional news programmes in whilst Iwas in Wales about the problem with cables on the old Severn crossing threatening to snap.
what do they do? grab you by the throat and say "Pay up or I snap"?>>
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This structure would have been designed with a minimum of 125 years design life and I'm very surprised at this happening. Obvious causes are during manufacture although quality assurance and inspection of the bridge components during manufacture should rule this out. Again problems causing this during construction should have been noted and corrected during the contract, the only thing left is vandalism. What! you may say but cables do get cut and vandalised throughout the country and bridges (mainly footbridges) are in need of constant repair. I also think that it it would take a fairly big disk cutter to get through one of the bridge deck ties!
I'm sure that's what the main contractor and the sub contractors/suppliers are hoping for as bridge should be under a 12 month maintenance period from the end of the construction programme.
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The story about the Severn Crossing
news.bbc.co.uk/1/hi/wales/south_east/6927852.stm
But the news article I saw was in the last week to ten days. Either is a filler story or something is happening for real !
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>>125 years design life
if that's the case, then, to fail within the elapsed life would mean that you would need about 4.5 times the level of fluctuating stress than the bridge was designed for.
Depending upon how load is shared between adjacent cables, and if there are any other exacerbating factors - like some environmental issue that wasn't taken into account (I'm thinking of stress corrosion cracking of some types of stainless in a chloride atmosphere), then it doesn't seem too far fetched.
I'm sure the failure has set a number of hares running in the offices of whoever manages this particular asset - from deciding the appropriate safety response protocol, considering other similar bridge types in use, metallurgical examination of the fracture surfaces and adjacent material (to determine if the correct material and heat treatment was actually used), modelling of the bridge dynamics, modelling the bridge's thermal response, defining and scheduling a repair, working out who's going to foot the bill.....
Number_Cruncher
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This was reported on-line in the local evening newspaper and out of embarrassment, I am not going to post a link to it.
Comments have been added about how useful the council are (because there were guys with hi-vis on they must be council workers) and that why does it need to be closed, why can't they just put a new cable up!
I would imagine a big part of the "repair" process will take place in an office, analysing data, capacities, load bearings etc.
Apparently when the cable came down it absolutely wrecked the metal crash barriers, glad I was not under it!
Annoying thing is that its a great bridge, lit up nicely at night with a "squinty" arc over it. Have cycled over it a few times and it is very cycle friendly - ok it has no pot holes in it like most other roads about there! And only about a mile from my office.
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Here's the aforementioned evening newspaper story, I have no pride so have no shame in posting it and the usual assortment of barbed comments their stories attract! It contains a small Quicktime video clip narrated by someone rather bored sounding.
Clearly shows that the eye at the top anchorage of the cable has snapped clean in two horizontally. The barrier damage is also fairly evident, good job this happened so late at night with little pedestrian or vehicle traffic.
As regards stress, there were serious gales in Glasgow a few nights ago that caused much structural damage elsewhere, so there's some speculation that these might have been a contributing factor. Whilst fairly strong, however, the wind speeds were not unheard of, so it's hardly reassuring if this was the case.
As mentioned in most reports, the bridge was designed to withstand the loss of a cable for maintenance purposes, so it's not going to be plunging into the Clyde anytime soon - although Lord knows when we'll get to cross it again. Not that I used it often, it has the most draconian restrictions on what direction you can turn at either side!
www.eveningtimes.co.uk/news/display.var.1965936.0....p
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I know, I know!!!!!
In the past few weeks there has been the wrong type of cold (weather)!!
Thermal contraction etc leading to stress in excess of design. 1 in a million event, honest.
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Bet there was a massive shock loading on the remaining cables when that went.
A difference perhaps from being designed to have a cable removed progressively during maintenance?
Wonder what would have happened if it had been loaded with HGV`s and cars on it in the rush hour?
(Technically I mean, regarding the extra loading on the cables.)
Because surely it was only designed to have a cable slowly unloaded and removed on an empty bridge.
It would be interesting to know its designed maximum loading to contrast with a mechanical failure unloaded.
I wonder if the wind induced some form of loading that is unique to the structure due to its immediate surroundings?
Very lucky all round I think, makes you look at Brunel with even more appreciation.
en.wikipedia.org/wiki/Image:Clifton.bridge.arp.750...g
Wonder how many of our contemporary bridges will be around in 150 years time.
Regards
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It SHOULD be designed to allow for sudden failure of a cable, otherwise you have a single point failure which could cause collapse - not a good idea in bridge design.
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I guess my experience of suspenders is not transferable to bridges...........
Regards ;)
Edited by oilrag on 16/01/2008 at 08:56
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I'll bet there'll be a few white faces when they run the computer model of the loading on the other cables when that one suddenly snapped...
I wonder if they'll ever let on how close to a cascade failure it came - and whether they'll now change all the remaining cables as they have suffered a shock loading.
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I never have liked driving over suspension bridges - I mentally count them off till I get to the other side.
But as the engine driver said in Around the World in 80 days, a swig of whisky and the throttle open will clear most bridges.
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Glasgow did have 90mph winds last week and being a bridge it is very exposed. I think this cable was weakened in some way. Thats my theory anyway.
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Isn't it a little stupid, and dangerous, designing a bridge so a snapping cable WILL fall across the carriageway!
Simpler designs (QE2 and Severn Bridge) do not have this risk - the cable would fall at the side of the roadway.
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Hmm. Just tell me precisely which part of the cable will fail, what the weather conditions are and the loading on the bridge and it might just be possible to predict where it will land.
Failures under tension can go anywhere - and quite a long way sometimes. It does sound like a crappy design (most likely) or a poor execution though.
659.
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Is the root of this failure that form is now placed ahead of function. The designers want to create something delicate and pretty - and pare the sizes down to the bare minimum the computer model says will do the job.
Unfortunately; computers make mistakes too. Usually big ones.
I like Telford's approach: when told a prospective apprentice could help him with his calculations, as he held a degree in maths, he replied " That's a pity; I'll try not to hold it against him."
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>>and pare the sizes down to the bare minimum the computer model says will do the job.
Which is usually OK. The problem comes when either loads which were not analysed and accounted for actually happen, or, if the computer model is inadequate.
>>Unfortunately; computers make mistakes too.
It's extremely rare, computers themselves almost never make mistakes. The people programming and operating them make many mistakes. Garbage in = Garbage out.
The problems with using and relying upon modern systematic analysis, as opposed to old fashioned, excessive, engineering practice include (these are all true whether you use hand calcs or computers);
Over reliance on accepted building codes - the wobbly bridge over the Thames was code compliant, it's just that the code didn't really envisage bridges being built in that way.
Inadequate knowledge of the boundary conditions and loadings that the structure will experience - this is a practical limitation which you can only truly deal with by repeating the calcs for each combination of loading and support. This is costly, and almost never actually done.
Old fashioned bridge engineering rarely used the material effectively, for example, not using the capability of iron and steel to withstand tensile loading. Using these dated approaches, many modern structures, and vehicles!, would simply not be possible.
>>he held a degree in maths
Despite what many nay sayers beleive, it is only by using maths that designs can be systematically improved and optimised. Maths prevents you from performing many costly experiments that would result in failure. You can read tales from engineering of old (say, pre 1960s), where new developments were tried, and didn't work first time, and needed "fixing". Nowadays, with the well developed tools available, most well engineered items simply work first time - much of the debugging has been done cheaply, within the computer model.
The laws of physics, whether expressed by mathematics or not, dictate how engineered items behave, whether bridges or cars.
Number_Cruncher
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I image in a high wind they could fall almost anywhere. Why do they make slates that land on the ground when they blow off?
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seems surprising in the computer age that bridges still provide trouble when built.
Was it the footbridge across the Thames at St Pauls, that resonated to peoples feet and then undulated as though to toss them off.. or at least, give them the creaps?
I mean if a Foot bridge can`t do feet, what hope is there for a fancy `harp string` design other than perhaps playing `Bridge to Heaven` in the howling wind.
Regards;)
Edited by oilrag on 17/01/2008 at 13:28
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Isn't it a little stupid and dangerous designing a bridge so a snapping cable WILL fall across the carriageway! Simpler designs (QE2 and Severn Bridge) do not have this risk - the cable would fall at the side of the roadway.
Yes they do have this risk, one of those cables snaps, it could go anywhere, including across the road.
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To expand a little on the points made by NC and Screwloose, although a properly programmed computer will give the correct result if fed with the correct model or simulated loading information (and that's a very big "if") there are several factors which can catch out the unwary designer - Telford's point, perhaps.
I'll give two as an example, the first of which could and ought to have been spotted in the model.
Resonance as any electrical engineer will know, allows huge deflections to occur with very little applied force. This was the problem with the bouncy footbridge and I'm frankly astonished that the "engineers" were not ridiculed for allowing it to happen. Remember what happened to the bridge at Tacoma Narrows?
Corrosion is a bridge killer. This is clearly unlikely to be a factor in the present case of a new bridge, but there are few if any computer models which will spot a corrosion trap. I'm afraid that unlike Telford, a freshly groomed programmer who has never spent any time climbing around on old bridges and forming an understanding of what can happen to them, stands no chance whatever of avoiding a possibly disasterous design in terms of life. I strongly suspect we have a few of these in the pipeline.
The trouble is, this also seems to apply to car design. I wonder if the plenum chamber detail on my VAG car was designed on a computer...
659.
Edited by 659FBE on 17/01/2008 at 15:21
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Bridge will not open for at least a month according to the BBCwebsite
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That?s what happens when they make a cheap imitation of the millennium bridge on Tyneside ;0)
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>>Resonance as any electrical engineer will know,
Yes, but, in the case of the wobbly bridge, the mode shape associated with the resonance was one which was not envisaged to be excited, footfall excitation being assumed by the code to be primarily vertical, and the bridge codes which the bridge was designed to didn't include it. It's one of the dangers of designing to code, without taking a wider view.
As another example, of the problem of designing to code, railway bogies and axleboxes are typically designed to so-called Group Standards, in particulary, there are acceleration levels which must be withstood. Until we instrumented a train up, and took it over a number of different track types, and found that the Group Standard was not erring on the side of caution, quite a few bogie level failures were unexplained. Although I don't work in rail any more, I think the Group Standard has now been updated with these findings.
Of course, it takes both decent analysis, and decent practical experience to produce good quality items. 100 Telfords couldn't have got us to the moon, and certainly couldn't produce cars of the quality and performance we enjoy now for the price we pay.
Number_Cruncher
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And of course, in normal circumstances, the biggest loading that bridge components will experience will be duirng construction when not all the mutual support is in place. If the assembly is not carried out in accordance with the approved engineering schedule, this can lead to overloading and premature failure.
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The bridge has disappeared from the case studies section on this website, it was there two days ago. Although there are still pictures of it and mentions in the text.
snipurl.com/1xne1
Re: the millenium footbridge, on Arup's website there is some info on the testing carried out post closure and results.
snipurl.com/1xnf6
Edited by Rich 9-3 on 17/01/2008 at 16:20
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Also see
www.arup.com/MillenniumBridge/indepth/pdf/linking_...f
for more on the technical description of the vibration.
Number_Cruncher
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It appears that it wasn't just one faulty component - six months!
news.bbc.co.uk/1/hi/scotland/glasgow_and_west/7207...m
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That's six months just to evaluate.
Don't try to make exceptional excuses - a bridge should be designed and built with sufficient safety margin to cope with any possible conditions.
Since the Tay Bridge disaster, British bridge building had moved on a great deal - until this - it'll cost one of the parties dearly but that's what professional indemnity insurance is for.
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