This showed up tonight on stuff. Still nothing new as far as I can tell, except to re-state Ruth Dyson's call for a safety audit. http://www.stuff.co.nz/national/71786334/Safety-audit-urged-after-Waikeremoana-bridge-cable-gives-way
This post has been edited by the author on 6 September 2015 at 19:56.
@wayno
Basically you are right. Multi-stranding wire rope is done for several reasons; one to increase flexibility, the other to reduce the risk of progressive fatigue causing catastrophic failure.
That's why for instance, the ground anchor components (which are usually fixed and rigid) can be made of solid bolts or rods - but the bridge wires which need to move and be flexible are made of stranded wire.
Now any design is going to have a substantial safety margin. If for instance the total loads (static and dynamic) peak at say 2000kg, then the engineer is probably going to spec something good for 3000-4000kg. Maybe more.
This means that in a wire rope with say 40 strands, maybe 20 of them could fail before the structure was in any danger. Of course beyond this as each new strand failed excessive stress is now placed on the remaining strands - and at some point the rest will fail almost all at once.
So the short answer to your question is that I would expect any damage to a wire rope to be quite visible and obvious long before the bridge there was any risk of structural failure.
As I said above - this is not a clear-cut design failure. The bridge has functioned for many years, and the remaining cable held despite the massive increase in load it experienced at the moment it's partner 'released'. Logically this means there was plenty of design margin.
In principle the real enemies of a structure like this are metal fatigue and corrosion. And yet while I stand to be proven quite wrong here, it's my gut feeling that neither of these are likely candidates in this case. There are lots of these materials in use in many bridges around NZ and we haven't been seeing this kind of damage that any regime of regular inspection should be looking for. For corrosion or fatigue to show up the first time as a structural failure, rather than by inspection, would have to be very bad luck indeed.
(Apologies to any real civil engineers reading this - my coursework in this topic is well past it's safe by date.)
This post has been edited by the author on 6 September 2015 at 23:10.
"As I said above - this is not a clear-cut design failure. The bridge has functioned for many years, and the remaining cable held despite the massive increase in load it experienced at the moment it's partner 'released'. Logically this means there was plenty of design margin."
I'm also not a civil engineer and nor have I done any course-work, but I'm curious about how this works.
How much does the bridge itself weigh, or the relevant part of it, compared with a typical load of, maybe, the 10+ people it's designed to carry?
If cables A and B are expected to hold X kg of bridge and Y kg of people between them, then Y kg of people jump on and cable B gives way (because it can't successfully hold its half share of the total, being (X+Y)/2 kg, in its current flawed state), then everyone falls off, then the remaining cable A only needs to hold Xkg -- the weight of the bridge -- because the people are no longer a problem.
If X (the bridge) was <Y (the people) to begin with, then cable A isn't being tested with as much strain as what caused cable B to give way.
In any case, I guess they're probably fortunate that both cables didn't give way and the entire bridge didn't happen to fall on top of them after they fell into the river. That's yet another way something like this could have turned fatal, after flooded river, shallow river, being over land, being unable to swim, being incapacitated by or fatally tangled in the bridge on the way down, and the list goes on.
A design failure doesn't seem likely to me either, if only because it's survived so long and must have already been seen by so many engineers. But this bridge was supposedly built in 1994 according to one of the media reports, which was during DOC's dark phase for designing and building things, and exactly the same time that the CCP was designed and built in another DOC silo. I've wondered if it's a possibility.
[edit: oops, the comment above was a flawed attempt to post the one below. Apparently this forum didn't parse my input very well and cut off the comment, so I've edited and posted again.]
"As I said above - this is not a clear-cut design failure. The bridge has functioned for many years, and the remaining cable held despite the massive increase in load it experienced at the moment it's partner 'released'. Logically this means there was plenty of design margin."
I'm also not a civil engineer and nor have I done any course-work, but I'm curious about how this works.
How much does the bridge itself weigh, or the relevant part of it, compared with a typical load of, maybe, the 10+ people it's designed to carry?
If cables A and B are expected to hold X kg of bridge and Y kg of people between them, then Y kg of people jump on and cable B gives way (because it can't successfully hold its half share of the total, being (X+Y)/2 kg, in its current flawed state), then everyone falls off, then the remaining cable A only needs to hold Xkg -- the weight of the bridge -- because the people are no longer a problem.
If X (the bridge) was < Y (the people) to begin with, then cable A isn't being tested with as much strain as what caused cable B to give way.
In any case, I guess they're probably fortunate that both cables didn't give way and the entire bridge didn't happen to fall on top of them after they fell into the river. That's yet another way something like this could have turned fatal, after flooded river, shallow river, being over land, being unable to swim, being incapacitated by or fatally tangled in the bridge on the way down, and the list goes on.
A design failure doesn't seem likely to me either, if only because it's survived so long and must have already been seen by so many engineers. But this bridge was supposedly built in 1994 according to one of the media reports, which was during DOC's dark phase for designing and building things, and exactly the same time that the CCP was designed and built in another DOC silo. I've wondered if it's a possibility.
This post has been edited by the author on 6 September 2015 at 23:24.
@izogi
Good questions.
All structures need to consider; dead load - the weight of the structure itself; live load - the weight of the occupants, or traffic it must carry; and the dynamic load due to wind, oscillations, earthquakes and shock loads.
I'm making the assumption this bridge didn't completely collapse, rather one of the two main cables failed and the entire dead load of the structure finished up hanging off one of the cables instead of being shared by two.
Of course this meant the walkway tilted dramatically and four of the 'live loads' tipped off.
So you are correct, the dead load was doubled on the remaining cable, but the live load was effectively lost.
But what is much harder to quantify is the shock dynamic load on the remaining cable as everything bounced around wildly and all the stresses got redistributed dramatically.
It's easy to estimate the live load as say 10 people @ 120kg = 1,200kg. I don't have a good feel for the dead load - but it would have to be at least 3000kg, maybe 6000kg. That would depend a lot on the nature of the footdeck, whether it was light weight wire or timber.
So I'd guess that for this bridge maybe the dead load dominated and yet the remaining cable held it - along with the unknown shock loads that were generated at the moment of failure.
By comparison the loss of four people from the live load at a total of maybe 500kg seems modest by comparison. Or to put it in your terms above, X (the bridge) > Y (the people).
Well that's my gut feeling at the moment. I stand by to be corrected by someone with actual facts.
This post has been edited by the author on 7 September 2015 at 00:48.
Here's a new NZ Herald report. http://www.nzherald.co.nz/nz/news/article.cfm?c_id=1&objectid=11509158
There's still not a lot new in there except to state that a couple of investigations are starting. The entire information stream here continues to seem very tied to formalised media releases.
For those with an engineering bent, the Tracks and Outdoor Visitor Structures Handbook HB8630:2004 details the standards and loadings for structures and materials in the outdoors. A suspension bridge would be considered a cable structure.
A copy of which can be found here:
http://hutsandtracks.org.nz/wp-content/uploads/2014/09/SNZ-Tracks-and-Outdoor-Visitor-Structures-Handbook.pdf
Well, I've put an OIA request into DOC to ask for copies of any photos it holds of the Hopuruahine Bridge after its failure last week. I'd expect DOC must take and keep photos like this on record, and I'll report back if I get anything.
I'd expect it might be declined on the grounds that the information's about to be released as part of the upcoming report(s), but it might still be useful if the report doesn't include photographs, or not all representative photographs.
It might also be declined if there arent any photos.
Can you imagine them sending someone with instructions to be the scapegoat that sweeps it under the table or just sending him with the tools to fix it and a camera but no batterys
They might, but if they said that no photos exist then I'm going to the Ombudsman because I'd struggle to believe it. It might get overtaken by events in any case if an investigation report comes out with good photos.
I've basically just said I'd like them because the general media's proved itself incompetent with a camera, in so many words. I've generally found that DOC's quite good with this sort of thing, but legally it still has 20 working days to respond. I just hope nobody in the chain decides it's overtly sensitive or embarrassing and starts messing around trying to avoid responding.
