The Silver Bridge Disaster: The collapse
The 1967 collapse of the Silver Bridge was a human tragedy -...
The 1967 collapse of the Silver Bridge was a human tragedy - 46 people died - and an engineering mystery: why did a bridge built to last a century not make 40 years? Part 3: The collapse
- Duration: 10 mins
- Published on: Wednesday 8th August 2007
- Introductory Level
- Posted under: Engineering
VideoYou need the Flash Player (version 7 or higher) to view this clip - download Flash. http://podcast.open.ac.uk/open2media/forensicengineering/silver3.flv Copyright The Open University
The Silver Bridge collapsed because of a minor flaw in just one of the eyebars that formed the suspension chain.
The eyebar that failed, number 330, was positioned on the Ohio end of the bridge on its northern side.
Back at the West Virginia end, Charlene Wood was approaching in her car. But, with the failure of that eyebar, she was heading into trouble.
As I stopped, the bridge was coming forward like dominos, swaying back and forth, but the tower went to the north when it fell, and as I was seeing all of this, unbelievably, didn’t know what was really happening. I did know what was happening but it just hadn’t registered yet
So what did the forensic investigation conclude had happened to make the bridge fall?
Well, eyebar 330 was defective because it had particularly high levels of residual stress left after its manufacture.
The design of the eyebar assembly meant that water could pool at the bottom of the eye-hole, and the combination of the tensile residual stress and a corrosive environment had caused a stress corrosion crack to form. Hidden away, the crack had grown slowly, over 39 years, until it was about three millimetres long.
Also, the steel used in the eyebars had a low toughness at the near-freezing temperatures on the night of the disaster, making it susceptible to brittle fracture. Under a combination of the high live loading on the bridge and the reduced toughness of the steel, the relatively small crack caused a brittle fracture of the eyebar.
At the point of failure, a brittle crack grew almost instantaneously down to the outer edge. This overloaded the upper side which separated with some signs of ductility.
The resulting asymmetric load on the pin caused it to twist, and the single eyebar that was left vibrated off the other side of the pin, at which point the chain was completely severed.
The adjacent tower, being destabilized, started toppling and fell to the North. The road, below, twisted over and the other tower was pulled down into the river as well.
What, uh, what we thought about after the event was how it was inspected, and if it was and you found something, then what do you do to replace it, how do you…how do you get in here and replace one of these eyebars, or one of the joints if suddenly there is corrosion or you sort of found failure, and how do you go about replacing that.
But I guess that…that the people at the time had so much confidence in the process of this new high-strength steel that that wasn’t a fear, and I’m sure they built in factors, but, you know, we found out later that it wasn’t as purported to be.
Following in the wake of the Silver Bridge disaster, one immediate legacy was that the Hi Carpenter Bridge was closed.
The sister bridge, it had to be closed because it was a similar design, the same company built it. Um, we know now that, uh, they knew there wasn’t any way to inspect it and to correct any findings that they might have. So public opinion, uh, almost required that bridge to be closed to vehicular traffic.
So I sneaked through the bars of the barrier and walked across the bridge and the people that were waiting to get some way to cross, they just followed me like little chicks following a mother hen and there were just a trail of us but after Dick asked me, he said, “Aren’t you afraid to cross the bridge,” just to show you how the people were affected by the falling of the Silver Bridge, that they thought that bridge could fall at any time.
I suppose a case could be made for not having closed the St Mary’s bridge as soon as they did. It had different use, you know, it didn’t have the traffic lights at the end, it didn’t have the interstate traffic that our bridge had here, uh, the tractor trailers, all the heavy loads that it carried.
There could have been means of inspecting, uh, to find if there were failure, if there was failure on the bridge, if it could be used in a different manner, but I believe that public opinion would not have accepted that, there was too much pressure because of this terrible disaster we had here, I think public opinion would have overridden whatever they may have felt.
They couldn’t get it out of their minds, the fate of this bridge was sealed. The National Transportation Safety Board had no other recourse than to say they had to condemn the bridge because they couldn’t prove it was safe, and they were no doubt right in coming to that conclusion.
So, apart from the subsequent demolition and removal of the Hi Carpenter Bridge, the Silver Bridge disaster did at least have a lasting legacy in terms of bridge safety in general.
When the Silver Bridge collapsed in 1967, President Johnson established National Bridge Inspection Standards, which are the guidelines that are used throughout the United States for all bridges that are inspected.
The National Bridge Inspection Standards require that every bridge be inspected on a two-year frequency, and if that bridge has any problems, it’s increased to 12 months, or it could be once a month depending on how severe the problems are with that structure.
The Three Sister Bridges have already been inspected over 20 times and they will continue to be checked for safety…
In the past the designers had designed this bridge to last 100 years with a factor of safety around two, and due to the heavy loads that are travelling across the bridge today, that factor of safety has probably gone down, but it’s still a safe bridge to travel.
To replace these particular eyebars because they’re in tension, you’d have to design another support system to support it while you’re removing it. So you’d actually build some false work, it would be another bridge next to the exact same bridge that you have, and then remove that false work when you’re all complete.
The different techniques that we use to inspect the steel members would be, um, non-destructive testing where we can x-ray the metal or we can use a sonogram where you use a gel over top of the metal with a sound probe.
In the near future we’re going to make some minor repairs to hold us over until we have time to develop plans for a major rehab on the bridge, and that major rehab we’re going to replace the deck... and all the steel members that are deteriorated and paint the structure over again.
In America alone, there are over one million bridges and thanks to Silver Bridge, they all now receive regular inspection and maintenance. In the case of the Three Sister Bridges, redundancy was built-in from the outset in the form of additional eyebars, reducing the criticality of highly stressed joints.
It’s just as important today for those engineers responsible for designing and maintaining bridges to be aware of the need for redundancy, where the inevitable weakest links occur in a structure. And also, knowing those weakest links, to protect them from the effects of corrosion and fatigue and thus ensure the integrity of the structure.
Copyright & revisions
Originally published: Wednesday, 8th August 2007
Last updated on: Thursday, 12th February 2009
- Body text - Copyright: The Open University
- Video - Copyrighted: The Open University
- Audio - Copyrighted: Used with permission
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