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The Silver Bridge Disaster: Stresses and strains

Updated Thursday 12th February 2009

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 2: Stresses and strains

Video

Copyright The Open University

Transcript

Narrator
Clustering eyebars together, to make a suspension chain, was a demanding job.

The pinning, or ‘threading’, was itself a difficult undertaking because of the tight tolerance between the pin and the eyebar holes. The material used was standard annealed mild steel which is susceptible to corrosion - like many steels.

Also, each eyebar contained high levels of tensile residual stress from the manufacturing process and that residual stress could be significant at the points where the eyeholes were drilled out.

A combination of tensile stress, a material like mild steel, and the corrosive environment of a bridge exposed to the elements (and industrial pollution), can lead to stress-corrosion cracking.

Incorporating more material in the form of multiple eyebars makes the overall design of the bridge safer against factors such as these.

The construction engineers in Pittsburgh knew what they were doing and could rely on their own tried and tested expertise.

At this time, the American Bridge Company also went on to construct both the ‘Silver’ and ‘Hi Carpenter’ bridges.

However those bridges were engineered by a different designer, who embraced a new high strength, high carbon, heat-treated steel which, presumably, he thought meant that you could build a less substantial structure. Undoubtedly he expected lower ‘live’ loading than in Pittsburgh.

But each bridge was a much longer span and so the loads at the tops of the chains in the towers would have been greater than in the bridges in Pittsburgh. The towers themselves were less substantial structures as well. Yet, the designer felt sufficiently confident in the new material to proceed.

The new design did have a safety factor of 1.5 when they were built but that didn’t account for the increasing weight, and amount, of traffic that each bridge would carry - as cars and lorries were to become heavier and more prevalent with time…

Jack Fowler - a resident of Point Pleasant at the time of the disaster, now runs the local museum.

Jack Fowler
Well, the new bridge, of course, it was owned by local people, and they publicised and promoted it as a very high-strength material bridge. So, the residents had no reason not to doubt that it was not going to be a nice strong bridge, similar, though thinner in design, not the big chunky bridge that existed in some of the areas, but they had complete confidence in it.

And, uh, this nice silver shiny bridge that we had here, uh, people loved it, and we, throughout the 39 years of existence, people had pride in the Silver Bridge.

Narrator
Following the Silver Bridge collapse, it was imperative to identify the cause precisely and identify the safety-critical parts of the structure. The big problem facing the investigators was that 90% of the bridge was submerged in fast flowing river.

Jack Fowler
The next day, people came from every place, the state, the Federal Government, Ohio, West Virginia. They had crews in here, the National Guard. And they were out trying to drag and recover the…recover bodies, and they were bringing in the Corps of Engineer derricks, boats to start removing the steel.

Well, after they recovered bodies, they wanted to reconstruct the bridge... as they pulled pieces out, they numbered them, marked them, and then they took them and laid them all out in a field... to try to find the culprit, what happened, where was the…the failure.

Well, when they recovered this…the 330 eyebar, when they found two pieces, that’s when they started realising that one of those must have fractured and separated or blew apart and caused the failure. So they focused on that. And I think, from the investigation, that appears to be what happened, we, that’s the analysis that we received about the failure, and that’s the one we promote and talk about here at the museum.

Narrator
Most of the material that was recovered from the scene has long gone but the museum did save at least a sample of an eyebar assembly; albeit one that has been cleaned and painted to look like new.

Pete Lewis
This is a typical eyebar joint from the Silver Bridge, which was rescued after the disaster. Um, it comprises a central pin over which the eyebars would have been pivoted, and the whole assembly is encapsulated by the very solid end caps, which themselves are attached by one-inch bolts through the centre of the pin.

There’s some interesting evidence of pitting corrosion on the surface, the bearing surfaces. This would be the bearing surface of the pin over which the, uh, eyebars would rotate, and you can ... there is considerable pitting under the track of the one of the outer eyebars, and also ... underneath the cap, even deeper corrosion ... pitting caused during its lifetime on the bridge itself.

Narrator
So what were the weather conditions on that evening of December the 15th 1967 and how was the bridge being used at the time?

Charlene Wood
The weather was kind of chilly. There were snow flurries coming, and it was getting dark. It was around, uh, well it was time when everyone was getting off work, in the evening time.

Jack Fowler
Traffic lights on a bridge change the patterns of the traffic flow. Once it changes then you’ve got all the traffic that’s backed up, and they come, a wave of them rush across the bridge, and whatever is in that line, that’s the load that you’re going to get at that time. And it was on both sides and it worked both ways.

So as you can tell from our model that there were a great many coming through…through from that last change of light in Point Pleasant.

Charlene Wood
When there was heavy traffic on the bridge, there was a motion of going up and down, but I was told that that was normal, so I wasn’t afraid of the bridge.

Jack Fowler
The bridge, I don’t think anyone ever crossed it that they didn’t feel movement. Uh, that was a discussion of the community, of the area, because it… there was always swaying. It had the up-and-down motion from so much weight on it, and everybody always said wow this bridge is going to fall someday. But then you look back at the design, it was a different design, and we felt that it… because of the design it had that built-in motion so to speak. So, you talked about it, but did you worry about it, no, but the motion was there, we experienced that personally.

Pete Lewis
This is where the bearing surface would connect with the pin, and it’s, uh, from roughly a position, um, a…at right angles to the shank of the bar where the critical brittle crack, which brought the bridge down, actually started.

On the inner bearing surface, there is extensive pitting corrosion, very similar to the one on the corresponding part of the pin ...and also traces of fretting marks caused by particles of rust wearing away the surface as the bearing moved.

Narrator
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.

In the third and final part of the video, we examine the collapse and look at its lasting legacy for America.

Audio

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