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The Silver Bridge Disaster: Eyebars in suspension

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 1: Eyebars in suspension

Video

Copyright The Open University

Transcript

Narrator
It’s 1928 and suspension bridges are being built all across America. New designs and new materials make for rapid construction.

This is the Silver Bridge, crossing the Ohio River at Point Pleasant, West Virginia on its opening day.

Walter Carpenter My father was Vice-President of the corporation that built the Silver Bridge.

It was a wonderful time to be a young American boy. The Roaring Twenties, they called it, and America had…had never been as prosperous, they just thought nothing but the best in the future and a wonderful time for America.

The St Marys Citizens Band marched at the opening of the Silver Bridge, and my father and mother went to that opening, and it was supposed to be a very gala event, but unfortunately right in the middle of the parade, to have rain and everybody was running and trying to get out of the rain, it dampened the fervor of the thing and the historic impact of it.

Narrator
Shortly after work commenced on the Silver Bridge another bridge, almost identical in design, was constructed at St Marys, about 70 miles upstream from Point Pleasant. The bridge was formally named the ‘Hi Carpenter’ bridge.

Walter Carpenter
It was really ready for a big thing to happen at St Mary’s, and people were just rousing to welcome this new thing called a bridge crossing the river at St Mary’s.

Narrator
In a suspension bridge, the uppermost supporting chains, strung from tower to tower, are members in tension and they exert a downwards force on the towers. The deck is supported from the cabling system using a series of vertical hangers. These hangers are also in tension.

The bridge should be designed so that the degradation of any one tension element of the structure doesn’t immediately lead to collapse.

Nowadays, suspension bridges use cables spun from many individual wires but, in fact, the suspension chains in the Hi Carpenter and Silver bridges were formed out of long lengths of steel, with holes drilled out at either end. These ‘eyebars’ were put together in much the same way as the links in a bicycle chain.

A bolt is used to join the eyebars together. The resulting joints in the suspension chain can then move in response to the forces placed on them.

Walter Carpenter
The bridges were both painted with an aluminum colour, and described as a beautiful silver colour, although the St Mary’s Bridge never lost the name, the Hi Carpenter Bridge, it always had that, never alluding to the paint at all but the Silver Bridge was very proud of that silver sheen that they got from their aluminum coat, and when the bridge was even repainted, every time, so far as I know, always silver, or aluminum, but always referred to as a silver bridge.

But the word aluminum and the fact that the Silver Bridge will be painted aluminum, I think, had more of an impact on the psyche of the public than did the fact that it was steel.

Narrator
But after barely 40 years, the design and materials used came to haunt them. On December the 15th, 1967, the bridge fell in less than one minute, with the loss of 46 lives.

A 20 year-old eyewitness at the time, was Charlene Wood.

Charlene Wood
I was actually on the bridge when it fell that night. I was going home from work, and a trembling an…of the bridge and a…a noise that you couldn’t…I couldn’t even describe what the noise was like, but I realised that maybe something had hit the bridge, and I decided I wasn’t going to cross it, so I threw the car in reverse, and as I was backing, the car stalled on me, but my…it was on an incline that... it kept going, and when I was able to get it stopped, the bridge had fell in front of me, and my wheels were on the ledge here.

Narrator
What was the cause? Was it an accident, carelessness, or inadequacy in the design?

The problem couldn’t just be that this bridge was a suspension bridge using eyebars because that was a known technology with many contemporary examples.

The city of Pittsburgh has three suspension bridges spanning the Allegheny River. Known collectively as the Three Sister bridges, all employ eyebars in their suspension chains. They were constructed at much the same time as the Silver Bridge and are clearly still standing. They do, however, have a marked difference in their design.

This one is the Sixth Street bridge. The eyebars are configured together in clusters - meaning that several eyebars are used to form each chain and so the failure of any one eyebar won’t precipitate a collapse of the structure. In fact, the steel used here is of a lower strength than that used in the Silver Bridge but these bridges are said to have a safety factor of at least two; that means that they’re designed to support more than twice the greatest expected load.

Tom Vena
The Three Sister Bridges were built in 1928, and they were expected to last 100 years, and at the rate that they’re going now, I’d expect them to last 125 years.

The major difference between the designers of the past and the designers you have today, the members that were designed on…on steel structures were over-designed where the steel members only needed to be an inch today, well back then they would make them an inch-and-a-half, so you had a…that half inch of material that could actually deteriorate before it even impacted the structural capacity of the bridge.

Narrator
With each eyebar weighing several tons, assembling them into suspension chains was no easy undertaking. But clustering them together had become a time honoured technique because, that way, a degree of redundancy comes from the way that multiple eyebar assemblies provide multiple load-paths.

Photographs from the time of construction, along with the engineering plans, are still preserved in the local county archive….

Bill Connery
This is a plan view of the Sixth Street Bridge, it’s approximately 995 feet long and 77 feet high and it gives general notes to the contractor or the erector on the assembly of the plan in giving the manufacturer’s name of the American Bridge Company.

Narrator
The Pittsburgh-based American Bridge Company was able to employ the practice of building from either bank using a cantilever principle. It was necessary to stabilise the eyebars in each arm with additional diagonal braces – until, that is, the two arms met and the whole structure became independently stable.

Each eyebar cluster was, of course, clearly specified in the engineering plans.

Bill Connery
This sheet here shows you the assembly of the eyebar and the number of bars that are in the assembly which the pin goes through.

Narration
Once assembled, multiple eyebar chains need to be used, for safety, because there’s always the danger of failure in an individual eyebar - as you can see in part two.

Audio

Copyright Used with permission

Part two: Stresses and strains >

 

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