Tuesday, December 15, 2015

Silver Bridge, Deep Waters


December 15, 1967 the entire 1460-foot suspended portion of the Silver Bridge over the Ohio River at Point Pleasant, West Virginia collapsed.  46 lives were lost.  Two of the bodies have never been recovered. 

The bridge was suspended via eye-bar links.  I have attempted to understand this and in spite of the available information my mind seems unable to grasp the concept. Multi-strand cables I understand.
"The bridge failure was due to a defect in a single link, eye-bar 330, on the north of the Ohio subsidiary chain, the first link below the top of the Ohio tower. A small crack was formed through fretting wear at the bearing, and grew through internal corrosion, a problem known as stress corrosion cracking. The crack was only about 0.1 inches (2.5 mm) deep when it went critical, and it broke in a brittle fashion. Growth of the crack was probably exacerbated by residual stress in the eye-bar created during manufacture.The bridge was a victim of insufficient redundancy."  --Wikipedia
"Insufficient redundancy" I understand.  It is to say that there were not enough back-up or fail-safe bars built into the design.   In his 2012 book To Forgive Design, engineering historian Henry Petroski writes
 "If there is anything positive about the Silver Bridge failure it is that its legacy should be to remind engineers to proceed always with the utmost caution, ever mindful of the possible existence of unknown unknowns and the potential consequences of even the smallest design decisions."
Application may have a much wider scope than the discipline of engineering design.  "Unknown unknowns" lurk around the edges of every decision we make.

Words for life:  "Utmost caution" and "potential consequences"

2.5 fatal millimeters


Secondary Roads said...

It's not just for engineers. Each of us should be wary of "unknown unknowns."

vanilla said...

Chuck, exercizing caution in making judgments is desirable. Yet it seems that it is possible to be so rational that one becomes irrational. Case in point: my eldest child will not travel cross country by automobile lest she has to cross a bridge, the unthinkable of which she is terrified.
Your mileage may vary; hers is severely limited.

Grace said...

Speaking of bridges - you've heard of The Chesapeake Bay Bridge? They have people on site who will drive you across - one of the scariest bridges in the world so they say.

Utmost caution about potential consequences, while often wise, can result in no action at all.

vanilla said...

Grace, at over four miles I could see that as a "scary" bridge. The longest one I have driven across-- numerous times-- is over the Atchafalaya Basin in Louisiana. It is just over 18 miles in length, but lacks the height above water that would induce fear.

Lin said...

Yikes. I'm not usually scared to go over bridges...but now I am after reading this.

Grace makes a good point.

vanilla said...

Lin, don't adopt another phobia. Paralysis is the result, eventually. Hang in there and drive on across that bridge!