Friday, May 24, 2013

Silver Bridge Disaster December 15, 1967 What Happened?

The Collapse of the Silver Bridge:
NBS Determines Cause

Based on Richard Fields', The Collapse of the Silver Bridge, NIST Metallurgy Division, 2009

What Happened?

On December 15, 1967 at about 5PM the traffic signal at one end of the Silver Bridge turned red. The rush hour traffic, together with the Christmas shopping traffic, completely occupied the main span of the bridge connecting Point Pleasant, West Virginia with Kanauga, Ohio. Suddenly a loud cracking sound was heard and one of the main towers began to twist and fall.
In less than a minute, all three spans of the bridge collapsed into the icy Ohio River, carrying with them all the cars, trucks, and people. Forty-six died and many more were injured.
[Shown is a scale model reproduction of the actual bridge from the physical exhhbit inthe NIST Museum]
1. Scale model reproduction of the actual bridge from the physical exhibit in the
NIST Museum.

What Caused the Bridge to Fall?

The Silver Bridge (so named because it was painted silver with aluminum paint) was an eyebar suspension bridge. Instead of the cables that are so familiar on modern bridges, the highway was suspended by eyebar chains.
An eyebar is a long steel plate having large circular ends with an "eye" or hole through which a pin is used to connect to other eyebars (to make a chain) or to other parts of the bridge. The pins are held in place with bolted cap plates.
A unique feature of the Silver Bridge was that the eyebar suspension chains also were part of the top chord of the stiffening trusses and only one eyebar per suspension point was used.
This design results in light weight, cost-efficient construction, but depends on high strength steel eyebars. Only two other bridges were built with this efficient design: the St. Mary's, West Virginia bridge and a bridge in Florianopolis, Brazil. The high strength steel eyebars required only one-half of its ultimate strength to support the fully loaded bridge. So, why did the Silver Bridge fail?
Inspection of the Wreckage It was found that an eyebar had broken through the eye. The nature of fracture - a flat, brittle fracture on one side and a fully flexible bending hinge point failure on the other - indicated that this eyebar failed under normal services loads and initiated the collapse.
[An arrow points to the bridge's failed eyebar, which is located to the right of the tower]
2. An arrow points to the eye at the right of the tower (see picture 1. above) where
the fractures occurred.

The U. S. Army Corps of Engineers recovered both pieces of the eyebar (as well as most of the bridge) from the river and brought them to NBS for analysis. NBS was selected for this important phase of the investigation because of its competence in metallurgy and long experience in failure analysis.

Stressed to the Ultimate Visually, the two fractures on opposite sides of the broken eye (see picture above) are quite different. On one side, the fracture was straight across, indicative of a brittle fracture below the normal service load and caused by a flaw. There is no thinning or bending associated with this part of the fracture. It broke in a brittle fashion like a piece of glass. Furthermore, this part of the eyebar shows no stretching or bending that would have occurred in steel that was stressed to its ultimate strength.
On the other side, is a plastic bending hinge that indicates that this side was stressed to its ultimate strength and beyond. This must have occurred after the brittle fracture on the other side shifted all the load it was carrying to this side, essentially doubling the stress on this side, resulting in the visible, extensive deformation before failing.
Suspension Chain Unpins, Tower Rotates Once this eyebar failed, the pin fell out, unpinning this part of the suspension chain. The adjacent tower was subjected to an asymmetrical loading that caused it to rotate and allow the western span to twist in a northerly direction. This span crashed down on the western shore, folding over on top of the falling cars and trucks. Loaded by the whole weight of the center span, which had now become unsupported on its western end, the east tower fell westward into the river along with the center span. Finally, the west tower collapsed toward Pt. Pleasant and into the Ohio River, completing the destruction of the Silver Bridge.

Why Did the Eyebar Break?

The eyebar broke at a load below what was normally expected to occur. Examination of the brittle fracture surface that initiated the failure of the eyebar (and the ultimate collapse) revealed two small pre-existing cracks. These cracks were covered by an endcap that held the pin in place and could not be seen or detected during the regular inspections or while painting. (The last inspection was two years before the collapse.)
[Shown is a photo of the actual eyebar]
3. Photo of the actual eyebar, on display in the NIST Museum.
Extensive studies at NBS indicated that these small cracks started as corrosion pits. Under the action of the applied loads, rainwater combined with sulfur from air pollution to grow these pits into the cracks.
In addition, it was found that the high strength steel eyebars made in 1927 had a relatively low toughness - the property that makes steel resist fracture from cracks - and cold exacerbates this sensitivity to cracks. The longer the crack, the less stress is required to cause fracture and, in a sensitive steel, only a short crack is required to cause brittle fracture.
[Close-up of the eyebar shown above]
4. Close-up of the eyebar, shown in photo 3 above, on display in the NIST Museum.
Finally, on that cold evening in 1967, the crack length (between 3 mm and 4 mm), the load on the bridge, and the weight of the bridge itself combined to reach a critical condition. The fracture that led to the catastrophe began.

What Was Done As a Result of the Collapse?

The fact that the failure of one component led to the complete collapse of this bridge meant that other bridges might be at risk. A Presidential Commission was appointed by President Lyndon Johnson. They reviewed the design of similar bridges and made recommendations.
The sister bridge at St. Mary's, West Virginia was immediately closed. It was then torn down and replaced with one of modern design. Bridge design was improved to assure structural redundancy (that is, more than one part must fail before collapse becomes possible).
A program was started to identify bridge steels that might be particularly susceptible to slow flaw growth and avoid their use. Another program was started to develop a new generation of inspection equipment to detect flaws on heavy structures like bridges.
One year after the collapse, the U.S. Department of Transportation established standards for proper safety inspections of bridges and training programs for bridge inspectors. The eyebar suspension bridge at Florianopolis, Brazil was also closed to traffic, but still stands to this day. 

Other References

Collapse of U.S. 35 Highway Bridge
Point Pleasant, West Virginia December 15, 1967
NTSB Highway Accident Report 71-01
National Transportation Safety Board, 1971.
[Report Summary]
Metallurgical Aspects of the Failure of the Point Pleasant Bridge
J.A. Bennett and Harold Mindlin
Journal of Testing and Evaluation, Vol. 1, No. 2, pp.152-161. March 1973.
Point Pleasant Bridge CollapsePDF copy of the manuscript
National Bureau of Standards Technical News Bulletin
Vol. 55, No. 8, pp.196-197. August 1971.
Wikipedia - Silver Bridge

Date Created: July 8, 2009

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