The Point Pleasant Bridge Disastor
The Collapse of “Silver Bridge”
Group 19
James Gannon
Barry Maxwell
Kate O Dea
Point Pleasant Bridge Disaster
Introduction
The Point Pleasant Bridge was designed under the specifications of
the American Society of Civil Engineers and constructed, in 1928,
by the General Corporation and the American Bridge Company.
The U.S. Highway 35 bridge formerly linked Point Pleasant, West
Virginia and Kanagua, Ohio.
The bridge spanned 2,235 feet (0.7km) and consisted of two lanes,
measuring 22 feet (6.7m) in width, and one 5 foot (1.5m) footpath.
Details of Bridge
The structure was an eye bar chain suspension bridge and was
referred to as the Silver Bridge for its aluminium paint. The links
were composed of only two bars each, manufactured using high
strength steel. With only two bars the failure of one could impose
excessive loading on the other, leading to complete failure of the
structure, which is unlikely if more bars are used. While low
redundancy chains are cost-effective than say a cable-wire
suspension bridge, the safety is completely dependant on accurate
manufacturing and assembly processes.
The high strength steel developed by The American Bridge
Company and used in the eye bars was a new heat treated carbon
steel which, in theory, would allow the bridge to handle more
stress. This high strength, coupled with low redundancy balanced
out according to the designers.
The Towers
The 130feet (40m) towers used in the structure were “rocker”
towers, which allowed the bridge to respond to live loads and
changes in the length of the chains due to temperature variations.
This was done by placing a curved fitting next to a flat one at the
bottom of the piers. The rocker was then fitted with dowel rods to
keep the structure from shifting horizontally. With this type of
connection the piers were not fixed to the bases.
Anchorage System
The anchorage design for the structure was also innovative.
Ordinary gravity type anchorage was impractical at the site because
the bedrock was at too great a depth. The anchorage system
developed consisted of a reinforced concrete trough 200feet by 34
feet wide (61m x 10.4m) filled with soil and reinforced concrete.
The trough was supported on 405 16inch (40cm) octagonal
reinforced concrete piles.
The Collapse
The Silver Bridge allowed people to cross the Ohio River for 39
years. However, on 15th December 1967 at 5pm the bridge
suddenly collapsed. At the time of failure thirty seven vehicles were
crossing the bridge span and thirty one of those vehicles fell with
the structure. The death toll was forty six and nine people were
seriously injured. Most of the bridge's superstructure was hidden
beneath the water. In midstream only the concrete piers remained
as stark evidence of the total collapse of the structure.
Wreckage Analysis
Wreckage analysis showed that the failure of the structure was due
to a defect in a single link, eye bar 330.Fretting wear at the bearing
caused a small crack to form, which extended due to internal
corrosion, a phenomenon referred to as stress corrosion cracking.
Growth of the crack was also exacerbated by residual stress in the
eye bar created during manufacture. The lower side of the eye bar
failed and all the load was transferred to the other side of the eye
bar, which then failed by ductile overload. The joint was then only
held together by three eye bars, but another slipped of the pin at
the centre of the bearing, so the chain was completely severed. The
remaining steel frame buckled and fell due to the due to the newly
concentrated stresses.
Causes of Collapse
A major factor that caused the collapse of the Silver Bridge was the
increase in weight of more modern vehicles. When the bridge was
being designed, the design vehicle used was the model T Ford,
which had an approximate weight of less than 680kg. In 1967 the
average family car weighed 1800kg or more. The projected life span
for the project did not consider that traffic loads would more than
triple.
Another significant factor was the inspection difficulties associated
with the bridge. The only way it would have been possible to view
the fatal crack would have necessitated the disassembly of the eye
bar. The technology used for inspection at that time was not
capable of detecting such cracks.
Aftermath
The collapse focused much needed attention on the maintenance
and inspection of older bridges. Inspection protocols and were
developed and many unsafe bridges were replaced.
References
"The Collapse of the Silver Bridge", LeRose C.,
West Virginia Historical Society Quarterly Volume xv, No. 4
(October 2001)
Bridge Management and Nondestructive Evaluation
J. Perf Constr. Fac., Volume 19 Issue 1, pg. 3-16 (February 2005)
Policy and Procedure Manual: Fracture Critical Bridge Inspection
Policy for Local Units of Government
Reliability, Quality and Safety for Engineers, B.S. Dhillon
Practical Fracture Mechanics in Design, Alexander Blake
Why Buildings Fall Down: How Structures Fail, Matthys Levy, Mario
Salvadori, Kevin Woest
Structures: Or Why Things Don’t Fall Down, J.E. Gordon