The Mianus River Bridge Collapse
4A6 (1) Structures Report
Group 11
Peter Daynes
Mark Flanagan
Ciaran Markey
The Mianus River Bridge Collapse
Background:
On June 28th 1983, a 100-foot long suspended span of Route 95 Highway Bridge
over the Mianus River in Greenwich, Connecticut collapsed and fell 70m into the river
below. This engineering disaster claimed the lives of three people and injured another
three. The Mianus Bridge, on the busy interstate highway 95 between Connecticut and
New York is a multi-span structure consisting of two side-by-side three-lane bridges of
cantilevered construction. It has two 30m suspended spans on either side of a central
span, as shown below. On the morning of the 28th June it was the east end of the
eastbound suspended section that failed.
Design:
A pin and hanger assembly had attached the east corners of the suspended span
in question to the steel girders of the cantilever arm of an adjacent anchor span. The pin
and hanger assembly includes an upper pin attached through the two half-inch thick web
of the girder of the cantilever arm and a lower pin attached through the two half-inch
thick web of the girder of the suspended span. The Western corners of the span were held
in place by a pillow block cradle, on a horizontal pin attached to the end of the central
span cantilever arm.
The problem with this particular pin and hanger design is that the failure of a
single pin or bar could cause the span to fall; it had no redundancy. An indication of this
is that this particular method of construction had been recognized unsafe since 1968 and
no longer used in new construction. However the Mianus Bridge was constructed in 1957
when this method of construction was widely used. Later pin and hanger assemblies used
a second hanger in case of failure of the first.
Reason for Failure:
Although this design was flawed, another reason for the failure of the structure
was the deferred maintenance procedures in place at that time. Corrosion caused the pin
and hanger assembly to fail. This corrosion had accumulated throughout the 25 years
since the bridge’s construction with the constant wetting due to damp conditions and the
paving over of the floor drains. Lateral forces were applied to the hanger plates every
time traffic passed over the bridge. Finally, the southeast corner could no longer
withstand this lateral force due to the weakening of the plates by corrosion and a
restraining cap simply popped off causing a transfer of load to the outside hanger plate,
which could not hold the added load and failed.
Warning Signs:
There were many warning signs that a failure was imminent. Many locals living
on the banks of the river had reported to the Department of Transportation that pieces of
concrete and metal from the bridge had been found. Also, shrill noises could be heard
coming from the bridge. On the weekend before the failure a high pitched noise
accompanied the rumble of the traffic crossing the bridge.
The maintenance reports for the bridge also overlooked warning signs. The latest
report which was carried out less than a year before the collapse did not take into account
the heavy concentration of rust which had formed on various parts of the hanger
assembly which was easily identifiable from 6m away.
After the failure, the maintenance reports where forged by the inspector to try and cover
his negligence.
Conclusion:
This bridge failure was not only the failure of the design of the bridge but also the
procedures that were in place for the maintenance of the bridge. Before 1976 no funding
had been allocated to repair and maintenance, after the collapse there was a need for a
change, the Mianus bridge failure brought this issue to the forefront. Before the failure
only twelve engineers, working in two-man teams had to inspect the state’s 3,425 bridges
with inadequate equipment and man-power. As a result, the state embarked on an
unprecedented 10-year $5.5 billion bridge rehabilitation, maintenance and inspection
program.
The tragic collapse of the Connecticut Turnpike Bridge over the Mianus River in
1983 made engineers more aware of the dangers of negligence. One of the most
innovative structural engineers of the past generation, Lev Zetlin, summed up the feelings
of the engineering community with the following comments, ‘Even a fatal accident like
the Mianus bridge collapse can have a positive side. I hope and believe that the things we
find on the Mianus River Bridge will keep other bridges up. What is needed is preventive
engineering. I look at everything and try to imagine disaster. I am always scared.
Imagination and fear are among the best engineering tools for preventing tragedy.’