Collapse of the Mianus River Bridge.
Route 95, Highway Bridge, over the Mianus River Greenwich, Connecticut. June 28,
1983.
Introduction:
The Mianus river bridge is a multi span structure carrying more than 100,000 car
daily, over one of the most highly traveled routes in the world. It consists of two, side by
side, three lane bridges of cantilever construction. Steel plate girders run over two piers,
projecting beyond the face of the piers, to hold the suspended spans. The mianus bridge,
had two suspended spans, 30 metres long on each side of the central span.
Design flaws:
The east end of the easterly suspended span was hung by means of a pin-and-hanger
assembly from the ends of the east cantilever. The failure of a single pin or bar in this
assembly could cause the span to fail; it had no redundancy built in.
The assembly had been recognized as unsafe since 1968 and was not used in new
construction. Most new bridges built using pin-and-hanger assemblies were made safer
by using a steel yoke which acted as a second hanger in case of failure of the first.
The west end of the span sat in a cradle, called a pillow block, on a horizontal pin
attached to the end of the cantilever from the centre span. Each hanger assembly
consisted of two vertical steel plates pin at the top to the cantilevered end of the anchor
span and at the bottom to the end hung span. These hanging plates tended to be in a
vertical position but could swing from it, allowing shortening of the hung girder in cold
weather and it’s lengthening in the heat or summer.
Negligence:
Over the 25 years since it was built, Mianus River Bridge had fallen into disrepair,
less than one year before the disaster, an extensive survey was due to be carried out by
the Connecticut D.O.T. Unfortunately, due to equipment being out of service (a snooper
had been out of action for eleven months), they were unable to carry out the survey from
the deck of the bridge, instead resorting to the use of binoculars to inspect the structure
from the banks below.
In the late spring of 1983, the local residents reported shrill noises coming from the
bridge. For five or six years, they had been finding pieces of concrete and steel on the
local riverbanks, all of these findings had been reported to the Connecticut D.O.T.,
however in the time immediately preceding the accident, a newer, more urgent sound had
been reported, “ a high piercing sound, like thousands of birds chirping”.
The Collapse:
On the night of June 27th 1983 at 2a.m., news of the bridge collapse was received.
Locals were awoken by a loud noise, “like a clap of thunder”, which shook their homes.
In the accident two cars and two tractor-trailers lay crushed in the shallow water below,
as part of the bridge fell from its suspended perch. The disaster took the lives of three
people and left another three seriously injured.
Initial surveys of the wreckage seemed to lay the blame at flaws in the construction of
the bridge. One of the pins, connecting the fallen section to one of the cantilever girders,
was missing. A segment of the 18cm pin was eventually recovered in the river, while the
rest was found attached to the cantilevered part of the bridge.
The pins used to hold the construction together were held in place with locking caps.
The caps were found to have been dished out, changed from there normal flat shape to a
saucer like design, showing the presence of a large force pushing on them. Their
thickness of 8mm, was totally out of proportion with the large size of the pins, and was
less than one half of the design regulations in force at the time of construction, 1957.
There were also heavy concentrations of rust on the various parts of the construction.
This also may have lead to the collapse. Rust building up in confined spaces would exert
a tremendous pressure on the restraining structure, due to the increase in volume. This
rusting may have caused the dishing of the cap plates, resulting in the failure.
The unusually high
amount of rust found,
led the investigators to
search for a reason why.
It was found that, ten
years previous, drains
had been paved over,
allowing water, salt and
dirt to flow through the
joint between the
cantilevered section and
the suspended one. Dropping directly onto the hanger assembly and causing the
accelerated rusting of the steel.
This corrosion, combined with the excessive loading of the pins was a major factor in
the bridge collapse. Due to the bridges orientation to the river, it suffered far larger lateral
loading than most structures, estimates of up to six times greater forces acting on it. The
force exerted on the deck by cars braking and slowing down also would have induced
lateral forces on the bridge. These forces are believed to have contributed to the popping
off of the weakened pin cap.
The Conclusions:
As a result of the mianus
river bridge collapse, a number
of measures were introduced to
help stop this type of problem
from reacurring.
The type of design had
already been deemed unsafe, so
new constructions were made
without a redundancy measure in
place, but an extensive survey of
existing bridges with a similar
design was carried out to eliminate the same scenario from occurring.
New guidelines were drawn up for the inspection of bridges, detailing exactly what
must be covered and in how much detail. Bridges which had been paved in a similar
manner to this bridge were also checked for adequate drainage routes, to prevent
excessive corrosion to their structure.
New laws were introduced for the maintenance and inspection of critical elements of
bridge assemblies. And the standard inspection procedure in place at the time of this
accident was changed to include a face to face portion with the inspector, to prevent a
repeat of the attempted report tampering, which occurred here.
Group 11:
Paul Cawley
Katie Frost
Jack McInerny
Donal Salmon