The Mianus River Bridge Collapse
Introduction
The Mianus River Bridge is a section of interstate 95 that runs through
Connecticut and carries over 100,000 vehicles per day. On the 28th of June 1983 at
1.30am, a 30m section of the eastbound span collapsed and fell 21m into the River
Mianus below causing 3 fatalities. The bridge was constructed in 1957 and is a steel
deck bridge of welded construction that has 24 spans, 19 of which are approach spans,
and is 810 m long.
Bridge Design
The highway is six lanes wide across the bridge, but an expansion joint on the
centreline that runs the length of the bridge separates the structure into two parallel
bridges with three lanes on both that act independently of each other. The bridge
design consists of five spans over the River Mianus that are symmetrically arranged
around a 62.5m central main span. On either side of the main span there is a 30m
suspended span as well as a 36.6m anchor span. Both the anchor span and the main
span are cantilevered 13.7m beyond their piers and the suspended span is connected
between them by using a pin and hanger system.
Design Flaws
The deck of the bridge consists of two parallel steel girders with floor beams
that frame into the girders. The continuous five span girders contain four internal
hinges where they joined to the suspended span section. As a result of this, the
suspended spans are therefore statically determinate meaning that there is no
alternative path for the load. The design of the suspended span consisted of two
different pin assemblies. On the western edge, where it joined to the cantilevered arm
of the main span, a pillow block cradle pin system without hangers was used.
But on the northeast and southeast corners where the suspended span joined to the
anchor span a pin and hanger system was used. The spans were joined together by a
pin in the 2 ½” web of each girder, with the upper one attached to the cantilever arm
of the anchor span and the lower level arm attached to the suspended span. The two
pins were connected either side of the web by a steel hanger. There were no redundant
members in the bridge design as it was statically determinate structure which meant
that if a member failed, the structure would fail.
Failure
The bridge failure started to occur when the inside hanger at the southwest corner of
the suspended span dislodged from the end of the lower pin, due to the corrosion
packout in the assembly. This meant that all the weight and forces of the southeast
corner of the span were transferred to the cantilever by the outside hanger only,
doubling the load it had to take. The outside hanger gradually moved outward on the
pin and deformed. This caused fatigue cracks to form at the top of the upper pin.
With the repeated loading and unloading of traffic passing over the bridge, as well as
the weight of the span itself, the cracks grew and eventually the upper pin fractured
off, causing the suspended span to fail and fall into the river.
Causes of Failure
The official highway accident report by the National Transportation Safety Board
concluded that the span failed due to corrosion induced forces and poor inspection
and detection methods.
The corrosion induced forces occurred primarily due to changes to the original design
of the rainwater draining system. The original curb drains were covered up in 1973
with plates when the road was resurfaced. This meant the rainwater had nowhere to
go except down the bridge deck towards the expansion joint between the suspended
span and the cantilever arm. During heavy rainfall a lot of water would leak into the
pin and hanger assembly area and begin to corrode the assembly. The corrosion
effect led to the build up of rust, resulting in extensive corrosion packout between the
outer washers on the upper and lower pin assembly. (See fig.1 (c)) This then led to
the deformation of the retaining plate and hanger, and high tensile forces in the bolt.
The bolt eventually fractured allowing the hanger to slip off the end of the pin,
starting the chain of events that led to the span failing.
The corrosive action that started the failure took place over a period of time and
should have been detected with properly conducted inspections. However the
inspections were lax, with the pin and hanger assemblies on the outer girders
inspected from the ground using binoculars and here were also only 12 engineers
working in pairs to inspect 3,425 bridges in the state of Connecticut. The rusting on
the pins was noted, but no action was taken after the last inspection in 1982. The
damage to the assembly could have been repaired and the failure averted if there was
a good inspection and repair program in operation.
Conclusion
After the bridge failure, the report into the accident suggested that the paved over
drains on all bridges in the state of Connecticut be reopened and new drainage
systems be put in place to divert rainwater runoff away from the steel members of
bridge structures. They also suggested that better inspection procedures be put in
place, including the use of specialized equipment to access the bridge area being
inspected, and the recording of displacement measurements and comprehensive
inspection report reviews.
Although the type of design was deemed unsafe and new constructions were made
without a redundancy measure in place, more blame was proportioned to the state for
not maintaining and inspecting the critical elements of the bridge’s assembly. This
was exemplified when the engineering company were cleared of responsibility for the
partial collapse by Connecticut’s superior court.
Group11
Paul Hennessy
Andrew McCreddin
Adam William Kearns
Sources
http://www.ntsb.gov/publictn/1984/HAR8403.htm
http://media.wiley.com/product_data/excerpt/60/04714143/0471414360.pdf
From: Metal Failures: Mechanisms, Analysis, Prevention
Arthur J. McEvily
Wiley
http://media.wiley.com/product_data/excerpt/83/04716975/0471697583.pdf
From: Design of Highway Bridges: An LRFD Approach, 2nd Edition
Richard M. Barker, Jay A. Puckett
Wiley