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